Description
Circuit operation: When the circuit is switched on with S1, the pulse at junction R4-C2 resets the counter and counting starts, when the count reaches bit 14 (Q14), pin3 goes high so that the self oscillating piezo-electric buzzers, a 12V type is turned on via driver T1. The timer is powered by 9V battery. Light emitting diode D1 does not affect the operation of the circuit and is included merely to show that the timer works. Diode D1 and resistor R3 are, therefore optional components.
With the buzzer actuated, the timer draws a current of about 10 mA.
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