| The circuit below is powered by three 1.5v alkaline AA cells. With a finger tap to the pushbutton trigger switch, a cluster of 6 wide angle white LEDs is turned on. The lights remain on for about 3 minutes, then will turn off. The circuit’s standby current is only a few microamps. A set of fresh batteries should last at least 200 light applications. |
The circuit uses a Schmitt trigger inverter and two transistors. When the pushbutton switch S1 is pressed, the 10uF capacitor C1 is discharged. When the switch is released, the capacitor is charged through R1. With the capacitor C1 discharged, the Schmitt trigger output swings to 4.5v, which turns on transistor Q2. The combination of R4 and Q1 form a constant current control circuit. The current is set about 15ma per LED. When the capacitor charges up to about 2.3v, the Schmitt trigger changes state, turning off the LEDs.
Click on Drawing Below to view PDF version of Schematic
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