How to build homemade high-precision thermometer

by Shawn Carlson

Last month I described how to build a triple-point cell, a device that reproduces the unique temperature, defined to be exactly 0.01 degree Celsius, at which water can exist with its solid, liquid and vapor phases all in equilibrium. The cell can be used for calibrating state-of-the-art thermometers, but few amateur scientists can afford such expensive instruments, which cost thousands of dollars.

Fortunately, George Schmermund, the creative genius from Vista, Calif., who developed our triple-point cell, has also designed a thermometer capable of measuring temperature to within a few thousandths of a degree C. What is more, you can build this remarkable instrument for less than $100.

Schmermund's thermometer uses something called a resistance temperature detector (RTD), which relies on the fact that the resistance of platinum changes with temperature in a precisely known way. For each degree C of temperature change, these sensors typically change their resistance by 0.00385 ohm per ohm of resistance. For example, if your RTD has a resistance of 100 ohms, each degree C change in temperature will alter the resistance by 0.385 ohm. So if you know the probe's resistance at a particular temperature, such as the triple point of water, you can then convert any measured resistance into a corresponding temperature.

EXACT TEMPERATURE MEASUREMENTS to within millidegrees can be made with a thermometer that relies on a resistance temperature detector (RTD)--a sensor that exploits how the electrical resistance of platinum changes as the material becomes hotter or colder. The relation is linear and is given by the equation shown (above), where a (Alpha) is typically 0.00385 and RTP is the resistance of the sensor at 0.01 degree Celsius: the triple-point temperature of water. A digital multimeter measures the RTD's resistance.

In the past, RTDs were always made of wire. Because the wire had to be thick enough to withstand the manufacturing processes and because a larger-diameter wire has less resistance than a smaller one made of the same material, the operating resistance was limited to about 100 ohms. Recently, though, a new breed of RTDs has been constructed by laying an ultrathin platinum coating on a ceramic substrate. The resistance of some of these devices tops 2,000 ohms. You'll find a smorgasbord of these marvels in the catalogue of Omega Engineering in Stamford, Conn. (; 800-826-6342). For this project you'll need a model like the F3141, a small, unencapsulated 1,000-ohm unit that sells for $19.

These new RTDs can bring exquisite sensitivity into the home-based laboratory. Using a high-quality handheld digital multimeter that can measure 1,000 ohms of resistance to within 0.02 ohm, amateurs can now resolve temperatures to within 0.005 degree C, or 0.009 degree Fahrenheit. That performance bests liquid-filled thermometers by 20 times and trumps any thermocouple by a factor of 10.

And you can do much better. In practice, the sensitivity of an RTD-based thermometer is limited by how accurately you can determine its resistance, which is measured by observing the voltage drop associated with a known current. With a typical digital multimeter, the lead wires are part of the circuit, and so their resistance affects the results. This error can be eliminated by measuring the voltage drop directly across the resistor with an independent set of wires. Such instruments, called four-wire ohm meters, have separate inputs for a current source and a volt meter.

Hewlett-Packard's snazzy HP 34401A multimeter (priced at $995) uses the four-wire technique to measure 1,000 ohms to within 0.001 ohm. And top- of-the-line instruments costing $30,000 apiece enable professionals to resolve temperature differences as slight as 10 millionths of a degree C. In such a four-wire configuration, an RTD-based thermometer is called a standard platinum resistance thermometer (SPRT).

To build Schmermund's thermometer, contact a local glassblower to purchase a Pyrex tube 30 centimeters (12 inches) long and eight millimeters (0.3 inch) in diameter. At one end of the tube, have the glassblower form a receptacle that is five centimeters (two inches) long for the RTD sensor.

Next, attach lead wires to the sensor. If you solder the leads or use wires insulated with plastic, you'll be restricted to temperatures below the melting point of those substances. That's not a problem for many applications. To allow the maximum possible range of temperatures, however, Schmermund spot-welds the RTD to bare 10-mil nickel wires that he then insulates in thin Pyrex sleeves. He gets these sleeves in 46- centimeter lengths from a local glassblower, but capillary tubes, which are available from any scientific supply house, work equally well when strung on the wire like beads on a necklace.

For a thermometer that will be used with a four-wire ohm meter, Schmermund bundles four of his long tubes and delicately tapes them together at one end. He then bends two one-meter lengths of nickel wire in half and threads each half through a different tube from the untaped end. Finally, he spot-welds the RTD to the bends in the two wires. (Note: If you will not be making four-wire measurements, simply connect one wire to each of the RTD leads.)

To secure the insides of the device and to thwart convection currents from forming, Schmermund packs the instrument with tiny glass beads that are only about 25 microns in diameter. These are expensive and must be purchased from a scientific supply house. Fortunately, fine silica sand (grit 30 or greater) also does the job. You can purchase a 23-kilogram (50-pound) sack from a hardware store for just a few dollars.

Because any moisture that becomes trapped inside the thermometer will distort your readings, all water must be driven from both the filler and the glassware before assembly. Bake everything, including the entire sensor assembly, at 250 degrees F for approximately two hours.

