How to Build a Simple Fire Door Alarm

This is a simple - and easy to build - alarm circuit. For power - I used a small 9 volt battery. But the circuit will work from 5 to 15 volts - just choose a buzzer that is appropriate for the voltage you are using, and the standby current is almost zero.

It is very important that the fire door is kept closed at all times. If the fire door is connected to SW1 - the alarm circuit will allow you to open and close the door without sounding the buzzer. However - if the door is left open for more than about 30 seconds or so - the buzzer will start giving a warning series of short beeps.

The length of the initial delay is fixed by R1 and C1. The length and speed of the beep is determined by R2 and C2. At the values ​​shown in the diagram - at an initial delay of about 30 seconds - the buzzer will switch on and sound at intervals of about half a second.

I drew a reed switch for SW1 - but you can use any type of switch that fits your application. You can use multiple switches. Just wire all the switches in series.

Change output time

Generally speaking - the length of the initial delay is proportional to the value of R1 and C1. In other words, if you double the value of either R1 or C1 - you will double the length of the initial delay. If you halve the value of either R1 or C1 - you will reduce the length of the initial delay by half.

The same applies to the length of the beep. In general - the beep length is proportional to the value of R2 and C2. In other words, if you double the value of either R2 or C2 - you will double the length of the beep. If you halve the value of either R2 or C2 - you will halve the beep length.

If you want an exact output time - use a variable resistor (or preset) in place of R2. Then just adjust the resistor until you get the length of beep you need.