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Momentary 12v on circuit for 12v trigger. Help!

Jaydriver

New Member
Hi, I apologize if this is elsewhere or out of place, but I've been out of diy circuitry for decades. I need a simple circuit, maybe a 555??, to output 12vdc for maybe 3 or 4 seconds when triggered by 12vdc, and then stay off until the trigger voltage is recycled off and back on again. Output would be very low current, only enough for a small LED. I can no longer figure out how to do this. Ideas? Thanks, Jay
 

billybob

Active Member
I believe I was looking for the same thing, check the thread labeled “Help with circuit design?” Visitor suggested a “one-shot circuit“
Hope this helps.
 

AnalogKid

Well-Known Member
Most Helpful Member
I'm not a 555-solves-all-problems guy, but in this case the datasheet monostable circuit will. With a bipolar 555 and a 12 V rail, the output will be approx. 10.5 V. Add this circuit to the Trigger input to extract only the negative edge of the input signal.

C1-R2 form a differentiator circuit. Because R2 is tied to Vcc, the Trigger pin rests high. when the switch is closed (or whatever is driving this goes low), the right side of C1 goes low and triggers the 555. R2 immediately starts to charge up C1, teturning the Trigger input high in approx. (R2 x C1) seconds. With the values shown, that is 10 microseconds. The circuit can sit there forever and the 555 will not retrigger. When the switch opens or the input signal goes high, the left side of C1 goes high. But the right side of C1 already is high, so nothing happens and everything sits. When the input goes low again, the 555 cycles again.

ak

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KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Many years ago (maybe late 1980's), I built exactly that. I even had lots of PCB's made. I had thought about selling on that auction site. . The 555 timer glitches. The main timer IC is obsolete, but obtainable. The caps I used are also hard to find, 1 uF polyester. A 2 Meg RA potentiometer are hard to find. A FET is nearly impossible to find as well. The case went south and I did find it again.
The output was like 1A or so, using a fully protected transistor. There was an internal 3A fuse. One stupid art, that makes it easy to add wires, you have to be a company to get it or they need a real company to send the invoice too. What I planned to use to test it never got completes. It would be a voltage bar graph display. Now, I know how to label.

Three flying leads. I never got around to finding a connector with screw terminals. Mounting could be through the bottom of the ase or a ty-wrap base so it could be attached to a wire bundle.

What mad it unique was the extremely quick reset time. Not 50ms. but on the order of 1 ms providing the power went away or drpped below a value. It would be on the order of a $80 to $100 timer module.

This started when I was approached by a automotive electronics installer, I used it in my vehicle for a long time. The design was proven rugged and reliable the fast reset and protections made it unique. It had +12, +12 trigger and ground. It was used on two vehicles/

I did have one glitch because the remote lead on my car radio was regulated, the 12V remote lead decayed slow, youCOULD end up with a thump. 99% of the time - no thump. A comparitor-based protected trigger would have fixed that.

It was a tough design. It's still hard to find timers with fast reset times.
 

Visitor

Well-Known Member
555 Timers obsolete? I think not.

From Wikipedia:

In 2017, it was said over a billion 555 timers are produced annually by some estimates, and "probably the most popular integrated circuit ever made."
 

AnalogKid

Well-Known Member
Most Helpful Member
So where do they all go?, it's a rare to ever see them in anything?.
I'm not so sure that it is. Beyond phones and tablets, the big wide world of electronics that was there before them still is out there. The LM324, another 1971 wonder-child, is the darling of secure security systems and a ginormous pile of industrial control stuff.

Note - I left TVs out of the list because both my Panasonic plasma and my Sony LCD TVs have LM324's in them.

ak
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I'm not so sure that it is. Beyond phones and tablets, the big wide world of electronics that was there before them still is out there.
Yes but where are the 555's? - you don't find them very often.

LM324, completely different beast - presumably it's a very cheap chip, and a bog standard low spec quad opamp, ideal for MANY purposes.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
The LM3905 (obsolete) was my favorite timer. The 555 is better suited for a cheap oscillator. Analog/LT's Timerblox series would be really cool if they worked to 30V. The software programmable 555 seems cool too. A 555 does not make a good power on reset timer.
 

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