A Solar Engine is a circuit that takes in and stores electrical energy from solar cells, and when a predetermined amount has accumulated, it switches on to drive a motor or other actuator. A solar engine is not really an 'engine' in itself, but that is its name by established usage. It does provide motive force, and does work in a repeating cycle, so the name is not a complete misnomer. Its virtue is that it provides usable mechanical energy when only meager or weak levels of sunlight, or artificial room light, are present. It harvests or gathers, as it were, bunches of low grade energy until there is enough for an energy giving meal for a motor. And when the motor has expended the serving of energy, the solar engine circuit goes back into its gathering mode. It is an ideal way to intermittently power models, toys, or other small gadgets on very low light levels.

It is a great idea which was first thought up and reduced to practice by one Mark Tilden, a scientist at Los Alamos National Laboratory. He came up with an elegantly simple two-transistor solar engine circuit that made tiny solar powered robots possible.

Since then, a number of enthusiasts have thought up solar engine circuits with various features and improvements. The one described herein has proven itself to be very versatile and robust. It is named after the day on which its circuit diagram was finalized and entered into the author's Workshop Notebook, Easter Sunday, 2001. Over the years since, the author has made and tested several dozen in various applications and settings. It works well in low light or high, with large storage capacitors or small. And the circuit uses only common discrete electronic components: diodes, transistors, resistors and a capacitor.

This Instructable describes the basic Easter Engine circuit, how it works, construction suggestions, and shows some applications. A basic familiarity with electronics and soldering up circuits is assumed. If you haven't done anything like this but are eager to have a go, it would be well to first tackle something simpler. You might try the The FLED Solar Engine in Instructables or the "Solar Powered Symet" described in the book "Junkbots, Bugbots, & Bots on Wheels", which is an excellent introduction to making projects such as this one.

Step 1: Easter Engine Circuit

This is the schematic diagram for the Easter engine together with a list of the electronic components that make it up. The design of the circuit was inspired by the "Micropower Solar Engine" by Ken Huntington and the "Suneater I" by Stephen Bolt. In common with them, the Easter engine has a two-transistor trigger-and-latch section, but with a slightly different resistor network interconnecting them. This section consumes very little power in itself when activated, but allows enough current to be taken out to drive a single transistor that switches on a typical motor load.

Here is how the Easter engine works. Solar cell SC slowly charges up the storage capacitor C1. Transistors Q1 and Q2 form a latching trigger. Q1 is triggered on when the voltage of C1 reaches the level of conductance through the diode string D1-D3. With two diodes and one LED as shown in the diagram, the trigger voltage is about 2.3V, but more diodes can be inserted to raise this level if desired.

When Q1 turns on, the base of Q2 is pulled up through R4 to turn it on also. Once it is on, it maintains base current via R1 through Q1 to keep it on. The two transistors are thus latched on until the supply voltage from C1 falls to around 1.3 or 1.4V.

When both Q1 and Q2 are latched on, the base of the "power" transistor QP is pulled down through R3, turning it on to drive the motor M, or other load device. Resistor R3 also limits the base current though QP, but the value shown is adequate to turn the load on hard enough for most purposes. If a current of more than say 200mA to the load is desired, R3 can be reduced and a heavier duty transistor can be used for QP, such as a 2N2907. The values of the other resistors in the circuit were chosen (and tested) to limit the current used by the latch to a low level.

Step 2: Stripboard Layout

A very compact embodiment of the Easter engine can be constructed on ordinary stripboard as shown in this illustration. This is a view from the component side with the copper strip tracks below shown in gray. The board is only 0.8" by 1.0", and only four of the tracks must be cut as shown by the white circles in the tracks.

The circuit depicted here has one green LED D1 and two diodes D2 and D3 in the trigger string for a turn-on voltage of about 2.5V. The diodes are positioned upright with the cathode end upward, that is, oriented toward the negative bus strip on the right hand edge of the board. An additional diode can be easily installed in place of the jumper shown from D1 to D2 to bump up the turn-on point.

The turn-off voltage can also be raised as described in the next step.

Of course, other board formats can be used. The fourth photo below shows an Easter engine built on a small general purpose prototyping board. It is not as compact and orderly as the stripboard layout, but on the other hand it leaves lots of room for working, and space for adding diodes or multiple storage capacitors. One could also use just plain perforated phenolic board with the necessary connections wired and soldered below.

Step 3: Trigger Voltages

This table shows the approximate turn-on voltages for various combinations of diodes and LEDs that have been tried in the the trigger string of various Easter engines. All of these trigger combinations can be fit onto the stripboard layout of the previous step, but the 4-diode and 1 LED combination would have to have a diode-to-diode joint soldered above the board.

The LEDs used in making the table measurements were older low intensity reds. Most other newer red LEDs that have been tried work about the same, with maybe a variation of only about plus or minus 0.1V in their trigger level. Color has an influence: a green LED gave a trigger level of about 0.2V higher than a comparable red. A white LED with no diodes in series gave a turn-on point of 2.8V. Flashing LEDs are not appropriate for this engine circuit.

A useful feature of the Easter engine is that the turning-off voltage can be raised without affecting the turning-on level by inserting one or more diodes in series with the base of Q2. With a single 1N914 diode connected from the junction of R4 and R5 to the base of Q2, the circuit turns off when the voltage drops to around 1.9 or 2.0V. With two diodes, the turn-off voltage measured approximately 2.5V; with three diodes, it turned off at about 3.1V. On the stripboard layout, the diode or diode string can be located in place of the jumper shown above the resistor R5; the second illustration below shows one diode D0 thus installed. Note that the cathode end must go to the base of Q2.

