Hi I have done the exact samething as Sangee.
I built the constant to momentary circuit ,to find out that it only works when power is applied and not when power is removed.
I have a good back ground in electronics though it has been years.
My problem is I don't understand the examples given.
Can someone link me to schematic that shows where I pull off the momentary signal or how the two relays connect.
Thanks

 

Welcome to AAC.

I have placed your post in its own thread to prevent confusion. This is the old thread - http://forum.allaboutcircuits.com/showthread.php?t=23987

 

Hello and welcome to AAC!

I could help you, but I have a couple of questions I need to ask you in order to do so. What are you trying to achieve? Do you have a schematic?

 

Hi guys, thanks for the warm welcome and the thread move.
Here's what I have.
I've got a 12v constant source/trigger from a projector.
What I want to turn on/off is a garage door opener.
So essentially I am replacing the button on the wall that activates the garage door opener.

So I built this constant to momentary switch.

This activated the opener when the trigger/switched power was applied but did not activate the opener when power was removed.
When power is applied I need a pulse to the opener and when power is removed I need a pulse to the opener.
(It is hard to search for something when you don't know what it is called.)

I found this circuit and think it will do what I want it to.
Can you confirm this for me, thanks.
These are two SPDT Relays, they said the resistor could be elimanated and I might need to go to a 2.2K capacitor.

 

I think you were moved from :http://forum.allaboutcircuits.com/showthread.php?p=147012#post147012

The attached is a schematic of my circuit using your pin numbers. No diode and no resistor. Are you using automotive relays? These have a coil resistance of ~90Ω. The on-time of the second relay is determined by the capacitor and the coil's resistance, plus the mechanical delay. 90Ω and 2200μF will have a time constant of ~0.2 seconds. I think the attack and release delay times (I can't find the data) are about 0.1 Sec. So the contacts should be closed ~0.1 sec when the power is switched on or off.

Ken

 

It seems you are using automotive relays, right? I see what you are trying to achieve. You are using the coil in series with a capacitor so the coil is momentarily energized when power is applied. However, in the first circuit, if the power is removed, the capacitor won't discharge. You will need a second relay, but connect the NC contact of that second relay so the capacitor will discharge via the first coil. Use a diode to prevent a short.

See the diagram:


The diode is very important, since it will allow the discharging of the capacitor through the relay coil, but it will prevent a short circuit when power is applied.

Edit: I think Ken's schematic is better (wasn't there when I posted).

 

I am using little, inch long, relays from radio shack, they may be called reed relays.
The schematic does represent an automotive relay though.
I will build this circiut this weekend and let you know how it goes.
Thanks for the input.

 

Thank you Cume and moffet for the help.
When Cume put my pin numbers on the schematic I was able to understand both the schematics posted here, thanks.

Here is what I did incase someone stumbles over this post with the same issue.
Starting on the left the first relay is a Radio shack SPDT-Micromini 12VDC relay # 275-0241.
Second is a Radio Shack SPST-12VDC Reed Relay #275-0233
I used the original two relay schematic that I posted, though all the schematics will work.
I kept the 1K uf capacitor and got rid of the resistor.
If you use a diode across the second relay the circiut doesn't work properly.
I was able to use a diode across the first relay to stop the spike when the coil field drops.

In all three schematics power to pin 87, was provided by the door opener.
I wanted to isolate the two sides of this circiut so, I jumped the power from pin 86 to 87, on the first relay.
In this way the door opener is not physically connected to the projector.
It works perfectly.
Thanks again for the help. I hope this helps others too.
Hound

 

The attached is a schematic of my circuit using your pin numbers. No diode and no resistor. Are you using automotive relays? These have a coil resistance of ~90Ω. The on-time of the second relay is determined by the capacitor and the coil's resistance, plus the mechanical delay. 90Ω and 2200μF will have a time constant of ~0.2 seconds. I think the attack and release delay times (I can't find the data) are about 0.1 Sec. So the contacts should be closed ~0.1 sec when the power is switched on or off.

Ken

Ken: Using this schematic I've got it working as you discuss in the quote above. Latch times are pretty variable, anywhere from .1 to 1 sec depending on I don't know what....

Problem is I need a 1-2 sec "latch" for each edge... using the math above I'd like to add a resistor to increase the time accordingly if that's how it works, but can't seem to figure it out on my own so am asking for some help to lay it out....

Using standard 12v SPDT relays (automotive type), 2200uF/16 cap, so... I come up with 2200 x .000001 x (80ohm (relay) +(500ohm resistor) =1.276sec... close enough. But where to add the resistor? And I'm not even sure if that math is right. I've tried adding a 500ohm resistor between C1 and Relay2. Nothing happens.... any suggestions?

 

Adding a high ohm resistor (reletive to the coil resistance) in series will act as a voltage divider. You will not get enough voltage across the coil (1.655VDC) to activate the relay. Maybe try a low coil current 12v relay instead of those 80Ω automotive relays.

 

Do you mean something like this? https://www.westfloridacomponents.com/REL052APH02/12.5mA+12V+Low+Signal+Relay+Omron+G5V-1-DC12.html

 

Yes, that's worth a try. Will the contacts handle (1A) what you are trying to switch?

 

The project is to 'autostart' a push button start / push button stop generator for a battery wall system to recharge when it gets low... the battery wall sends a 'constant on' signal to the gen. when it's hungry for a charge and when it's charged cuts the signal. The signal can be either dry contact, 5v, 12v. I'm planning on using the dry contact to act as the switch in your schematic and need to convert that into a pulse to mimic the push button start. So the coil will only see what a seperate powersupply gives it to energize. The NO/NC contacts of the relay will only see the push button power which I assume is also low power as it's got its own relays etc for it's start procedure... but I've not measured it.

I actually went the other way first and replaced the 80 ohm relays with ~480ohm relays and got much closer to the 1 sec, but not quite close enough to get the stop to register (takes a little longer, say 1.5-2sec) adding resistors between relay 2 and cap in series didn't seem to change the timing any. So Im confused by that. small resisters make no noticable difference, and big ones make the nothing at all happen.... the break point seems to be around 150ohm.... but timing doesn't seem to change. I've got a couple 2mA trigger current relays on the way so I'll give that a go.... otherwise I'm guessing I've been outsmarted and need to get a 555 timer and work it out that way?? ugh!

Any suggestions gratefully appreciated, otherwise I may just drink till I don't care!! Cheers.

 

Hopefully getting closer, but still need a little guidance.... I've got a setup that's giving me 37sec latch times... so cap/resistance combo is too big I'm guessing.

Using your capacitor charge & time calculator I backwards engineered the resistance to be around 17000ohm using the 2200uf/16cap. If I change the cap to a 100 and leave resistance the same (in the online calculator) (no added resistor... so can't change that) I get 1.7 sec. which is close enough. Before I go buy a bag full of caps am I on the right track or is my thinkin' stinkin'?

Appreciate the assistance!