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Quad Comparator Help

duke37

Jan 9, 2011
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Show us your circuit.
Perhaps you have ambient noise, either electrical or light which is triggering the comparator for a short time.

Some capacitors on the inputs could help as would hysteresis.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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No, the problem is that the output can't pull low enough to turn the transistor off completely.

Using a pnp transistor would help (or a mosfet). Another solution is to place a diode in series with the emitter of the transistor.

With a relay this doesn't matter so much because the very low current won't pull it in. However the siren obviously makes a noise with very low currents.

With a PNP transistor the pullup resistor isn't required, but a resistor is required between the output of the comparator and the base (use the same value that you used for the pull-up resistor in this example).
 

randyrogers04

Apr 21, 2013
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Apr 21, 2013
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ok so i have the circuit soldered together and running, using a PNP 2n3906 but im still getting feedback from somewhere. What is weird is that when i had it on my bread board i had no feedback. what should i look for that may cause this to happen.

When i say feedback i mean i am getting a constant static like noise even when the resistor has a large light source(comparator output off).
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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OK, show us the circuit and if you can do so, a video showing the "static".

Also show us your assembled circuit.
 

randyrogers04

Apr 21, 2013
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Apr 21, 2013
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well i managed to solve the problem. I went to sleep and woke up to begin working on my circuit again, well it works and no "static". Not sure what changed but its working now, ill post a video of it so yall can see what you helped me make haha. Thanks again for all of your help, it is greatly appreciated. Im sure ill be back real soon for more help.
 
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