Auto Power Off

We are surrounded by battery operated equipment of all kinds, and this array is growing still. Manufacturers and designers lean over backwards to make sure that their equipment draws a small current and can thus be operated by a battery. This has its flip side, too. because even if the equipment in question draws only a small current, when it is not switched off, the battery is flat after a few days or weeks. The circuit presented here can prevent this happening. It may be added to all kinds of equipment operating from a 9 V battery and switches this off automatically one minute after a preset time has elapsed. The peak switching current is 20 mA, which is more than enough for most applications.

Circuit diagram:
The switch is formed by a p-n-p darlington, T1, which is actuated by push-button switch S1. The very high amplification of the darlington enables it to be kept on fairly long with the aid of a relatively small-value capacitor, C1 (= 100 µF). Resistor R3 limits the charging current of C1 to ensure a long life of S1. Resistors R1 and R2, in conjunction with C1, determine the switch-on time. When this time has elapsed, R1 ensures that T1 is switched off. Since the darlington can handle a UBE of –10 V, a polarity protection diode is not needed.