# Request help from the electronic gods!!!

Discussion in 'The Projects Forum' started by robert24b, Mar 8, 2011.

1. ### robert24b Thread Starter New Member

Mar 8, 2011
10
0
Hello all!
I read your posts very religiously and find myself needing to request some help. I am having a SERIOUS brain fart.

I have a "unit" w a N/C relay terminal output. When this circuit is broken these terminals go to "open". How can I use this "open" as a trigger to tell a relay (or whatever I should use) to turn on 12vdc?

In other words, my N/C will turn "open" and this "open" performs the rest of the action by allowing 12vdc to flow to the rest of the circuit.
What do I need between my N/C relay terminal and the rest of the circuit?

Last edited: Mar 8, 2011
2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,166
1,125
how much do you actually know about Ohms law and simple DC current flow/ voltage drop through resistors?

I take it from your post that you have NO background in electronics whatsoever.

That will make it very difficult to discuss 'what you need between the relay and the rest of the circuit'.

I would like to help, but hesitate to get involved. Severe lack of knowledge in this area means pages and pages of explanation would be necessary to convey the simplest of concepts.

If this is a wrong assumption on my part, please reply and let us know.

3. ### robert24b Thread Starter New Member

Mar 8, 2011
10
0
Thanks for the concern and the response.
I know enough to get myself 1/2 way to a desired result and I can do the math for ohms law. I have done a fair amount of reading and it seems as though most individuals are using a 555 circuit or some variation of to accomplish similar applications.
I feel like there is an easier way and I just don't know how to ask the right question.
Through my reading, the most similar application I can find is a home security system.
When motion is detected through a PIR, there is a N/C contact that goes open and tells the security system to "sound the alarm". What is between the PIR and the siren? The most essential item to convert that "open" to the 12vdc siren signal?
Please forgive my ignorance, I'm just a weekend hobbyist like many. (no lives are at stake).

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
4,166
1,125
When your relay is closed. You need to have a voltage on it that is grounded when the relay is closed. You will need some resistance in series with it as well to limit the current drawn while the terminal is grounded.

When the relay goes 'open', the ground is removed and that terminal now will have a voltage on it. This would be used to activate or trigger the next device in your setup.

5. ### Jaguarjoe Active Member

Apr 7, 2010
770
91
I don't know what your circuit looks like, but you might be able to put a sensitive relay across the NC troublemaker contact. When the contact is open, the relay will energize, when the contact closes, the relay will be de-energized. This is a common way to indicate power failure in a control circuit for backup systems that are not often used. Don't know for sure if this will work until you provide a diagram of what you're dealing with.

6. ### robert24b Thread Starter New Member

Mar 8, 2011
10
0
Kermit,
I follow what your are saying (i think). So will I need a transistor for voltage injection to the closed circuit?
As for the resistance, what values will I be using to solve for R? I assume the resistor is for a nominal drop and not an "exact" value?

I'm probably farther from the answer than I think aren't I?

7. ### robert24b Thread Starter New Member

Mar 8, 2011
10
0
Hey Joe, I like the "sensitive relay" idea. How is this powered?

8. ### R!f@@ AAC Fanatic!

Apr 2, 2009
9,634
1,109
How about the NC opening allows a FET gate to charge and apply the necessary Power to the load.

This way OP won't need a heavy duty relay but a heavy duty FET like those used in Mother boards. Low Voltage High current types. Simple and effective.

9. ### robert24b Thread Starter New Member

Mar 8, 2011
10
0
Is this a relatively inexpensive component to obtain? Do you have a link you could post so I could review the effectiveness/versatility of this FET gate?
Thanks!

10. ### R!f@@ AAC Fanatic!

Apr 2, 2009
9,634
1,109
Get a broken mother board and you will get plenty of FET's

11. ### robert24b Thread Starter New Member

Mar 8, 2011
10
0
So my gate is going to be the N/C from my relay and the source is 12vdc with the drain being 12vdc after the gate is "opened"?
Are there specific values for these components? A certain "size" I should be looking for?

Last edited: Mar 9, 2011
12. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
13
Just use another relay SPDT relay with N.C. contacts (when not powered). When your first relay opens it will remove power from the 2nd relay which will close the contacts (which are held open when powered by first relay).

13. ### robert24b Thread Starter New Member

Mar 8, 2011
10
0
Nomurphy,
Correct me if I am wrong but I don't believe the existing Normally Closed contacts can be used to energize another relay.
There is no voltage on these terminals (dry contact). In order to use another relay, I need to inject voltage to the contacts so that the 2nd relay is energized and subsequently de-energized when the contact is open. If I had voltage on these contacts, that would make life easier. Then I could just amplify.

Apr 2, 2009
9,634
1,109

15. ### robert24b Thread Starter New Member

Mar 8, 2011
10
0
12VDC not to exceed 148mA

16. ### R!f@@ AAC Fanatic!

Apr 2, 2009
9,634
1,109
Ok then find a mobo, post a pic of the processor are and I'll tell what to desolder

17. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
13
You need another relay and voltage thru the contacts.

Are you saying the contacts are not connected to anything? Or, just GND?

If GND, then the closed contacts can be used to ground the coil of another relay that hold its contacts open. When RLY1 opens, then RLY2 closes -- this is the same principle as using a FET or xstr.

18. ### robert24b Thread Starter New Member

Mar 8, 2011
10
0
This may be completely ignorant of me but can't I just use a 12V Normally open SPDT relay?

Last edited: Mar 11, 2011
19. ### robert24b Thread Starter New Member

Mar 8, 2011
10
0
I just had a thought! Instead of using the change from N/C to N/O as the trigger for a relay, can't I use the voltage from a disconnected LED to trigger a relay?
Wouldn't that make this whole circuit easier? (for me)