If someone in your family has the habit of not completely closing a drawer – or perhaps the food freezer’s door – you’ll appreciate this design. It senses an ajar door and, if the situation isn’t corrected within 20 sec, sounds a beeping alarm.
The circuit, shown in Figure 1, is controlled by a magnetic reed switch that mounts within the cabinet (food freezer in this case) and the magnet on the door. So long as the door remains closed, the switch is closed and the alarm is disarmed.
|Figure 1.||If the door and its switch are open, the low-frequency oscillator (C and D) pulses the transducer's 3-kHz
driver ON and OFF.
Opening the door in turn opens the switch, and C1 starts charging up through R1. Approximately 20 sec later – the delay allows for authorized usage – the voltage at pin 9 is high enough to turn on the oscillator formed from C, D, R2, R3, and C2. This oscillator, operating at approximately 1 Hz and a 50% duty cycle, in turn pulses the piezoelectric transducer’s 3-kHz oscillator.
Closing the door allows C1 to discharge through R6, an action that disables the low-frequency oscillator and, therefore, the transducer’s oscillator. You can override the alarm via S1 when the door must remain open.
You might want to consider using other values for R1 and C1. The values shown for R1 and R6 result in a continuous 27-μA battery load when the door switch is closed. This drain is approximately 10 times greater than what the rest of the circuit consumes in standby. Changing R1 to, say, 66 MΩ (3×22 MΩ) and C1 to a 1-μF Mylar capacitor preserves the 20-sec delay and reduces the resistor’s loading to approximately 0.1 μA. Additionally, by using the 1-μF Mylar unit rather than a 60-μF capacitor, you considerably reduce the possibility of the 60-μF device’s leakage current adversely affecting the timing.