Economy Timer 555 IC

Posted on Feb 5, 2014

This circuit detects when a window is open (it can also be used with a door), indicates that the window is open by means of a red LED or a blinking LED, and emits a loud acoustic signal from an intermittent electronic buzzer to remind you to close the window. The active components consist of a pair of type 555 timer ICs. Switch S1 is a reed switch that is attached to the window frame, and when the window is closed

Economy Timer 555 IC
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the switch is closed by a magnet attached to the window casement. When the window is closed, the reed switch connects resistor R1 to the 4. 5-V supply voltage. If the window is opened, S1 opens as well and the voltage on R1 drops immediately to 0 V. As a result, the trigger input of IC2 is briefly pulled to ground via C1. IC2 is wired as a monostable flip-flop, and it is triggered by this pulse. After C1 charges, the supply voltage is again present at the trigger input of the monostable flip-flop (via R2). This prevents retriggering and allows the mono-stable to time out normally. The red LED or blinking LED (user option; select the value of the series resistor accordingly) indicates that the timer is running (pin 3 is logic High). The output of the second 555 IC, which configured as a Schmitt trigger, also goes High when its trigger input is pulled to ground. As a result, the DC buzzer connected between the outputs of the two 555 ICs is not energised because both outputs are High. If the window is closed within the time inter-val determined by the R3/C2 network, the output of the Schmitt trigger returns to the Low state. If the output of IC2 is still High, diode D1 prevents any current from flowing through the DC buzzer. After the monostable times out, the out-puts of both 555 ICs are Low and the buzzer remains silent. Things are different if the window is still open when the monostable times out. The Schmitt trigger...

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