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Tele Timer Circuit

2017-08-17 15:23  
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The circuit is built around a 555 oscillator/ timer. The circuit provides two time periods. The long-running time period is adjustable from about 1 to 10 minutes, and the short time period is preset to about three seconds. Here`s how the dual timer operates. When the power is switched on, G2 begins to charge through R3, Rl, Dl, and R4 to start the long-term timer period. When the voltage across C2 reaches the 555`s internal switching point, the long-term timer times out, discharging C2 through R2, D2, and pin 7 of the 555.
Tele Timer Circuit

During that time, pin 3 of the 555 is pulled to ground, activating the piezo sounder. To set the short time period to about four seconds, use a 10 k resistor for R2, and for about twenty seconds use a 47 k resistor. The timing capacitor, C2, should be a good-quality, low-leakage unit.



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