Solar Engines
The circuitry that make your solarbugs go. The circuitry described here is for low powered solar panel use. The robugs described on this site (ozkal's robugs) use more powerful solar panels, so i don't use any solar engines in them and directly drive my control circuitry or microcontrollers with the output of solar panels. Thanks god, this solar panels are cheaper here in turkey. :)

After lot of reading I finally made the solar engine. It is the same solar engine found almost everywhere . I was amazed that something actually worked and this did a lot to my confidence. Before free forming I bread boarded the circuit. I used two 4700F Capacitors for the 9400F.Unfortunately capacitors available in Chennai with high farad rating also have high voltage rating which increases the size to an extent that the capacitors become really heavy and cannot be used for BEAM. I didn't incorporate it into a robot because somehow the pager motor didn't seem to be healthy enough to take load. Just a little touch while the motor rotates and the shaft doesn't move. Didn't know whether the robot will move if I use a pager motor. For those in Chennai -I found spoilt pagers in Moore Market for as low as Rs.20 after hard bargaining. Try to get pagers that don't work. U don't need a working pager anyway. U can get for cheap those with spoilt LCD displays. Take out the motors from that and have fun. I spoilt 3 motors trying to take the weight out of its shaft. Tried all things but couldn't help. The last time it was almost out, but I ran out of patience and pulled out the whole shaft. Best of luck to you guys and gals. Have a good shaft pull out time. You can find a lot of solar engines based on the 1381 J voltage regulator in the net. But I haven't been able to make one as I haven't found this component anywhere in Chennai. I even ripped open a working Panasonic cordless phone, but as it always happens I didn't find the 1381 J. The red FLED I have used I found it in Apex Electronics, Ritchie street, Chennai. About the solar cells I took them from solar calculators. I ripped about 5 calculators I found in Moore market (next to central station), took out the solar cells glued them together and connected them in parallel. The Sanyo solar cell in one of the calculators was really good. It generated a good voltage and current in bright light. While testing the solar engine always do so in bright sunlight. Tube light won't be enough. Be careful while removing the solar cell from the calculator frame. Don't pull out the connecting wires stuck to the solar cells. I did that in one of the cells thinking I could put it back, but I couldn't. Its almost impossible to stick it back. We get conducting ink in Ritchie street. Didn't try it out though.

This is the free formed engine I used for Moby

How it Works?

First of all you should know these facts

1. In a NPN transistor current flows from emitter to collector. For this N(emitter) should be negative, P(base) should be positive and N(collector) should be negative.

2. In a PNP transistor current flows from collector to emitter. For this P(emitter) should be positive, N(base) should be negative and P(collector) should be positive.

3. Certain components like Flashing LED's , LED's etc. let current flow through them only at a particular voltage across their terminals. Let us call these components trigger elements.

In Detail

The need to use a capacitor in the solar engine arises out of the fact that the solar cell doesn't generate enough current or charges at an instant to overcome the resistance of the motor and run it efficiently. So sufficient charges are stored in the capacitor and are then discharged to the motor whenever required.

As you can see the capacitor is charged by a solar cell and continues charging till the maximum voltage of the capacitor or the solar cell (whichever is minimum) is reached, if no connections are made to the capacitor.

Let us assume that the motors used in the solar cells work most efficiently when supplied with 3 volts. So the solar cell must be capable of generating minimum 3 volts and the capacitor must also be able to store charges up to more than 3 volts.

If the motor is directly connected across the capacitor the stored charges are immediately discharged to the motor and hence the capacitor has no significance. Hence we need a circuit that will automatically discharge the energy in the capacitor to the motor when the charge in capacitor has reached the required level to run the motor efficiently.This is accomplished using the transistors, FLED and the resistor.

Charges build up in the capacitor starting from 0 volts. The base and the emitter of the PNP are positive (emitter +ve directly through the positive terminal of the cap and base through the resistor and the motor). Since both base and emitter are +ve the PNP transistor doesn't work (current doesn't flow from collector to emitter).

The PNP doesn't work until the voltage across the cap equals trigger voltage of the FLED or LED or diode. At this voltage the current flows through the trigger element. Since current flows through the trigger element ,not to the base of the PNP, the base of the PNP becomes negative. Hence the PNP conducts and current flows from the collector to emitter

Similarly the NPN doesn't conduct as the emitter(N) is negative( through the -ve of the cap) and also the base is negative. For the NPN to work current must flow from emitter to collector. For this the base must be positive. Such a situation arises only at the trigger voltage of the diode when the PNP conducts and thereby makes the base of the NPN positive. At this point current from the capacitor flows through the NPN to the motor and causes it to rotate.

