Need help building a NCTC timed/delayed relay

Thread Starter

shainiac

Joined Mar 26, 2013
7
I'm a mechanical engr student and a bit out of my element, so please bare with me. For a project, I am trying to build a circuit that will allow a NC relay to stay open for an amount of time if a circuit is briefly completed in a relay via a momentary switch.
Basically, I want to interfere the NC relay by quickly pressing a button.
It can operate off of either 12V or 5V DC, there will be power supplies for both in the circuit.
Current through the relay will be less than 0.5A, but could be used to operate a high current relay if it's an issue.
I would like to have adjustment of the time delay.
I think this is referred to as a NCTC relay, but searching didn't bring me any results.
Can anyone help me with a parts list/schematic?
Attached is a rough picture of what I have in my head. Please excuse my awesome MS paint skills.

 

#12

Joined Nov 30, 2010
18,210
This relay will hold for about a second if you use 4000 microfarads of capacitance across the switch, but, "an amount of time" might be anything from a millisecond to an hour and a half. Personally, I don't have time to design an adjustable, all purpose circuit for a complete guess as to what you want.
 

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Thread Starter

shainiac

Joined Mar 26, 2013
7
I am trying to delay the release of the relay. I want to be able to quickly press the momentary switch to open the relay and for it to remain open after switch has been released.
 

tracecom

Joined Apr 16, 2010
3,944
I am trying to delay the release of the relay. I want to be able to quickly press the momentary switch to open the relay and for it to remain open after switch has been released.
Most relays are SPDT (also called C-form,) so when a relay operates, the wiper moves from the NC (Normally Closed) contact to the NO (Normally Open) contact. Thus, it's better to simply talk about operating and releasing the relay rather than opening and closing it.

I think you simply want to press a button, which operates a relay, and have the relay stay operated for about 10 seconds. If that is correct, then the best way to do that is with a 555 monostable circuit, also called a one-shot. Is that what you want?
 

Thread Starter

shainiac

Joined Mar 26, 2013
7
Most relays are SPDT (also called C-form,) so when a relay operates, the wiper moves from the NC (Normally Closed) contact to the NO (Normally Open) contact. Thus, it's better to simply talk about operating and releasing the relay rather than opening and closing it.

I think you simply want to press a button, which operates a relay, and have the relay stay operated for about 10 seconds. If that is correct, then the best way to do that is with a 555 monostable circuit, also called a one-shot. Is that what you want?

That's exactly it.
 

tracecom

Joined Apr 16, 2010
3,944
Here is a schematic for a 10 second one-shot. When you press the pushbutton, the 555 will output a high on pin 3, and pin 3 will stay high for 10 seconds. You could connect a suitable relay to the output, and it will stay operated for as long as pin 3 is high.
 

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Thread Starter

shainiac

Joined Mar 26, 2013
7
Here is a schematic for a 10 second one-shot. When you press the pushbutton, the 555 will output a high on pin 3, and pin 3 will stay high for 10 seconds. You could connect a suitable relay to the output, and it will stay operated for as long as pin 3 is high.
Thank you! I will try and find those parts locally and build one on a bread board in my uni's lab this week. Can you tell me which capacitor is responsibly for the duration of the delay? Also, does increasing the capacitance increase or decrease the delay?
 

tracecom

Joined Apr 16, 2010
3,944
Thank you! I will try and find those parts locally and build one on a bread board in my uni's lab this week. Can you tell me which capacitor is responsibly for the duration of the delay? Also, does increasing the capacitance increase or decrease the delay?
See note 2 on the schematic: R1 and C1 control the duration of the high at pin 3. If you do decide to build it, remember to put a counter emf diode across the coil of the relay to avoid damage to the 555.
 
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