1381-based Solar Engines
From BEAM Robotics Wiki
The 1381 solar engine uses a 1381* voltage detector (a.k.a., a voltage supervisor) IC to drive a voltage-based (type 1) solar engine. The 1381 is normally used to reset CPUs and Micros when the power supply drops too low for reliable operation. So 1381s detect and switch when the input voltage crosses the rated upper and lower threshold voltages. The upper- and lower-switching voltages are slightly overlapped so that the turn-on voltage is a few hundred mV above the turn-off voltage. This hysteresis keeps input noise (around the switching threshold) from resulting in multiple output cycles as the transition occurs.
The 1381 SE is designed to increase the 1381 hysteresis from 0.2 - 0.3 V to a much larger value (2 - 4.6 V). This is done by essentially dropping the turn-off voltage to zero, while allowing SE to "fire" at the 1381's rated turn-on voltage.
Here's the basic circuit:
Note: This is just a more complex variant for the Miller Solar Engine
 How it works
As the solar cell charges the (4700 uF) storage capacitor, the voltage across the capacitor increases with time. Eventually it reaches the 1381's trip point, and the 1381 applies voltage to the base of the 2N3904. Since this is an NPN transistor, it "trips" and applies current to the motor. Meanwhile, it has brought the base of the 2N3906 "low," which causes it to conduct to the 2N3904's base as well (so at this point, the 1381 is essentially out of the circuit). This state of affairs will continue until the capacitor is fully drained, at which point the 2N3906 and 2N3904 both go "quiescent," and the solar cell resumes charging the capacitor.
- Increase the size of the resistor for longer running time (generally more-efficient), but lower starting torque
- Decrease the size of the resistor for shorter running time, but higher starting torque
If you want to build a "freeform" version of this circuit, here's a very compact layout (note that the layout shows the transistors and 1381 in "dead-bug" fashion, i.e., with their legs pointing towards you).
 Circuit variants
You can also build a variant of this circuit with a photodiode to provide light sensitivity:
This version of the circuit is often used in pairs in photopoppers.
 Calculating the 1381 Voltage
To calculate the matching voltage of the 1381 you for 80% of the output given in ambient light (rather than 80% of the rated output). Put the cell in the environment that you expect the robot to work under. Measure and determine 80% from there.
So, measure the cell's voltage in room lighting. Multiply that number by .8.
The result will give you the target voltage for the 1381.