The 1381 Solar Engine

This circuit (found here: works with a MCP112 voltage supervisor to save up solar energy until it is significantly large to make turn a motor. With a small solar panel charging a large capacitor, it is possible to make something move even on an overcast day!

The circuit works by charging the capacitor with a solar panel until it reaches the MCP112’s tripping point (in this circuit I’m using the 2.7V tripping point flavor but they range from 1.8V to 4.38V) at which point the MCP112 activates the PN2222 transistor which powers the motor. The PN2907 turns on at this point and latches the PN2222 in the on position until the capacitor voltage level runs below the PN2907 turn on threshold (of about 0.6V). Without the PN2222 the MCP112 would deactivate at 0.3V below it’s trigger threshold (2.4V in this case).

I’m using a 37x33mm Monocrystalline Solar Cell from ( which produces 6.7V and 20mA. It is at maximum efficiency of 106.335mW with a load of 300 ohms.

By changing the size of the capacitor from 2200uF to 4700uF you can change the amount of charge that will be passed on to the motor.

On an overcast day the little motor is activated every 5 seconds or so for about half a second, not bad for a tiny solar panel!

Here’s the trace image showing a comparison of the same circuit with different capacitor sizes and voltage trigger flavours:


I am interested in using this circuit to power something other than a motor. However, simply replacing the motor with a given circuit (like a microchip) does not work.

Replacing the motor with a 220 ohm resistor appears to allow the circuit to cause an LED to blink and to charge and discharge, however.

The Complete Solar Power Smart Head Rev.3 by Wilf Rigter

Here is a photo of the full assembly in action!

I found the circuit at this website:

I am building Wilf Rigter’s 74HC240 circuit but replacing the 74HC240 with two 74HC540s (and replacing the 1381 with a MCP112) because I couldn’t source a 74HC240 from my local electronics store. The two logic chips are inverting octal buffers but the 74HC540 has one enable for all eight inverters while the 74HC740 has two for two sets (A and B in the circuit above). To make the translation, first I connected the two AND input enables of the 74HC540 to behave the same way as the single enable of the 74HC240, then I remapped the pins from one 74HC240 to two 74HC540s.

Here is the comparison of the 74HC240 with the 74HC540 and my remapped circuit:

![]( chip equivalency-01.jpg)

Once the voltage trigger is reached (2.9V in my case) the MCP112 sends a high signal to inverter 1 which turns on the Standby Flasher circuit.

Meanwhile the High/Low oscillator circuit which was oscillating is pushed into either the clockwise or anticlockwise motor turn configuration depending on the values of the two light dependent resistors (LDRs). Either the low voltage that exists to the bottom will win out or the high voltage that exists at the top.

This configures the motor drivers to turn on the motor in one direction or the other.

I have “air wired” this circuit after redrawing the circuit here:

Here is the play by play of the assembly:

Here is how I glued the solar head to the post:

And here is another circuit which claims to do the same thing but which I failed to make work: