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Voltage monitor - need help with the circuit
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tthoang86



Joined: 17 Feb 2009
Posts: 3
Post17 Feb 2009 4:06   

Voltage monitor
Hi, I need help.

I have no idea how to build this one. I need a circuit that can read a voltage drop
below a certain voltage then make a close circuit between 2 point.

aka
example: voltage > 10 V , open circuit between A and B

voltage < 10 V , short circuit between A and B

The voltage input source is 22V which could be reduced if necessary.

Any help is appreciated.

Thanks
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IanP



Joined: 05 Oct 2004
Posts: 6490
Helped: 1542
Location: West Coast
Post17 Feb 2009 5:35   

Re: Voltage monitor
You can use a comparator (or an opamp in comparator configuration) and a relay ..
Comparator will compare 10V input with a reference and if the voltage is lower it will activate the relay and the relay's contacts will shorten A with B ..

Another option is to employ 555 timer and configure it as comparator, and the reason I suggest this IC is that it can directly drive a relay ..
See this link:
http://www.zen22142.zen.co.uk/Circuits/Switching/vcs555.htm
One thing to remember is that the input has to be at 2/3 of the 555's suply voltage, so you may need a potentiometer connected between "10V" and GND to precisely adjust at what level would you like to activate the relay ..

Rgds,
IanP
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tthoang86



Joined: 17 Feb 2009
Posts: 3
Post17 Feb 2009 5:50   

Re: Voltage monitor
can you please explain a bit how exactly i can use an op-amp connect with a relay to do it? as i dont know what is 555 timer Sad
Sorry for being such a noob.


p/s: i got UA741CP op-amp
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IanP



Joined: 05 Oct 2004
Posts: 6490
Helped: 1542
Location: West Coast
Post17 Feb 2009 7:17   

Re: Voltage monitor
Goto:
http://www.uoguelph.ca/~antoon/gadgets/741/741.html
and have a look at "12V Battery Monitor" (for proper operation you will need to swap input +[3] and -[2], so the output goes high if the voltage is below "10V") ..

Instead of connecting LED try to connact a relay in the same way as it is connected in the "Auto-Fan" circuit (via NPN transistor) ..

Rgds,
IanP
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lats



Joined: 27 Oct 2005
Posts: 199
Helped: 7
Post18 Feb 2009 3:41   

Voltage monitor
simply use a pic microcontroller feed the input o ADC ( make it in a range of 0-5Volt) take whatever decision make a out high/low as desired.
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