This circuit performs a similar function to the ‘sleep’ button on a radio alarm clock; pressing the button connects the battery supply to some external equipment or circuit (represented by RL) for a preset time period. The period can be extended by pressing the button again before ‘time out’. The circuit will avoid the situation where you forget to turn off some battery powered equipment and return to find the battery is flat. Unlike a digital alarm clock sleep function the circuit here is based on a simple analogue timer which uses very few components. Pressing button S1 rapidly charges C1 via R1. When the voltage on C1 exceeds the threshold voltage at the gate of FET T1 it conducts and switches the battery to RL. The drainsource voltage drop introduced by the FET is negligible for the two types of FET specified (for a maximum load current of either 100 mA or 1 A).
T1 remains conducting as long as the voltage on C1 is greater than the FET gate threshold voltage (around 2 V for the FET types specified). The length of the ‘on’ period depends on three factors; firstly the value of R2 which governs the capacitor discharge current, secondly the capacitance of C1 and finally the supply voltage from the battery BT1. When C1 is charged to a higher voltage it takes longer to fall below the threshold level. The component values given will produce an ‘on’ time of around 10 minutes with a supply of 5 V. The FET turns off relatively slowly at the end of the ‘on’ period; this should not cause a problem if the switched equipment uses only analogue circuitry but can lead to a momentary malfunction if the equipment contains digital circuitry.