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Old 19th May 2004, 08:07 PM   #1
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Question Check: Simple Circuits

I've just put together some simple circuits. I'd very much apreciate it if someone could them me if they will work, especially the 1st, which I put together myself. The others are pretty much standard design.

I tried to simulate the first one, but SIMetrix wouldn't let me. All it has to do is: if there's a signal on one of the input signals the relay has to switch on. Signals on the diodes will be at least 9V, with a maximum of 12V (maybe 18V, but the chance is pretty small).

Ok, so here it is:
Click the image to open in full size.

Here are the 2 proven designs, they should work, but I figured I might as well post them to make sure I didn't make any stupid mistake.

Power supply:
Click the image to open in full size.

On/off signal with push switch:
Click the image to open in full size.
The out signal is going to be one of the inputs on the first circuit.
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Old 20th May 2004, 11:03 AM   #2
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Ah, c'mon, sure there must be someone who can tell me if it will work by just looking at it? Half of you guys are EEs.
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Old 20th May 2004, 11:11 AM   #3
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If you put the relay coil in series with the collector rather than the emitter, the relay will pull more reliably with input voltages that are less than the relay's coil voltage. You might also want to put a 100k resistor in parallel with a .1uf capacitor from the transistor's base to ground to make the circuit less prone to false triggering due to noise.
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Old 20th May 2004, 01:43 PM   #4
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Quote:
Originally posted by Frank Berry
If you put the relay coil in series with the collector rather than the emitter, the relay will pull more reliably with input voltages that are less than the relay's coil voltage. You might also want to put a 100k resistor in parallel with a .1uf capacitor from the transistor's base to ground to make the circuit less prone to false triggering due to noise.
Thanks . One question about the resistor & cap to earth though. Is this really necessary? There will either be a signal or no signal at all on the diodes, nothing in between.

*Edit: Ah well, nevermind, I managed to added it to the schematic and I only had to make the PCB 25 mils longer .
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Old 20th May 2004, 01:55 PM   #5
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Yes. The resistor and cap are necessary. Without them, the base of the transistor will see an extremely high impedance when none of the diodes are conducting. The circuit will be very sensitive to RF interference and other "noises."
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Old 20th May 2004, 02:19 PM   #6
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Quote:
Originally posted by Frank Berry
Yes. The resistor and cap are necessary. Without them, the base of the transistor will see an extremely high impedance when none of the diodes are conducting. The circuit will be very sensitive to RF interference and other "noises."

Ok, thanks for the info!
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Old 6th June 2004, 11:24 AM   #7
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Hm, OK, I got kind of a problem. I made the circuit about 2 weeks ago, but hadn't tested it yet. Yesterday, I decided to do so and the thing has kind of a very annoying flaw. It only turns on, I can't turn of the relay, no matter what (well, OK, shutting down the power helps ).

Why is that? Here's the schematic I used:
Click the image to open in full size.

Should the cap & resistor be in front of the 10K resistor or is it something else?

Oh, btw, I used a 47nF cap instead of a 100nF cap, but I can't imagine that that's causing the problem. I even took out the cap and tested again and the result was the same: it only turn on, not off.

Ah, and you can find the PCB layout on my website: http://members.lycos.nl/anthonyvh/index.php?page=pwrsw
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Old 6th June 2004, 11:34 AM   #8
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Lightbulb Darlington

Quote:
Originally posted by Devil_H@ck
Hm, OK, I got kind of a problem. I made the circuit about 2 weeks ago, but hadn't tested it yet. Yesterday, I decided to do so and the thing has kind of a very annoying flaw. It only turns on, I can't turn of the relay, no matter what (well, OK, shutting down the power helps ).
Hi, Use two transistors Darlington connected!
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Old 6th June 2004, 12:03 PM   #9
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2 darlingtons in series in front of the relay coil? Or 1 in front & 1 behind?
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Old 6th June 2004, 12:19 PM   #10
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Quote:
Originally posted by Devil_H@ck
2 darlingtons in series in front of the relay coil? Or 1 in front & 1 behind?

Two transistors Darlington connected before the relais or one Darlington transistor at the same position.
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