General Purpose Alarm, Pulsed


I hate "alarms" that sound continuously - unless they are smoke alarms. Smoke alarms should be annoying, but others should not. I wanted an alarm for a function in my home, but I wanted it to pulse on and off at about a 1 per second rate.

So I took an LM556 chip (a "dual timer") and used one side as an oscillator to generate a 2kHz audio tone and the other side to generate a pulsing signal of about 1 Hz to control the oscillator. Powered it with 9 volts, ran it to a small speaker (2" dia), and configured it so it could be activated either with a switch closure or with a small NPN transistor (such as a PN2222). Mounted it all on a small printed circuit board (PCB) - and there you have it. With a 9V supply it is fairly loud and very effective, and it will function down to about 2.5V although the tone is very "soft" at that supply voltage. When activated, it draws 104 mA from the supply at 9V and 15 mA from the supply at 2.5V. When not actively sounding, it draws no current at all.

The kit includes most of the parts you need, including the PCB (with mounting holes ), the chip, the "silence alarm" switch, and the speaker. A 9V battery is supplied, but you may use another power source. You will get some hook-up wire for jumpers and mounting the speaker, and your switch or other circuit for triggering it. This gives you a handy audio alarm module to use any way you need to.....