You must complete the next steps while everything is hot, so be sure to exercise the proper care by wearing gloves, an eye shield and protective clothing. Secure the large tube in a vise. A clean rag wrapped around each jaw will allow you to hold the glass tube firmly without breaking it. Insert the RTD assembly into the tube and use a small glass funnel to pour in enough of the desiccated sand to cover the sensor completely. Lift the assembly just a bit to make sure the RTD is suspended about two millimeters above the bottom of the well, without it touching the glass wall. Remove the tape and slowly fill the tube with hot sand to within about half a centimeter from the top, stopping frequently to tap the glass with a pencil to consolidate the material.

Hermetically seal the thermometer by topping off the sand with glue from a hot-glue gun. If you're using uninsulated wires, heat them with a hair dryer for a few seconds before the adhesive sets so that the wires will seat themselves into the glue.

To minimize signal interference, connect the probe to your ohm meter through a stereo microphone cable, which consists of two twisted pairs of wire shielded inside a metal sheath that you must ground. Use a four- wire terminal strip to connect each twisted pair across the device. Solder the wires and protect the strip inside a plastic canister from a roll of 35-millimeter film.

This homemade instrument, which is functional up to about 400 degrees C, can open up fascinating avenues of research. Although the device is a bit cumbersome for fieldwork, you can use it for accurately calibrating other thermometers. In the laboratory, it will also help you probe the nature of phase transitions and measure the strength of chemical bonds (for ideas, see the March 1996 Amateur Scientist). With a little imagination, this thermometer can become a powerful weapon in your arsenal of research techniques.

How to build a combat robot

Experience the excitement of building your own champion battling bot!

Build a powerful and invincible robot--for full-blown competition or just for fun--using this authoritative robot resource. This team of experts gives you an inside look at the innovative new world of robotic combat, explaining the origins of the sport as well as all the elements that go into constructing a fighting robot. You'll learn technical basics from motors and wiring to locomotion, and read builders' true stories from the front lines of robot competition. Whether or not you're mechanically-minded, you'll find this book both entertaining and informative. Fully capturing the spirit of the sport, this detailed guide shows you how an imagination and a few bits of metal can start you on your way to constructing your own champion bot.

  • Plan, design, and build your own battling robot
  • Discover how electric motors work and choose the right type for your design
  • Learn about both wheeled and walking robots
  • Incorporate creative weapons and armor on your bot--learn the pros and cons of spinning hammers, jabbing spikes, buzzing saw blades, and more
  • Minimize radio interference--so you can effectively control your bot to do what you want, when you want
  • Understand what goes into building fully autonomous robots
  • Save money on parts and equipment by enlisting help from company sponsors
  • Read true stories from combat roboteers and learn which types of constructions work and which don't

About the Author
Pete Miles (Graham, WA) has worked in robotics for over 12 years. He holds a Master's degree in mechanical engineering and is currently working as Senior Research Engineer in advanced machining technologies for Ormond LLC in Kent, WA. Miles is active with the Seattle Robotics Society and was recently appointed to the SRS Board of Directors.

Tom Carroll (Eastsound, WA) has been involved with robotics for over forty years. He built his first robot at age 14, and later worked as a robotics engineer for Rockwell International for nearly thirty years. He built the robots that were featured in the movies Revenge of the Nerds and Buck Rodgers in the 25th Century, and founded his own company, Universal Robot Systems, which focuses on personal robots to assist the elderly and disabled.

Full Read & Save

How to make biodiesel

What is biodiesel? Simply put, it is diesel fuel that is made from vegetable oil. It will run in any unmodified diesel engine. It has many advantages over petroleum diesel fuel such as: 1) It burns cleaner 2) It has a higher cetane rating (less knocking) 3) It has better lubricity 4) You can make it yourself from used vegetable oil (a waste product) often for less than the cost of petroleum diesel.
You will need the following things to make your first batch:
  1. At least 1 Litre (1.1 Quart) vegetable oil. Canola oil, corn oil, soybean oil, etc will suffice.
  2. A variable speed blender with a slow speed option. Use one with a glass pitcher only. The methanol that is used in this process will "eat" a plastic pitcher. Make sure that this blender will never be used for food products again.
  3. A scale that will accurately measure 3.5 grams (.12 oz). I use a triple beam balance available through Edmund Scientific. Search the site for the keyword "balance". A good scale will cost between $100 and $200 and is a good investment. However, if you are on a budget, you can get the Edmund Scientific "Carry-With-You Twin Beam Balance" (Stock Number: CR30360-28) which will weigh up to 4 grams. This costs $25.
  4. 1 bottle Red Devil Lye Drain Cleaner (Sodium Hydroxide) available from you local hardware store. Make sure the label says "contains sodium hydroxide". Most other drain cleaners are chlorine (Calcium Hypochlorite) based and will NOT work! Notice: Lye is poisonous! Take all necessary safety precautions!!
  5. At least 200 milliliters (6.8 fl. oz) of methanol (Methyl Alcohol or "Wood" Alcohol). Methanol is widely available in 12 oz. quantities as "gas tank antifreeze" in auto parts stores, hardware stores and even some grocery stores. Popular brands include "Heet" and "Pyroil". Read the label carefully and make sure it says "contains methanol"! Many gas line antifreeze products contain isopropyl alcohol or "isopropanol" and will NOT work! Methanol is available in larger quantities as racing fuel through some racetracks that cater to drag racers and some "high performance" auto parts stores. Keep in mind that Methanol is both poisonous and flammable. Take all necessary safety precautions!!
  6. A glass container that is marked for 200 milliliters (6.8 fl. oz). We like to use a beaker.
  7. A glass or plastic container that is marked for 1 liter (1.1 Quart)
  8. A wide mouth glass or plastic container that will hold at least 1.5 litres
  9. A common spoon (preferably plastic or stainless steel).
  10. Safety Glasses and Rubber Gloves! Methanol and Lye are extremely poisonous and must not come into contact with skin or eyes! Methanol is a poison that attacks the eyes (ocular nerves) even if it comes into contact with your hands. Use extreme care when blending the methanol and lye, as the blender can spash the chemicals around. Put on your glasses and gloves BEFORE opening the chemicals! Do your work close to a sink or hose, or have a bucket of water handy to wash any part of your body immediately if it comes in contact with these chemicals.
Get organized in a well lit, well ventilated area! This process is best done at or above room temperature (70 degrees F or 21 Degrees C). Temperatures lower than 60 F or 15 C may cause an incomplete reaction. Plan for spills by spreading paper or plastic on your work surface. Put your safety glasses and gloves on before opening any chemicals!

