Constant to pulse, how?

**Injector** · 11-29-2007, 02:50 PM

well before anyone tell me to search, i have searched and i found how to do something similar to what i want to do,

but this will make a pulse when the key is switched on, what i want to do is to pulse when the key is turned off, anyone have ideas?

**KB5WID** · 11-29-2007, 04:37 PM

Add a double throw relay. Connect the arm to the circuit and 12 volt fused to the contacts. This way you get a negitive pulse each time the relay is turned on or off.

**Injector** · 11-29-2007, 09:47 PM

Originally Posted by KB5WID

Add a double throw relay. Connect the arm to the circuit and 12 volt fused to the contacts. This way you get a negitive pulse each time the relay is turned on or off.

could you go into a little more detail? i've read your post about 20 times but i just feel like banging my head against the desk

, cause i can't figure it out.

**nserpa9248** · 11-30-2007, 12:31 AM

diagram belows shows a second relay which will do the same that KB5WID was talking about.

This diagram shows the first relay which will normally have +12v coming out of (87A) and going to the second relay.

The +12v wire connected to the first relay would actually be the accessory or ignition wire you are using. when the car is turned on, the relay sends +12v to 30 (which is not connected to anything). Then, as the car is turned off, and the 87A terminal gains power again, it sends power to the second relay.

That relay will in turn take that voltage, and make a momentary pulse from it. (You can make this pulse a negative or positive pulse).

If you have any questions, feel free to ask

**ldiamond** · 11-30-2007, 12:11 PM

Why use a relay? Do you have very high current going through this? Wouldnt a MOSFET be simpler and more reliable?

**KB5WID** · 11-30-2007, 01:59 PM

This is not exactly what I was thinking.

Here is whe correct way.
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**ldiamond** · 11-30-2007, 03:04 PM

And wouldnt a simple LM555 do the trick...? Why bother with all that?

**Injector** · 11-30-2007, 06:38 PM

Originally Posted by ldiamond

Why use a relay? Do you have very high current going through this? Wouldnt a MOSFET be simpler and more reliable?

because i don't know what a MOSFET is.

And wouldnt a simple LM555 do the trick...? Why bother with all that?

because in a small town it's really hard to find parts to build one, not to mention that i wouldn't know where to start to build one.

**Injector** · 11-30-2007, 06:41 PM

nserpa, kb5wid, thnx for the help i know what direction to head.

**eastvolt** · 12-01-2007, 07:43 AM

Originally Posted by ldiamond

And wouldnt a simple LM555 do the trick...? Why bother with all that?

i agree. and i'm not sure what your application is, but a relay is going to create a train of pulses, not one (due to the mechanical properties of the relay and contact bounce)

If you're interfacing a logic circuit, this would NOT be good.

Go 555 - cheaper, smaller, easy to use, and much more reliable