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Help with 555/556 monostable timer circuit

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oldchap

New Member
I would like to use a 556 or two 555s in order to simulate the pressing of a pushbutton on a radio. I powerup several scanners via the 12 supply, and when powered, they require an additional keypress after a few seconds to get them to start scanning. My thought is to powerup my circuit with the same power supply as the radios, (or even feed it from the radio) then use the first 555 to delay about 5 seconds, then it would initiate the second 555 to simulate a .25 second keystroke by wiring the output of the second 555 to the wires on the scan button.

My question - How do I trigger the first 555? I would like it to "self-trigger" by powering up the whole circuit. I could just ground the trigger terminal, but that would cause indefinite triggering, and from what I read, the trigger time has to be less than the delay time.

Other uses - This circuit could be adapted to get any equipment to powerup in a mode other than the manufacture designed.

Any help or circuits would be appreciated. Thanks, Oldchap
 

laroche73

New Member
power-up triggering 555

A simple RC connected to the 555's trigger input should work. The component values aren't critical, the values shown give a 1 mS time constant.
 

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oldchap

New Member
Thanks! Can you help me a little further? I need about 5 seconds for the first 555, so I guess I could increase R to 1meg and C to maybe 4.7uf. Then I'd like to tie the output to a second 555 to give me about a quarter second pulse which I need to connect to some kind of switch to give me a switched output. Can I tie pin 3 of the first to pin 2 of the second?
 

laroche73

New Member
cascaded 555s

Yes, the ON time of a 555 monostable is about 1.1RC, so those values give you ~ 5 seconds. The pulse on the trigger input should go away before the 555 times out, so use a simple differentiator (cap & resistor) to connect the output from the first 555 to the trigger input of the second. You might want to look over the circuits at this site to get a few more suggestions & ideas on using the 555: **broken link removed**

(The 2-stage time delay circuit does what you want) **broken link removed**
 
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