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.
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
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