Auto Switch-Off Torch


An elderly acquaintance who couldn't cope with battery recharging was getting through dry cells rapidly by leaving their torch switched on, so I built the following LED torch circuit which switches off automatically after about five minutes. Switching the torch off manually then resets the circuit.



Operation

On closing the switch G1 output goes high and the capacitor charges slowly through R2. The high output of G3 provides base current for Tr1 via R3. If the voltage dropped across R4 exceeds 0.6V Tr1 begins to conduct and robs Tr2 of base current, thus lowering the collector current of Tr2 and the R4 voltage. Tr1 and Tr2 hence provide an approximately 20mA constant current source for the LED. After about 5 minutes the capacitor voltage reaches the Schmitt threshold voltage and gates G2, G3 and G4 change state. The now low output from G3 switches off Tr2 and hence the LED. If the switch is then opened G1 discharges the capacitor rapidly through the diode across R2, resetting the circuit.

Notes and Modifications

The value of R1 is not critical (anything ~1 meg or more will do, and higher values will give greater protection against static damage to G1 if the battery terminals are touched).
Tr1 and Tr2 are any general-purpose npn transistors, e.g. 2N2222, 2N3904.
By removing Tr1, R4 and the LED and connecting Tr2 emitter directly to ground the circuit could be used to switch other loads. R3 might then need to be reduced in value.

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