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

Quote: 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.
Name: Pulse.JPG
Views: 159
Size: 19.1 KB

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

Quote: 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.

Quote:

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

Quote: 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

Injector · 12-01-2007, 10:26 AM

well if anyone knows how to wire the 555 chip to fits my needs i would certanly look into it, because i'v been looking at diagrams online of the 555 and i have no idea how to even start wiring it.

nserpa9248 · 12-01-2007, 11:11 PM

Quote: Originally Posted by KB5WID

This is not exactly what I was thinking.

Here is whe correct way.
Attachment 46586

You are correct, you would want +12v constant at terminal 87 relay #1, not switched as i had shown originally
However,

A-This is an unnecessary connection. Connecting the same wire at 87 and 87a is not required, since it is a normally closed (NC) connection.

B-You would want the wire shown (pink that is coming out of 30 on relay #1) to go to 87a. If the OP (Injector) wanted a pulse when the car was turned on you would make the conection at 30, however, connecting the wire to 87A will send +12v back to the #2 relay when the #1 relay goes back to its NC position (when the car is shut down or the +12ACC wire connected to 86 on relay #1 loses power).

nserpa9248 · 12-01-2007, 11:20 PM

the way i was saying you would want to wire it (revised :-p) i re-drew below.

You never specified if you needed a negative or positive pulse.

Injector · 12-02-2007, 07:50 AM

Quote: Originally Posted by nserpa9248

the way i was saying you would want to wire it (revised :-p) i re-drew below.

You never specified if you needed a negative or positive pulse.

i never specified it because it's not really going to be neither, its going to the the shutdown button push for my mobo, so on relay 2, 87 will be on side of the cable and 30 will be the other, so that when relay #2 gets power it will do a temporary button push.

loginfailed · 12-02-2007, 02:39 PM

I bet one of these would work:
see option #6