I was following the instructions on this page: solarbotics.net/library/circuits/se_t1_mse.html
to try to build a "Miller" solar engine, using a solar panel generating ~4V to drive a small DC motor. This is the motor I used:
The schematic of the circuit is as follows, with the 2N3906 and 2.2k resistor removed, and R1 = 0 (as instructed):
I didn't have a 1381 voltage detector, and used a TC54VC3002 instead. Besides that everything was exactly the same. But after connecting the solar panel to the circuit it just won't work, no matter how I tried.
For debugging, I tried replacing the solar panel with a 9V battery and the circuit works well, the motor sucking ~150mA of current, but the 2N3904 was getting extremely hot. So I switched in a TIP120 for the 2N3904 instead. What happened was that when I switched the solar panel back for the battery, the motor actually turned!
Here's the working circuit with the TIP120 in place of 2N3904:
I am very befuddled by this and hope that someone can explain to me, what is the difference between using a 2N3904 and a TIP120 in this circumstance, such that one won't work and the other one does?