Feb 28, 2013


The complete circuit diagram for the simple Games Timer is shown in Figure below.Essentially, it comprises one building block based on a single 555 timer IC (integrated circuit), wired as a monostable. The circuit produces an audio alarm after a preset time delay.

Push switch S2 is pressed momentarily to start the timing. Preset potentiometer VR1 (wired as a variable resistor) and capacitor C2 determine the time delay, which is between about ten seconds and three minutes using the component values shown. At the end of the timing period, the voltage at pin 3 of IC1 falls and the buzzer sounds.

When the circuit is switched on by means of S1, the buzzer sounds at first, but stops once the circuit is triggered to come on again after the time delay. When LED1 is lit, it shows the circuit is ready to be triggered or the timing has ended, while LED2 lights during the timing period. You can increase the values of VR1 and C2 to provide longer time delays.Capacitor C1 offers a short circuit when S2 is pressed, momentarily causing trigger pin 2 of IC1 to be connected to 0V to start the timing. Resistor R4 connected across capacitor C1 ensures that the capacitor discharges once it becomes momentarily charged by pressing S2.Resistor, R1, ensures that the voltage on the trigger pin is normally at the positive supply voltage.

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