OPAMP as voltage comparator issue

[iPanos]

Hey guys...

I needed to check a variable voltage DC output (1-95V) and give a high voltage visual warning when voltage goes above 40 to 50V. With my little experience (I am a hobbyist) I thought it was within my skill level to design the circuit with the LM358, so I went like the this:



I used as reference voltage a 4.3V Zener and a 1:10 voltage divider on the variable voltage in order to make the alarm go at 43V. The blue led is supposed to be on when the voltage is under 43V and the red one will be on when the voltage is over 43V.

All works fine, except that when the red led lights up, the blue only dims (instead of turning completely off). I measured the opamp output and it gives 1.3V when it was supposed to go low.

Any clues as to how to fix that?

 

[Arouse1973]

Put a pull down resistor at the base of the BC548 down to 0 Volts. If you have another 68K try that, if that doesn't work you will have to lower the value of the pull down until the LED is off. It's always better to use a dedicated comparator to do this sort of thing. Although OPAMP will work, it's not really the right component to use.
Adam

 

[iPanos]

Thanks a lot Adam, and appreciate the advice too.
68K was too big, it worked great with 20K.
Thanks again!

 

[cjdelphi]

Says 35v so are you powering this from the 1k/zener ?

 

[Arouse1973]

Thanks a lot Adam, and appreciate the advice too.
68K was too big, it worked great with 20K.
Thanks again!

Ok no probs.
Adam