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Post by badtube » Sat Mar 01, 2008 5:47 pm

I built a 555 timer mono stabe(one shot?
It times for 6 sec and powers a relay. Works fine. Except when I first power the ckt up the timer starts its cycle. And if there is power surge, lights blink etc it self triggers. How can I make it work ONLY when a low is put to pin 2?

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Post by SETEC_Astronomy » Sat Mar 01, 2008 5:54 pm

I always have the same issue with one shots made with a 555 or with other techniques. I'm looking forward to a solution. In the past for critical apps I had to use a uC to ensure proper and desired operation without adding more circuitry to get the 555 to work as intended.

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Re: 555

Post by Bigglez » Sat Mar 01, 2008 7:30 pm

Greetings Roger,
badtube wrote:I built a 555 timer mono stabe(one shot?
..... How can I make it work ONLY when a low is put to pin 2?
Do you have a schematic? Can you post it?

One technique would be to hold the trigger pin above 1/3Vcc
with a resistor divider and AC couple the trigger pulse.

Comments Welcome!

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Post by dyarker » Sat Mar 01, 2008 7:46 pm

Try this - 10K resistor from RESET# to Vcc, and 10uF capacitor from RESET# to common. That will hold reset low while power supply rises at power-on.

By "surge" do you mean lights blink off, or brighter?

Dale Y

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