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ArcticMyst Security by Avery

Quick NPN switch question

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Feb 5, 2008
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Greetings, have a friend and he has a problem,

6gTgDMKRi2fC5kdKgoZ8h1ZcmrK92.jpg


Basically he cannot find a way to make this work. Tried a 4k and 10k ohms in-line, it doesn't appear to be switching the NPN properly.

The comparator is powered and calibrated, and works correctly.

Ideas for connecting it to NPN?
 





Things

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Try 1K, in fact sometimes you don't even need a resistor there, many transistors have base resistors built in now. Try it without one too.
 
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What are the exact TL081 output voltages at High (ON) and
Low (OFF) switched conditions..??


Jerry
 
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What are the exact TL081 output voltages at High (ON) and
Low (OFF) switched conditions..??


Jerry

10 and zero volts respectively.

He says he did manage to switch a simple LED on/off, with this:
heizung2.jpg


But if he tries to drive a relay (0.5 amps rated) to switch something else, he says it doesn't work.

PSU supplying the needed juice is from a PC, so it's sturdy enough to take the current demands.
 
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Remove the coil L. You may be shorting out the transistor
and damaging it by shorting it if the coil resistance is too
low.
Also lnterchange the 2 resistor values and it should work
if the Transistor is still good. The 2K2 should go to the base.

What part number of transistor is in the circuit ??


Jerry
 
Last edited:

djQUAN

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10 and zero volts respectively.

He says he did manage to switch a simple LED on/off, with this:
heizung2.jpg


But if he tries to drive a relay (0.5 amps rated) to switch something else, he says it doesn't work.

PSU supplying the needed juice is from a PC, so it's sturdy enough to take the current demands.

Is the op amp used really TL081? It is a fet input op amp but has limited output current capability and not really designed for single supply operation.

Try using an LM358 (since it is a dual package, you'd only use one side) since it is designed for single supply operation, can work down to 2V and input common mode and output swing includes ground.
 
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It's all fine folks, in combination with Jerry's advice and little bit of experimenting we managed to solve the problem.

Thanks all!
 

djQUAN

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glad it worked. :) I would still recommend changing the op amp with something more suited to single supply operation unless a FET input is necessary, but hey, if it works. ;)
 




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