Quick NPN switch question

[Eudaimonium]

Greetings, have a friend and he has a problem,

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]

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.

 

[lasersbee]

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


Jerry

 

[Eudaimonium]

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:

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.

 

[lasersbee]

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

 

[djQUAN]

10 and zero volts respectively.

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

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.

 

[Eudaimonium]

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]

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.