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  #1  
Old 02-22-2008, 03:01 PM
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Electronics question. specifically a circuit.

I was wondering if anyone who has a bit more of a history in electronic engineering might be able to verify for me that this circuit would work fine being bumped up to 9V. The relay would be replaced with a 12V version.

I tried a comparator circuit for this application, but couldn't seem to get it to work. So I figured I'd try something a tad more simple for this. It'll get the job done just the same.

Here's a link to the circuit diagram.
http://www.uoguelph.ca/%7Eantoon/circ/actrelay.htm


Thanks,
Matthew
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Old 02-22-2008, 03:35 PM
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You think an op amp would do the trick? Feedback, with gain set to 0.5?
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Old 02-22-2008, 04:14 PM
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I have no idea. Unfortunately I'm not well versed enough to design these things. I can build them, and get a pretty good understanding of how they work when I see a design, but I've never learned enough to design much beyond a 555 circuit.
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Old 02-22-2008, 09:11 PM
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Give this one 24 hours and see what happens.
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Old 02-22-2008, 09:15 PM
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GAH, I'm not sure why, but this one doesn't seem to work. I've built, re-built, and re-built ten times and it just doesn't do a darn thing. Only difference is that I'm running a 9V battery and a 12V relay.
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Old 02-22-2008, 09:38 PM
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circuit may be working now

If the relay does not trip until it sees 12 volts then the ciruit could be working now, but not have the current at 9 volts to throw the relay.
I sugest you first substitute a 12v bulb in place of the relay coil.

Then you can see it glow when Q1 turns on.
If you want to limit the amperage through Q1 to about 1 amp.
then just put a low value resister in the drain say 10 ohms below q1 and ground.
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Old 02-22-2008, 09:38 PM
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The relay does fire on 9 volts.
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Old 02-22-2008, 11:06 PM
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Quote:
Originally Posted by techy101 View Post
GAH, I'm not sure why, but this one doesn't seem to work. I've built, re-built, and re-built ten times and it just doesn't do a darn thing. Only difference is that I'm running a 9V battery and a 12V relay.
Rather difficult to trouble shoot for you, without hands on what you have done. It is a matter of checking voltages as they appear within the circuit as a means of finding out what is happening.

A lot depends on the sensitivity of the photo resistor you are using. I do not at all like the suggestion of using a signal diode. The diode connected across the relay to stop reverse EMF can be any cheap diode, but not a signal diode.

Trying to keep the description simple ---

The photo resistor reduces in resistance when illuminated, and changes the voltage applied to switch the transistor, so that the balance of R1 against R2+ R3 is critical. The higher resistance of R2+R3 the greater the sensitivity. When increasing the operating voltage the resistance must be increased to compensate.

The proper approach would be to experiment with both R2 and R3 so as not to make the control via R2 too coarse/sudden. However I would say the easy way will be to increase R2 to 100K ohms.

Hook up a voltmeter across the relay and you will be able to see whether or not the transistor is switching the negative circuit to ground. If you are getting say eight volts or better and the relay is not operating, you will have to consider a lower voltage or a more sensitive relay. If the relay is an open type, you may be able reduce the spring pressure.

I have done my best to show you in simple terms how things are working, which is about all I can do from so far away.
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Old 02-22-2008, 11:23 PM
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You could try it using a 5 volt regulator, or an adjustable regulator to get 6 volts. I wish I still had a working copy of Electronics Workbench or I'd load it up and test your circuit.
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