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:

Automatic Power Off Circuit

Automatic Power Off 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.

Author:H. Bonekamp
Copyright: Elektor ELectronics

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