Thus it is possible to effectively use the Easter engine with motors that do not run well near the basic turn-off of about 1.3 or 1.4V. The solar engine in the toy SUV in the photos was made to turn on at 3.2V and turn off at 2.0V because in that voltage range the motor has good power.

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Step 4: Capacitors, Motors, and Solar Cells

The capacitor used in the toy SUV is like the one shown on the left in the illustration below. It is a full 1 Farad rated for use at up to 5V. For lighter duty applications or shorter motor runs, smaller capacitors give shorter cycle times and, of course, shorter runs. The voltage listed on a capacitor is the maximum voltage to which it should be charged; exceeding that rating shortens the life of the capacitor. Many of the super capacitors intended specifically for memory backup have a higher internal resistance and so do not release their energy rapidly enough to drive a motor.

A solar engine such as the Easter engine is fine for driving motors that have an internal static resistance of about 10 Ohms or more. The most common variety of toy motors have much lower internal resistance (2 Ohms is typical) and so will drain all the energy from the storage capacitor before the motor can really get going. The motors shown in the second photo below all work fine. They can often be found as surplus or new from electronic suppliers. Suitable motors can also be found in junked tape recorders or VCRs. They can usually be singled out as having a diameter larger than its length.

Choose a solar cell or cells that will provide a voltage somewhat higher than the turn-on point of your engine under the light levels that your application will see. The real beauty of the solar engine is that it can collect low grade apparently useless energy and then release it in useful doses. They are most impressive when, from just sitting on a desk or coffee table or even on the floor, they suddenly pop to life. If you want your engine to work indoors, or on cloudy days, or in the shade as well as in the open, use cells designed for indoor use. These cells are usually of the amorphous thin film on glass variety. They give a healthy voltage under low light, and the current corresponds to the illumination level and their size. Solar calculators use this kind of cell, and you can take them from old (or new!) calculators, but they are quite small these days and so their current output is low. The voltage of calculator cells ranges from 1.5 up to 2.5 volts in low light, and about a half a volt more in the sun. You'll want a number of them connected in series-parallel. Wire Glue is excellent for attaching fine wire leads to these glass cells. Some solar rechargeable keychain flashlights have a large cell that works well indoors with solar engines. At the present time, Images SI Inc. carries new indoor cells of a size suitable for directly driving a solar engine from a single cell. Their "outdoor" solar cell of the same type works quite well indoors as well.

More commonly available from many sources is the crystalline or polycrystalline type of solar cell. These types put out a lot of current in sunshine, but are specifically intended for life in the sun. Some do modestly well in lower light, but most are pretty dismal in a room lit by flourescents.

Step 5: External Connections

To make the connections from the circuit board to the solar cell and motor, pin tail sockets taken from inline strips are very convenient. The pin sockets can be easily emancipated from the plastic setting in which they come by careful use of nippers. The tails can be snipped off after the pins are soldered in the board.

Solid 24 gage wire plugs into the sockets nice and secure, but usually externals are connected via flexible stranded hookup wire. The same sockets can be soldered to the ends of these wires to serve as little "plugs" that fit into the sockets on board beautifully.

Board sockets can also be provided into which the storage capacitor can be plugged. It can mount directly into the sockets, or be remotely located and connected via wire leads plugged to the board. This makes it possible to easily change and try different capacitors until the best one is found for the application and its average lighting conditions. After the best value of C1 is found, it still can be permanently soldered in place, but rarely has this been found necessary if good quality sockets are used.

Step 6: Applications

Perhaps our favorite application of an Easter engine is in the toy Jeepster SUV illustrated in Step 3. A thin plywood bottom was cut to fit the body, and large foam wheels were made to give it a "Monster Wheel" look, but in operation it is quite docile. The underside is shown in the photo below. The axles are set to make the car run in a tight circle (because we have a small living room) and the front wheel drive setup greatly helps it stick to the intended circular path. The gear train was taken from a commercial hobby motor unit shown in the next photo, but it was fitted out with a 13 Ohm motor.

A 1 Farad super capacitor gives the car about 10 seconds of run time each cycle, which takes it almost completely around a 3 foot diameter circle. It takes a while to charge up on cloudy days or when the car happens to stop in a dark spot. Anywhere from 5 to 15 minutes is usual during the day in our living room. If it finds direct sunlight coming in a window, it recharges in about two minutes. It travels around in a corner of the room and has logged many revolutions since being built in 2004.

Another amusing application of the Easter engine is "Walker", a robot-like creature that waddles along by means of two arms, or rather, legs. He uses the same motor and gear train setup as the Jeepster with the same 76:1 ratio. One of his legs is purposely shorter than the other so that he walks in a circle. Walker also carries a blinking LED so we know where he is on the floor after dark.

An simple use for a solar engine is as a flag waver or spinner. The one shown in the 5th photo below can sit on a desk or shelf and every now and then it will suddenly, and rather wildly, spin a little ball around on a string thereby attracting attention to itself. Some embodiments of these simple spinners had a jingle bell on the string. Others had a stationary bell mounted nearby so that it would get smacked by the flailing ball - but that tends to become annoying after a few sunny days!

Step 7: NPN Easter Engine

The Easter engine can also be made in the complementary or 'dual' version, with two NPN transistors and one PNP. The complete schematic is shown in the first illustration here. The stripboard layout can have the same component locations and the same track cuts as the first or 'PNP' version, the essential changes being switched transistor types and reversed polarity of the solar cell, storage capacitor, diodes and LEDs. The NPN stripboard layout is shown in the second illustration and incorporates an extra diode D4 for a higher turn-on voltage, and a diode D0 from the base of transistor Q2 to the junction of resistors R4 and R5 for a higher turn-off voltage as well.