What you work
  • Measure 200 milliliters (6.8 fl. oz) of methanol
  • Pour the methanol into the blender. Notice the glass pitcher on the blender.
  • Weigh out 3.5 grams of lye on your scale. Notice that we use a white piece of plastic to hold the lye. The weight of the plastic is 4 grams, so we set the scale to 7.5 grams.
  • Turn the blender on "slow" speed and slowly add the lye to the methanol. You now have a mixture called "sodium methoxide". The methoxide must be used right away to make biodiesel. Do not plan on making a large batch of methoxide and storing it for use later. It loses its potency over time.
  • After the Lye has completly dissolved into the methanol (about 2 minutes), add 1 liter of vegetable oil to the blender. Blend on low speed for 20 to 30 minutes. The ideal speed for this process just barely creates a vortex or "tornado" in the oil without spashing the mixture around or frothing it up.
  • After the blending is complete, pour the mixture into the wide mouth jar. It is advisable to label all containers used in this project as "POISON"! And of course, keep all of this stuff away from children!
  • After about 30 minutes to 1 hour, you will notice a layer of darker colored glycerin settling to the bottom of the container. The lighter layer on top is biodiesel. Wait another few hours for complete settling. At that point, you can carefully pour off the lighter biodiesel from the top and discard the glycerin (or save the glycerin to use in soapmaking). An alternative would be to use a pump to remove the biodiesel from the jar. You are done!
It is always wise to use a "diesel fuel filter/water separator" with any diesel engine. These are available through some auto parts stores or A good model is the Racor 120AS diesel fuel filter/water separator (West Marine # 411348).

Biodiesel has a solvent effect on natural rubber hoses and seals. While newer diesel engines have polymer hoses and seals (such as Dupont's "Viton" brand), older engines may need to be outfitted with new hoses and/or seals made of viton. Since most diesel injector pumps don't have rubber parts directly in contact with the fuel, it is usually easy to replace hoses and seals without any major dissasembly. A fuel mixture of 20% biodiesel and 80% petroleum diesel (called "B20") will have no effect on older natural rubber hoses.

Biodiesel will "cloud" at temperatures below 55 degrees F (13 degrees C). While this "clouding" is easily reversible by raising the temperature of the fuel again to above 55 degrees, it may cause temporary clogging of your fuel system, thus stopping your engine. Petroleum diesel fuel (Diesel #2) can be used down to -10 degrees F (-24 degrees C). It is advisable to use a blend of at least 50% petroleum diesel with your biodiesel if you are going to be operating in cold weather. You can experiment with different blends of biodiesel and petroleum diesel to determine what works best. Simply mix up batches of fuel with different ratios of petroleum diesel and biodiesel in glass jars and put in a freezer. Use a thermometer to determine the temperature of the fuel. Periodically check on the fuel to determine at what temperature it gets cloudy. This temperature is the "cloud point". It is best to determine this point at home before you head out on the road and get stranded in a snowstorm because your mixture is too rich in biodiesel. Of course, if you are going to be operating during the warm months, or in a warm climate, you can use 100% biodiesel with no problems.

How to build simple seismograph

A seismograph is a device used for recording earth tremors. Basically, it is a heavily weighted horizontal rod (pendulum) suspended from a pole. This rod is free to swing from side to side if the earth shakes. One end of the rod rests against the pole, while the other holds a pen or stylus. This stylus marks a slowly moving roll of paper. If there is no shaking, the passing paper is marked with a straight line. If there is a tremor, the paper is marked with a squiggly line.

The waves that reach the seismograph are, in order: the P or primary waves, which are caused but compression of rock, and which travel straight through the earth; S or secondary waves, which are shear waves caused by rock shaken from side to side; and L waves, which are surface waves caused by rolling motions of the surface. L waves, which travel along the surface, are the last to arrive. They are also the most destructive. Since these waves travel at different speeds, seismologists can pinpoint the origin of an earthquake (epicenter) by comparing their arrival times.

