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Old 1st March 2005, 09:51 AM   #1
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Default Relay switching circuit please

I need a simple circuit that will allow me to activate a 5 volt relay (max 200m coil) lusing a momentary action PCB mount tactile membrane (keyboard) type switch. I want the relay to remain on until the power supply is switched off, and then revert to the off position, ready to be activated agian by the switch.

Power supply for the circuit willbe +5V.

This relay will be used to force a NEC CDROM to read the TOC of a CD. Currently a simple microswitch on the CD transport tray does that job but I have converted the CDROM to top loading and do not want to use the microswtch.

Can anybody help?

Cheers,

Joe Newbold
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Old 1st March 2005, 10:22 AM   #2
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The easiest solution is to use a set ot N.O. contacts on the relay to keep the coil energized until the power is switched off.

Wire the contacts across your momentary switch.
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Old 1st March 2005, 01:10 PM   #3
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Hi Frank,

I think I see what you mean (I am not an EE by the way). Does using a relay with N.O. contacts mean that the relay is open until I depress the momentary switch, and then, when the switch is dpressed, the contacts remain closed until power off?

Cheers,

Joe
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Old 1st March 2005, 01:38 PM   #4
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Old 1st March 2005, 01:44 PM   #5
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Quote:
Originally posted by jnewbold
Hi Frank,

I think I see what you mean (I am not an EE by the way). Does using a relay with N.O. contacts mean that the relay is open until I depress the momentary switch, and then, when the switch is dpressed, the contacts remain closed until power off?

Cheers,

Joe

Absolutely. It should do exactly what you need.
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Old 1st March 2005, 01:59 PM   #6
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1. Hi Frank Berry,

Thank you for your help. I will try the NO relay in the morning.



2. Hi jacco vermeulan,

Thnaks for the URL you quoted above. Very useful information.


Cheers,
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Old 1st March 2005, 02:40 PM   #7
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Joe,

This is the circuit. Pardon the sloppy drawing. I did it in MS Paint.
Attached Images
File Type: gif latching relay.gif (1.5 KB, 229 views)
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Old 1st March 2005, 03:23 PM   #8
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A very easy way to do this without any other electronics, is to use the membrane switch to close the relay using the 5V. Also wire the 5V through one of the spare contacts (if it is for instance a double relay) back to the coil of the relay. This way the relay will "keep itself on" until the 5V drops away.

I've built a couple of circuits using this technique, from car engine immobilizers to keypad security locks.

Hope this helps.

Regards
Paul
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Old 1st March 2005, 07:21 PM   #9
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Quote:
Originally posted by psmithmx
A very easy way to do this without any other electronics, is to use the membrane switch to close the relay using the 5V. Also wire the 5V through one of the spare contacts (if it is for instance a double relay) back to the coil of the relay. This way the relay will "keep itself on" until the 5V drops away.

I've built a couple of circuits using this technique, from car engine immobilizers to keypad security locks.

Hope this helps.

Regards
Paul

That's exactly what I told him.
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Old 2nd March 2005, 01:38 PM   #10
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Hi Frank (and Paul)

Thanks for your help. I constructed a test swith/relay circuit as you described. It is exactly what I needed.

Thanks again,

Joe
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