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Author Topic: How to make simple off delay for piezo alarm?  (Read 2627 times)

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How to make simple off delay for piezo alarm?
« on: April 05, 2012, 07:42:14 AM »
Hey all,
I'm working on something that gets a very brief signal through a mercury switch and I want to power a small 12vdc piezo horn for a second (or so)
after this momentary contact. Is there any clever way I could use a capacitor or anything to do this? I don't want to buy a 35$ off delay if I don't need to.

Appreciate any advice, thanks!


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Re: How to make simple off delay for piezo alarm?
« Reply #1 on: April 05, 2012, 15:17:20 PM »
hello mate

well, there is one simple way, not so "clever" though. You can use for example the 555 as monostable circuit:

Basic monostable circuit

The pulse is negative edge or positive edge? (i mean it goes from high to low or low to high)? If it is low to high, then you need to add one more transistor to the previous circuit, a BC548 or any other NPN transistor will do. You connect the collector of the transistor to the input of the circuit (pin 2 of 555) through a 220 Ohms resistor. Then the emitter to the ground directly. And then, you drive the pulses to the base of the transistor through a 1 KOhm resistor. This will invert the signal and make it high to low.