A simple seismograph, as shown here, is easy to build and will record local vibrations such as passing trucks or people walking past it. Later in the article are modifications to this design that will make an instrument sensitive enough to record distant earthquakes. The text and photos for this model are from Science Equipment, by William Moore, 1962. You will need:
  • 1 piece steel pipe, threaded at least at one end, 1 inch by 3 feet (25 mm by 100 cm)
  • 1 steel floor flange 3 1/2 inches (88mm) in diameter (should fit the pipe)
  • 1 steel rod 1/4 inch by 24 inches (6mm by 60cm)
  • 1 piece steel wire app. 3 feet (100 cm) long
  • 1 wind up alarm clock
  • 1 piece of wood, 3/4 inches by 1 foot by 3 feet (20mm by 305mm by 915mm)
"Begin by screwing the flanged plate to one end of the large board. Next drill holes through the top end of the pipe for the wire. Also drill a hole for the wire at about one inch (25mm) from the end of the steel rod. The steel rod must be pointed at both ends, so place it in a vise and file on alternate sides of the rod. After this is done, drill a small dimple about six inches (150mm) above the threaded end of the pipe in which one pointed end of the steel rod may rest Attach the wire to the end of the pipe and the end of the rod next, and after screwing the pipe into position tightly, place the pointed end of the rod in the dimple. Now wire some heavy weights to the steel rod. These may be lead blocks as shown here, or bricks, or even sash weights.

The recording device should be prepared next. This is done by removing the glass or plastic cover on the clock face and cementing a white cardboard disc to the (hour) hand of the clock. Use a candle or small lamp to cover the disc with soot. Now mount the clock on a piece of scrap wood, so that the point of the steel rod barely touches the disc at the nine o'clock position on the dial. Experiment with the position of the clock until the correct location is found before fastening it permanently. For greater accuracy, be sure to clamp the baseboard to the bench or table."

Once the basic design is understood, it is possible to design and construct a much more sensitive seismograph. There is a great deal of leeway for experimentation. A few tips are:
  • The base should be solid and positioned on the ground. A large paint drum half filled with concrete works well; a concrete umbrella stand works even better.
  • The upright support should be very solid. Metal pipe or angle iron works well as long as it can stand upright and remain very steady. The post should be anchored firmly in the concrete base. Three or four feet high (90-125 cm) should be sufficient.
  • Any rigid length of metal can be substituted for the steel rod arm (pendulum) in the above plan, as long as it rigid enough not to bend when suspended from the upright. A thin strip of metal can be folded to make a strong, rigid channel. It should be about 3/4 long as the upright is tall. The end of the rod that contacts the upright comes to a sharp point. Ideally, the point of contact on the upright should be a bearing, which can be adjusted in or out for fine-tuning. This point of attachment should be about 6 inches (150mm) up from the base (this will depend on how high your recording apparatus is positioned).
  • The wire supporting the arm/pendulum attaches to the top of the upright. It is best to make this attachment adjustable with an arrangement of angle irons, nuts, and washers so that the wire and pendulum can be easily raised, lowered, and moved from side to side in small increments.
  • The wire attaches to the arm/pendulum at about six inches from the upright. The weight should lie near this point of attachment. A metal food can filled with plaster is convenient to work with; the arm/pendulum can pass through it, and the wire can attach to the can itself.
  • The other end of the arm holds the recording stylus. What is used here depends on the material being recorded on. A needle works well for smoked metal, aluminum foil, or glass. If paper is the recording material of choice, pens must be chosen that will write with light pressure, but will not readily dry out. The type of pens used in plotters are one option. Whatever is used, it must be remembered that it will be striking the recording surface very lightly.
  • There is more room for improvisation with the recorder than with the rest of the seismograph. The clock dial method above works well; there is an article on a cylinder type phonograph in these pages that may give the adventurous designer an idea. If you do it this way, though, remember that the drum must rotate easily if it is to be turned by the minute hand post of an electric or wind-up clock. The drum should rotate away from the stylus. It is also possible to record on a roll of adding machine paper using two rotating drums. The recorder should be firmly set so that it does not get knocked around easily.

How to build vacuum engine

Power is transmitted in everyday life most often by electricity. There are other means of power transmission such as high-pressure air, high-pressure hydraulic oil, and, on industrial sites, steam. However, electricity dominates: it is the most versatile form of energy. It can be converted efficiently to any other form of energy, something that is not true of other types.

Most kinds of power transmission have a certain degree of tangibility. The rotating propeller shaft of a large truck leaves no doubt that considerable power flows from the engine at the front to the axles at the rear. Wander near a highpower electric system, and you’ll readily hear the low but insistent hum at the 50 or 60 Hz line frequency; sometimes you’ll even feel the hairs on your body react. The apparently inert wires and cables of high-power electric systems can produce huge and mortally dangerous flashes and sparks if they are disturbed. Similarly noisy and spectacular gas jets signal the presence of even small leaks in compressed air or steam systems.

By comparison, the transmission of power through a vacuum in a pipe seems a peculiarly intangible concept. How can power be apparently transmitted by nothing? But in this project we show that a vacuum can indeed transmit power, and that we can demonstrate a motor rather like an old-fashioned steam engine, an engine that can turn the power transmitted by a vacuum in a pipe into mechanical energy.

