Miller Solar Engine
From BEAM Robotics Wiki
On December 18, 1999, Dave Hrynkiw announced on the that had licensed the Miller Solar Engine from Andrew Miller of "AM Innovations". This solar-engine came about from Andrew Miller's playing around with the then-new 1381 voltage detectors.
The Miller solar engine (also called Millerengine) uses a 1381* voltage detector (a.k.a., a voltage supervisor) IC to drive a voltage-based (type 1 solar engine) 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 Miller SE is designed to increase the 1381 hysteresis to a larger value. This is done by putting a small capacitor across the input legs of the 1381, and a diode between the 1381 and the "true" ground.
 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. This state of affairs will continue until the 1381 sees a voltage that looks like its trip point minus 0.3 V, at which point the 2N3904 goes "quiescent," and the solar cell resumes charging the storage capacitor.
- Larger values for C1 result in longer (but less-frequent) "bursts" of motor activity.
- smaller values result in shorter (but more-frequent) activity.
- Larger values of R1 reduce the power going through the motor, as well as lengthening the "bursts" of motor activity. If the load is an LED or something similar, you'll need less power to drive it, and probably then want a non-zero value for R1.
If you want to build a "freeform" version of this circuit, here's a very compact layout (note that the layout shows the 2N3904 and 1381 in "dead-bug" fashion, i.e., with their legs pointing towards you).
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.
Also, remember to add in the diode's voltage drop. So, measure the cell's voltage in room lighting. Multiply that number by .8. Subtract something like .7V for the diode (it is better to test this first).
 Also See:
- Miller Solar Engine Kit, Rapid Fire SCC2433b-MSE Versions
- SCC2433b-MSE Powerful Bundle 2
- SCC3733a-MSE Powerful Bundle 2