Opening windows for ventilation should be brief, minimizing the risk of break-ins during prolonged periods when nobody is home. This circuit serves to detect open windows (or doors), utilizing a pair of 555 timer ICs as active components. A reed switch (S1) affixed to the window frame is closed by a magnet on the window casement when the window is shut. In this state, the reed switch connects resistor R1 to the 4.5-V supply voltage. When the window is open, the circuit activates: a red or blinking LED indicates the open window, and an intermittent electronic buzzer emits a loud acoustic signal, reminding users to close the window. This setup enhances security while allowing for controlled ventilation.
Window Opened: Triggering the Monostable Flip-Flop (IC2)
When the window is opened, S1 is activated, causing the voltage on resistor R1 to immediately drop to 0 V. This action briefly pulls the trigger input of IC2 to ground via capacitor C1. IC2 is configured as a monostable flip-flop and responds to this pulse.
Monostable Operation: Preventing Retriggering
After C1 charges, the supply voltage is restored at the trigger input of the monostable flip-flop via resistor R2. This setup prevents retriggering and allows the monostable to time out normally. The status of the timer is indicated by the red or blinking LED (user-configurable), showing that pin 3 is in the logic High state.
Schmitt Trigger Configuration: Ensuring Buzzer Silence
The second 555 IC is configured as a Schmitt trigger. Its output goes High when its trigger input is grounded, ensuring that the buzzer remains silent since both outputs of the 555 ICs are High. If the window is closed within the time interval set by the R3/C2 network, the Schmitt trigger’s output returns to the Low state.
Preventing Buzzer Activation: Diode D1 Operation
If the output of IC2 remains High, diode D1 prevents any current flow through the DC buzzer, ensuring it stays inactive. After the monostable times out, the outputs 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 output remains High, but the monostable output goes Low. As a result, a positive voltage is applied to the buzzer, and it generates an acoustic signal until the window is closed. As befits an intermittent buzzer, it generates an intermittent signal. The time-out interval of the monostable can be calculated reasonably accurately with the formula
t = 1.1 × C2 × R3
With the indicated component values (1 MΩ and 470 μF), the alarm sounds after approximately nine minutes if the window is still open. Instead of the reed switch, you can use a light-dependent resistor (LDR) to detect the light from the refrigerator lamp. If you replace R1 with a trimpot and adjust it so that the monostable is triggered. When the refrigerator lamp goes on (when the refrigerator door is opened), after the monostable times out the buzzer will remind you to close the refrigerator door (which is often left open). A nice side effect here is that you can use this circuit to definitively answer the age-old question of whether that refrigerator lamp actually goes off when the fridge door is closed.