The Industrial Revolution that transformed the Western world, starting about 1700, needed mechanical power. At first, increased use and more efficient designs of watermills and windmills could provide that power. But it gradually became evident that the continuous power which steam could provide was going to be needed. It is easy to appreciate the expansive force of steam when you see a kettle boil. However, none of the early steam engines used that expansive power. Instead they used atmospheric pressure (they became known later as “atmospheric” engines), with the steam being used to create a vacuum so that the atmosphere could push a piston. We might today, perhaps less accurately, call them vacuum engines.

There have been times when vacuum power transmission has been used. Perhaps the first example was the system used by Matthew Boulton and his partner James Watt. Near the Boulton and Watt engine factory in Birmingham, England—the world’s first engine factory—was the Boulton and Watt mint, a coin factory operated by one of the company’s own engines. Engineer John Southern devised a system in which a steam-driven vacuum pump partially evacuated a huge pipe, known then as the “spirit pipe.” Individual coin presses were powered by cylinders and pistons connected to the spirit pipe.

Since the time of Watt and Southern, vacuum power distribution has occasionally resurfaced in different places. Vintage automobiles from the 1920s on were sometimes fitted with a kind of vacuum engine to operate the windshield wipers, using the vacuum from the gasoline engine’s inlet manifold. It cannot have been an ideal system: if an engine turns over slowly, the vacuum from the engine would decrease and the wipers would operate more slowly. If you were driving one of these old cars and saw an approaching hazard, you would naturally slow down. And just when you needed more wipes of the windshield to see what was going on, the opposite would happen: the wipers would slow down and you would be left peering through rain-swept glass at exactly the wrong moment!

Today this principle is still being used in at least one application (albeit rarely): vacuum cleaners. In some models of cylinder vacuum cleaners with a rotating brush, the brush is powered by a simple turbine device that is turned by air sucked into a vacuum created by a centrifugal fan in the cylinder.

Our vacuum engine is a “steam engine” type of device. Unlike most steam engines, however, it does not require a fully equipped workshop with lathe, milling machine, and so on. Neither does it need the thousandth-of-an-inch accuracy required of a working model steam engine. The vacuum engine only requires a few hand tools, pieces of wood, plastic tubing, and easily obtained metal hardware, and you don’t need to make anything more accurately than within a millimeter. You won’t burn your fingers, either—because you don’t need steam! It is also easy to make—you can probably assemble one in an afternoon. Nevertheless, it well illustrates all the main working principles of steam engines: piston and cylinder, crank, flywheel, valve gear, and valve timing. Take a look at books like that of Semmens and Goldfinch if you want to know more about steam engines.

What you need
  • Vacuum cleaner (ideally the horizontal cylinder kind)
  • Short section of 18-mm (3/4-inch) hose
  • 300-mm-long, 32-mm-diameter plastic pipe
  • ca. 150-mm-long, 31-mm-diameter round section of wood to fit snugly in pipe
  • Flywheel pulley from an old washing machine
  • Brass rod that will roughly fit the hole in the flywheel
  • Metal shaft and brackets
  • Conrods (e.g., 8-inch by 1/2-inch Erector set strips)
  • Wood pieces
  • Electric drill
  • Bolts and nuts
  • Hot-melt glue

How to build
The basic idea of the vacuum engine is that a piston is propelled up and down to push a crank that connects to a flywheel. The piston is activated by atmospheric pressure on its connecting rod (conrod) side, with periodic pulses of vacuum applied to its piston-head side. The pulses of vacuum pressure are applied by intermittently connecting the low pressure from a vacuum cleaner to the piston. The intermittent connection is made by a slide valve. The valve is synchronized to the flywheel rotation and hence to the piston movement, by being actuated 90 degrees out of phase with the piston in terms of flywheel position.

I found a piece of wooden dowel that fit snugly inside the drainpipe I had chosen. I then used this rod and pipe for both the piston and the slide valve. I suggest that you aim for a piston that is about 1 mm smaller in diameter than the cylinder, both for the piston and for the slide-valve assembly. Try to find plastic pipe that is close to precisely round. (You will find occasional pipes or sometimes even entire batches that are appreciably noncircular; perhaps they have been squashed in storage or loading at the factory or supplier.)

The piston, if it is the right size, needs no preparation at all other than to bevel the edges and to screw on the conrod bracket. The slide valve and its cylinder are more complicated. The cylinder needs two or three holes (an air inlet hole is optional) as shown in the diagram, which all need their edges smoothed. You must drill through the valve body for the vacuum port and then make a slot with a chisel for the transfer port. The transfer port allows air into the drive cylinder after it has completed its power stroke.

I have two suggestions for alternative, simpler slide-valve designs: First, you can omit the air-inlet hole and the transfer port channel, relying on air leaking around the piston and valve. Second, you can omit the air hole and transfer port from the valve body and also cap its end. You can now switch the valve on and off with a simple cylindrical piston (exactly like the power piston), by arranging that the piston just uncovers the holes in the valve body as it reaches top dead center (TDC). You will need to cap the end of the valve cylinder in this design too.

