The all-transistor T3SE works much like a conventional SE: like the Tritium, it fires when the charging current drops below the minimum and after the circuit triggers, positive feedback is used to latch the circuit, resetting at a fixed voltage on the main cap.
How the T3SE works:
When the cap is discharged, the solar cell starts to charge the main cap mostly through the base – emitter junction of Q1 and some current passing through the 1K resistor with a maximum forward voltage drop of .6V. This causes the collector of Q1 to be saturated and the voltage on the collector will be close to the Q1 emitter voltage. This clamps the base emitter junction of Q2 and cuts off the Q2 collector current which would otherwise flow into the base of Q3.
When the voltage across the 1K resistor drops below .6V (i.e., 1 KOhms x 0.6 mA), Q1 comes out of saturation and “unclamps” the Q2 base. This allows Q2 to turn on with base current supplied through the 220 K resistor. When Q2 turns on, it supplies base current to Q3 and the load (here a motor) is picked up.
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