Create a trigger "pulse" from a constant 12V+ ?

Thread Starter

nka

Joined Jul 27, 2013
4
Hello,

I have two timer switch (Labco TM-5) that can be trigered from a Ground "push" switch. I would like to replace this switch with an automatic "pulse" when a constant 12V is received. Only one pulse, no repeat.

Got an idea how to do this?

Note that I'm very beginner in that! :)
 

LDC3

Joined Apr 27, 2013
924
Hello,

I have two timer switch (Labco TM-5) that can be trigered from a Ground "push" switch. I would like to replace this switch with an automatic "pulse" when a constant 12V is received. Only one pulse, no repeat.

Got an idea how to do this?

Note that I'm very beginner in that! :)
Yes, you can use a 555 timing circuit.
Obviously this means very little to you, but we cannot provide you with the correct circuit without knowing more detail of what you want to do and what it will be used for.

I don't know what you mean by a ground push switch. Could you post a picture?
 

wayneh

Joined Sep 9, 2010
16,399
Your request is confusing.
The timer is triggered by pulling one of its terminals to ground?
You want to trigger the timer when something goes from 0 to 12V and stays there?
Can the timer and this 12V source share a common ground connection?
Would the timer behave properly if the terminal is pulled to ground and held there, or does it need to be brief connection?

If it can tolerate being held low, then a single MOSFET will do the job.
 

Thread Starter

nka

Joined Jul 27, 2013
4
Hello,

Sorry for the confusion.

When I apply the ground (close circuit) to the Timer, it does not switch from NO to NC and the timer doesnt start until I release the ground (open circuit).

What I mean by "grounded switch", it's that when the switch is close, the circuit goes ground.

I guess it could only send a .5 sec pulse, but can't be forever as it wont trigger correctly.

Yes, they can use the same 12V. I jumped the ground directly to the triggers with the switch (PIN 3 to 2).

Not sure if low could do.
 

MaxHeadRoom

Joined Jul 18, 2013
21,396
Hello,

Sorry for the confusion.
Still not clear.. to me anyway? when you say 'Ground' do you mean the supply common or Earth Ground?
If you mean earth ground then the common must be referenced to it somewhere?
If your circuit is isolated from E. Ground then it is best or clearer to say 12v common.
Max.
 

wayneh

Joined Sep 9, 2010
16,399
You need a monostable multivibrator, or "one shot" circuit using a 555 timer IC. This will see a negative-going trigger and produce a signal for whatever length of time you want, say 0.5 second.

You may need a transistor to invert the incoming +12 trigger into a negative-going edge for the 555.
 

LDC3

Joined Apr 27, 2013
924
Hello,

Sorry for the confusion.

When I apply the ground (close circuit) to the Timer, it does not switch from NO to NC and the timer doesnt start until I release the ground (open circuit).

What I mean by "grounded switch", it's that when the switch is close, the circuit goes ground.

I guess it could only send a .5 sec pulse, but can't be forever as it wont trigger correctly.

Yes, they can use the same 12V. I jumped the ground directly to the triggers with the switch (PIN 3 to 2).

Not sure if low could do.
It appears that the TM5 starts on a low to high transition.
If all you want is a switch to start the TM5, then a resistor, a capacitor and a momentary switch is all you need.
 

Thread Starter

nka

Joined Jul 27, 2013
4
yes, but I wish to switch it automatically when the 12V+ goes live. I dont want to press a switch (like I do now).
 
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