I used Erector set parts to construct the crank plate and light steel strips for the conrods. The pipe work was completed with a washing-machine drain hose, which is typically a fairly generous-bore 18-mm (3/4-inch) corrugated pipe. You can minimize the Erector set parts by making your own bearing for the flywheel and fitting the crank pin directly into a small hole drilled into the flywheel. The bearing for the flywheel can be made using a piece of 6-mm (1/4-inch) steel and a piece of brass rod around 15 mm (5/8 inch) in diameter, glued with epoxy adhesive into the center of the flywheel central hole. Bore out the middle of the brass rod with a 6-mm (1/4-inch) drill, deburr it if necessary with an oversize drill bit or just a sharp knife, then run the drill up and down it a few times until the rod will fit snugly but freely rotate around the 6-mm (1/4-inch) steel rod.

The position of the cylinder on the base plate is not critical. The position of the valve body, however, is more sensitive: it must just begin to open to vacuum when the piston is closest to the flywheel (the position conventionally known as TDC).

You must ensure that every part can move freely. Check that the edges of the holes in the valve piston cylinder are smooth and that the pivots on the pistons and the crank are not binding. If rotated vigorously by hand without the vacuum applied, the engine should turn over at least three or four times. If you find that the engine slows more quickly than this, you should check for excess friction in one of the parts.

What you do
Without a plentiful supply of vacuum, your engine won’t work, so make sure that your vacuum cleaner has powerful suction. The stronger the vacuum—meaning the larger the negative pressure relative to atmosphere—the better the vacuum engine will perform. If you hold any doubts concerning the performance of the vacuum cleaner, try to find some means of measuring the negative pressure it produces. The flow rate that the vacuum cleaner can produce is rather less important, as the flow rate needed by the vacuum engine is fairly low and, unless your fabrication of the device is more precise than I have suggested, much of the air flow will go to supplying leaks rather than to propelling the engine. If your vacuum cleaner has a low flow rate, you can still operate a vacuum engine, but you must make the piston and valve pieces a tighter fit within their cylinders.

Now position the flywheel just a little past the TDC. Apply the vacuum. With luck, you should find that the flywheel should begin to turn of its own accord, rushing down toward bottom dead center (BDC) and then beginning to slow down. But it should be going just fast enough to rotate one complete revolution at low speed, after which the process can repeat. The next time the engine will reach TDC a little faster, and the flywheel will complete its revolution more quickly. With the dimensions given here and a reasonably powerful vacuum cleaner, your vacuum engine should build up in speed until it is whirling around at 300 to 400 rpm or more.

How it works
The vacuum engine works by atmospheric air pressure. When the flywheel is at TDC, air pressure is the same on both sides of the piston, so no force is applied. With the flywheel turned a little, so that the valve opens to the vacuum, air is removed from underneath the piston. With no air pressure below but atmospheric air pressure above, the piston is forced downward.

Curiously, in the engines I have tried, the rather rough-and-ready fit of the wooden piston to cylinder may help, in that the air inlet and the transfer passage in the valve gear did not seem to be necessary. As mentioned earlier, this means that you can simplify the engine and use a piston as the slide valve. With a better standard of construction, you will need a proper slide valve with an air inlet.

Like the original steam engines, your vacuum engine may need a little adjustment before it will run properly (or perhaps run at all). You do need to ensure that all parts run smoothly and are lubricated with a little light oil such as bicycle oil.

The highest friction forces, assuming that all your components are smooth running under freewheel conditions, will be developed when the vacuum is applied to the slide valve. With a fairly close-fitting slide valve, this force will be reasonably low. If, however, like me, you started with a rather loose-fitting slide valve, you will find that it tends to bind. What is happening here is that the valve piston is being pulled hard against the valve cylinder because of air pressure on the side opposite the vacuum cleaner connection. Some oil may fix the problem. If the fit is really loose, worse than 1.5 mm smaller than the cylinder, then it may be necessary to start again and make another slide-valve piston with a better fit. A simpler solution if the fit is not too bad is to glue a cap onto the end of the slide-valve cylinder. This blocks half the flow of leakage air to the slide valve and reduces the force needed to operate the valve. (Thanks to the kids at the Saturday Activity Center in Guildford, U.K., for that tip.) Of course, if you have used the simplified piston-style slide valve, then you will have blocked off the end anyway.

How to build simple motor stepper

The stepper motor is the rather complex type of motor at the heart of much modern equipment. Computers, printers, CDs, DVDs, photocopiers, and countless other machines right up to industrial robots all rely upon it. The stepper motor is designed to respond digitally, which of course makes an excellent match with computers. Send a command to move 713 steps to the interface circuit, and that circuit will pulse the several sets of coils on and off 713 times until the motor output shaft has carried out the instruction.

The stepper might superficially resemble an ordinary motor—it is just coils, iron, and magnets, after all. It differs, though, in several ways. First, it is specifically designed to be able hold a stationary position. Most ordinary motors can’t remain stationary; this project relies on a particularly simple kind of ordinary motor that does happen to have some position-holding ability. Second, the stepper motor is not, like an ordinary motor, designed to run continuously when you supply current; instead it is intended to move one step and then stop. Last, unlike ordinary motors, which connect directly to an ordinary electricity supply like a battery, stepper motors are useless without their matching driver circuit. (The sidebar for this project describes what that driver circuit does.)

In the early days of power electronics, stepper motors were expensive devices, and very large ones still are. Mass production has brought the price of small steppers down to only $10 or $20, which is why they are so widespread, even in simple computer printers costing only $70. Happily, we can demonstrate all the principles of a stepper motor and all its complex sets of coils and driving circuits with just a simple capacitor and a simple 50-cent motor.