Even if the voltage across the cap falls to less than the trigger voltage and the base of the PNP becomes positive, the motor continues to rotate.The rotation continues until the motor resistance becomes high enough to prevent further discharge of the capacitor. Now the voltage in the capacitor again rises until it reaches the trigger voltage of the trigger element and the above cycle repeats.

Upon notice, the starting of the motor is controlled by the trigger voltage of the trigger element and the stopping of the motor is determined by the resistance of the motor( 2 independent factors). Hence the solar engine acts like a silicon controlled rectifier (SCR).

Solar Engine Types

Type 1 - The motor turns when the voltage in the circuit reaches a preset level.

Type 2 - The motor turns at a preset interval of time.

Type 3 - The motor is "charge curve differentiated." It's sort of a combination of type-1 and type-2. When capacitor slows down it's charging rate, it triggers the circuit.

Most solar engines are type-1, because it is the easiest to get good efficiency using it. Type-2 solar engines are fairly efficient, and are useful in single neuron solar engines for creating phototropic behavior. Type-3 solar engines would be the most efficient, but they haven't been built yet.

Instead of using trigger elements like FLED , LED or diodes there are solar engines that use IC's like 1381 voltage triggers that are much more efficient than the FLED's , LED and diodes. Solar engines can be configured to trigger at particular time instants rather than voltage values using neurons. Such solar engines find use in circuits that activate a load at night , charging throughout the day.

Below are the schematics of most of the types of solar engines I have come across. The following schematics are the beautiful work of their creators and I don't take credit for any.


Diode triggered Solar Engine

This is the original circuit designed by Mark Tilden, and is the basis for all of the other solar engines created. The Zener diode can be replaced with diodes in series , a flashing LED, or even a resistor.

1381 Solar Engine

This very efficient Solar engine is more complex than the normal version, but is much more efficient. This circuit was designed by Andrew Miller.

Time triggered Solar Engine

The twist on this circuit is that instead of being voltage triggered, it's time triggered. Adjust the values of resistors R2 and R3, or the values of C2 and C3 to suit your needs. Increase the values of the resistors as a general rule, but to get really long time values you are going to have to increase the values of the capacitors

Micropower Solar Engine

Circuit diagram by Ken H.

If the circuit has a power source which will provide 2.5Vdc at 10uA this circuit should drive a pager motor. It turns on the motor
at 2.3 to 2.5Vdc and switches off at 1.2 to 1.5Vdc. This circuit was made by Ken Huntington.

PM1 Solar Engine

This is an SE that can drive a bicore. Good for solar walkers, heads and more. For C1 Ian recommends somewhere in the 4000uF range for a head and in the Farad range for a walker. If you would like to be able to control when your robot moves just put a switch across the source and drain of the 7000. Just make sure you turn the switch off when it runs out of juice or else it won't charge.

D1 Solar Engine

This is a neat circuit that comes alive when it gets dark. You can adjust the sensitivity using the 150K variable resister. The outputs can be connected to a LED bicore or whatever you want.

Solar Revolver

This SE made by Wilf Righter uses a 1381E voltage detector with a red LED in series to raise the trigger level to 4.0V. A 10uF cap between the 1381 power and ground pins together with the 1M resistor on the output pin causes the SE to reset after about 1 second. A single NPN transistor used used as a inverter to match the requirement for a low current active low enable. For this application the 1381tr SE works just like the Miller Engine and draws less than a uA of current during charging.


1. new 1381 SE with timed reset
2. revolve (turn) left/right
3. reverse
4. new ultra low power delay Nu
5. new delay Nu memory


  1. The solar panel will slowly charge up the storage capacitor C1 towards 6.8V. U1 will assert a ground ( believing the voltage is too low ) which keeps U2 ( a high gain darlington NPN transistor ) open and the motor OFF.
  2. When 6.8V is reached, U1 will open. The base of U2 will then be pulled high ( through R1 ) and U2 will turn on, allowing the solar energy in the capacitor to discharge through the motor. The motor spins.
  3. The motor will continue spinning and discharge the capacitor until the solar voltage falls to 5.5V.
  4. At 5.5V, U1 will assert a ground at its output (Out), believing that the voltage is too low, and it must apply a RESET. This ground turns off U2 and the motor stops spinning and the system is ready for another cycle!

Miller Solar Engine

A much more efficient solarengine compared to the 1381 SE , using fewer components

This article comes from Robotik & Elektronik

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