This is a image of most of part of my circuit (555 timer IC is cut from right side)... enter image description here using the pot (RV1) , I change the non inverting input voltages of the op amps. when non inverting exceeds inverting voltages of the op amps (which is different for different op amps) is should output 5V which should take BJT at saturation. Then current will flow from pin 7 to 6 in timer IC. But as you can see from the picture, though non inverting voltage is higher then inverting voltages output op amp voltage is still low.

why this is happening ??

again if I increase non inverting voltage a little from this value, LED start to blink (555 timer start operating) but still op amp output remain low .

enter image description here

**why this is happening ?

what can I do to made op amp work properly ??**


  • \$\begingroup\$ What is the lm324 supply voltage? \$\endgroup\$
    – markrages
    Oct 19 '14 at 21:23
  • \$\begingroup\$ 5 volt supply .. \$\endgroup\$
    – Anklon
    Oct 19 '14 at 21:24
  • \$\begingroup\$ Your voltage probe on R4(2) shows 0.05 V. And the probe on the transistor emitters shows 3.8 V. Both these nets connect off the page, so we can't see why they are that way. Are you trying to use the transistors in reverse-active mode, or do you need to fix something with those supplies? \$\endgroup\$
    – The Photon
    Oct 19 '14 at 21:36
  • \$\begingroup\$ I changed the power supply of op amp to 4 volt which change stop this reverse active mode and made R4(2) voltage changing. But still the same problem... op apms behave same . \$\endgroup\$
    – Anklon
    Oct 19 '14 at 21:47

The LM324 has an input common mode range of the negative rail up to two volts below the positive rail.

So with a five volt supply, you need to make sure your signals are in the 0-3 volt range for proper operation.

In no case should you apply a signal beyond the supply rails to the opamp's input, like 6.1 volts into a 5 volt-powered opamp.


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