What you need

  • A small (10 mm × 10 mm × 20 mm) two-magnet, three-coil DC motor, such as the kind used in countless motorized toys
  • Lightweight plastic wheel to fit on end of motor
  • Changeover switch, ideally a microswitch, 5A current rating
  • Capacitor
  • Battery
  • Wires

For the AC Jiggler

  • Transformer (e.g., 12-V AC output)
  • Resistor (correct value to be selected, but probably from 200 to 2000 ohms)

What you do

Suitable motors should be available from stores that supply small electronic parts, or you can order one from an educational supplier for as little as 30 cents. A simple motor of the type we need has two magnets and three iron-cored coils on its armature or rotor (the rotating part). You should find that when you rotate it, it tends to stop in one of six positions. It should just click into place, almost as if it had six mechanical detent stops. If you fit a wheel onto the motor shaft and mark a line on the wheel, you should be able to check this out precisely. The motor does not have any actual mechanical detents; this effect is due to one of the iron cores lining up with one of the magnets, a phenomenon described in my book Ink Sandwiches, Electric Worms (Experiment 21, “Motor Dice”). The most common motor of this type has a cylinder shape with two opposing flats around the circular cross section, typically about 15 mm across the flats, 19 mm in diameter.

If you can’t easily buy such a motor, you can take apart a few discarded motorized toys until you find one. There are other motors in which the spacing and small number of magnets and cores in the armature mean that they also have positive location angles around the axis. The small motor I recommend, however, has only six stable points; because these six points are basically equivalent in the motors I have, they are particularly suitable.

First you must set up the stepper motor circuit: the motor is arranged to be driven from the capacitor, not from the battery. Each time you operate the switch up and then down, the motor should jump to its next stable position. With each cycle of the switch up and down, the capacitor is first charged up, then discharged through the motor, causing it to hop along by one pole. I used pulses of 6 V from a 2,000 microfarad (µF) capacitor. With these values, I found that I could step six times per revolution, with a rate of 2 Hz. What seems to matter is the energy in the pulse provided. You could use a smaller capacitor, for example 470 µF, but charge it up to a larger voltage of about 12 V.

You can use a simple toggle changeover switch, but it will limit the speed of operation. If you can find a changeover microswitch or a push-button changeover switch, you can run the motor at higher stepping speeds.

Occasionally the 50-cent stepper will jam in a position between its stable positions. This situation becomes clearer if you put a small, lightweight wheel on the motor shaft, as suggested, with an index mark—a large arrow or something —so that you can clearly see whether the motor is stopping between positions. Friction from the commutator is probably mainly responsible for a motor’s stopping between stable positions, but the externally connected load and any gearwheels connecting the motor to that load may also have some effect.

A separate “jiggler” current supply can be connected to deal with a sticking problem as follows: Use an AC transformer connected to the domestic electricity to supply low voltage AC. The AC low-voltage value can be anything from 3 to 30 V, since you should connect it via a large resistor to the motor. Choose the resistor value to yield a suitably low jiggler current: try a 500 or 1,000 ohm resistor to start with, or otherwise select a resistor that yields a current of around 10 mA.

How it works

If you have access to an oscilloscope, you can see what is happening. Connect the motor to a battery and then connect the oscilloscope to it: you will see sharp, high spikes and longer, lower lumps in the voltage as the motor draws and then stops drawing current while the commutator rotates. Put a low resistance like 1 ohm in the motor lead (use a high-current resistor like a 1-W or 10-W type so you don’t burn the resistor out), and then you can precisely measure the current flowing. You should find sharp pulses coming from the capacitor and less spectacular spikes coming from the motor: we calculate below how wide the capacitor pulses should be, but something on the order of 5–20 milliseconds is probably what you will get with the recommended values below.

The sharp, high voltage spikes derive from the motor’s armature coil and its iron core. The motor coil stores small amounts of energy in the magnetic field in the iron core, and this energy is released in sharp spikes of voltage when that field collapses (i.e., when the motor is disconnected by the commutator). This is why electric motors are noisy, electrically speaking, and radiate radio-wave noise that can be picked up on radio and television sets. Most commercial commutator motors, like the ones in your vacuum cleaner or electric drill, use capacitors to suppress this effect.

How to build photochemical solar cell

The photochemical solar cell has grown out of an expanding branch of technology—biomimickry, looking at how we can mimic natural processes to make more advanced technologies. Rather than having a single thing to do all of the jobs, as in a conventional photovoltaic cell, photochemical solar cells mimic processes that occur in nature. Electron transfer is the foundation for all life in cells; it occurs in the mitochondria, the powerhouses of cells which convert nutrients into energy.

Titanium dioxide, while not immediately springing to mind as a household name, is incorporated in a lot of the products that we use every day. In paints, as a pigment, it is known by its name titanium white. It is also used in products such as toothpaste and sunscreen. Titanium dioxide is great at absorbing ultraviolet light. [You might find titanium dioxide referred to as “Titania” in some references.]

