555 countdown timer

Thread Starter

Hereathome

Joined Sep 7, 2014
2
I am new to this forum. It looks very informative.

I have a question about a 555. I am trying to make a countdown timer that starts when power is applied and sets off a piezeo buzzer after a duration of about 10 minutes. I have tried different thing circuits I found on the web but none of them really do what I need. Some require start up switches, etc. I just want it to start timing when power is applied.

Could someone refer me to a page or post a circuit that would do that please? I would appreciate it.
 

Dodgydave

Joined Jun 22, 2012
11,284
You need to use it in monostable mode , and use pin 3 to pin 8 for the output, like this diagram, when power is applied the piezo will be off until the expired time, then will be on .timer.pngtimer.png
 

MrChips

Joined Oct 2, 2009
30,708
Welcome to AAC. I am sure you will find useful information for all of your electronic projects right here on AAC.

The 555 timer circuit is a simple and easy way to create short delay times of the order of seconds and shorter.
The duration of the delay is determined by two components, a resistor and capacitor. If we use a very crude approximation for the time delay, RC time constant is given as the product of R times C.

In the 555 timer circuit provided by Dave above, with R = 1M ohm and C = 1000uF,
R x C = 1M x 1000u = 1000 seconds which is about 16 minutes.

The problem with such long time delays is that large value capacitances are not very stable and accurate. For capacitances greater than 10uF, electrolytic capacitors are almost the only choice. Such capacitors suffer from a memory effect in the dielectric whereby even after complete discharge the capacitor will gradually show some additional charge.

For long time delays using a 555 timer circuit or any other type of oscillator, more repeatable results are obtained by using an oscillator running at much shorter periods (higher frequency) and then using counter circuits to achieve the desired time interval.

A better solution in a single package is to use a microcontroller.

Here is a series of blog entries that began with a request for a long delay timer using a microcontroller:

http://forum.allaboutcircuits.com/blog/msp430-interrupt-on-input-change.553/
 
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Brownout

Joined Jan 10, 2012
2,390
It's a standard default to say use a microcontroller, but not everyone is on the UC bandwagon. There is a tried and true, not to mention easy and low part count method that doesn't require and code, compilers, programmers, etc. Take a look at this page: http://www.onsemi.com/pub_link/Collateral/MC74HC4060A-D.PDF This will get you exactly what you want, although it doesn't include a 555, but as already been mentioned, that chip isn't the best way to go for your application.
 
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