You will need
● Berries
● Motor
● Alligator clips
● Wires
● Nanocrystalline TiO2 Degussa P25 powder in mortar and pestle
● Glass plates

● Petri dishes
● Tweezers
● Pipette
● Pencil

We need to get our titanium dioxide ground down so that the particles are as small as possible—this maximizes surface area, and so allows our reactions to take place quickly. To do this, we will need the mortar and pestle mentioned in our “tools” list. Be careful not to inhale any of the fine titanium dioxide powder as you are grinding, as it won’t do you any good!

Now that we have prepared our suspension of titanium dioxide, we need to coat it onto our glass plate using a glass rod. The next thing that we need to do is sinter the titanium dioxide film in order to reduce its resistivity. To do this, we hold it in a Bunsen flame and allow the gas to do the work! We need to hold the plate at the tip of the flame where the temperature is approximately 450 C or 842 F. Hold it steady for around 10–15 minutes.

Now you need to produce the dye which will sensitize our photochemical solar cell. There area number of suggestions for different substances which can be used for this cell. You can try:

● Blackberries
● Raspberries
● Pomegranate seeds
● Red hibiscus tea in a few ml of water

To produce the dye, you need to take the substance you are going to make the dye from, and crush it in a small saucer or dish. Once this has been done and a nice fluid has been produced, take the plate which has been coated in titanium dioxide, and immerse it in the dye. The titanium dioxide film should now be stained a deep red to purple color and the color distribution should be nice and even. If this is not the case, you can immerse the plate in the dye again. Once you have finished staining the plate, take a little ethanol and wash the film and then with a tissue, blot the plate dry.

Now we need to prepare the other electrode. To do this you will need another of the coated glass plates (the one with the conductive tin oxide coating—not the one with a titanium dioxide coating). You need to find which is the conductive surface. There are two ways of doing this—thetactile method is to simply rub the plate. It should feel rougher on the coated side. The other involves a voltmeter or continuity tester. The conductive side is the one which yields a positive reading when tested for continuity.

We now need to deposit a layer of graphite. The easiest way to do this is take a soft pencil, and simply scribble on the surface until a nice even coating of graphite is obtained. Just note that you need to do this with a plain pencil not a colored one!

Now if you have got this far, you are on the home run! The next thing we need to do is take some of the iodine/iodide mixture, and spread a few drops evenly on the plate that was stained with the dye. Once you have done this, take the other electrode and place it on top of the dyed electrode. Stagger the junction between the two plates in order that you leave a little of each exposed at either end—you can then use a couple of crocodile clips to connect the cell to a multimeter.

Now clip the sheets of glass together carefully to ensure they stay together (Figure 1) and connect a multimeter—we can start to think about doing some really cool stuff now! You might like to try a few different experiments—like seeing what way to shine the light through the cell for the most effective operation. You might like to repeat some of the experiments in the section on photovoltaic solar cells, and see what results you obtain with a photochemical solar cell.

Another educational idea is to use a multimeter to measure the amount of power from both a photovoltaic solar cell, and the photochemical solar cell you have made, and compare the results—now work out their relative efficiencies taking into account the area of the cells. Now we can take some measurements! This photochemical cell able to yield 6.0 mA; it could be used to drive a small motor and fan.

The materials required for this project are available from the Institute of Chemical Education from the following link or the address in the Supplier’s Index:

Where does it all go from here?
This technology has a lot of promise for the future. There is a growing trend for manufacturers to integrate renewable energy systems into building elements—this allows us to feed two birds with one crumb, rather than shelling out for roof tiles and solar cells, why not buy a solar roof tile! The exciting thing about photochemical solar cells is that unlike photovoltaic cells, they don’t necessarily have to be opaque. This opens up exciting possibilities—shaded windows and skylights which simultaneously produce electricity. How cool would that be!

When you consider all of the glazing that adorns the skyscrapers in our cities, you begin to realize that this technology has interesting applications for energy generation. It also allows us to make good use of daylight with our south-facing building areas, while generating energy at the same time.

There are also implications for consumer electronics, the watch giant Swatch has already built a prototype watch with a photochemical cover glass. This allows the glass which covers the watch to generate electricity all the time the watch is exposed to light. When you think that people wear watches on their wrists where they are permanently exposed to daylight, this becomes quite a sound idea! Of course, you also need some means of storing the electricity to enable the watch to run at night! It would be no good to wake up, put on your watch, only to find the time is set to the evening before.

Are there any limitations to this technology?
One of the problems with this particular type of cell is that the cell contains liquid which is essential for its function. Unfortunately, liquid is hard to seal and keep in—preventing the liquid from leaking is a real technical issue that needs to be solved. After all, you wouldn’t want leaky windows! If you have ever seen a poorly fitted double glazing panel with condensation inside, you realize how hard it is to seal building fixtures and fittings against the ingress or egress of fluid.

However, there is hope on the horizon, Michael Grätzel together with the Hoescht Research & Technology in Frankfurt, Germany, and the Max Planck Institute for Polymer Research in Mainz, Germany, have announced that they have developed a version of the cell with a solid electrolyte; however, efficiencies are low.

Photobiological solar cells?
Truth can sometimes be stranger than fiction. Realizing that conventional solar cells require expensive industrial processes, researchers at Arizona State University have initiated a project codenamed Project Ingenhousz which is looking at photosynthesis and how organisms can be used to harness solar energy to produce fuels that will wean us away from our carbon-based fossil fuels. Could your car one day run from hydrogen that has been produced by algae from solar energy?