Hello,
I have the following circuit which utilizes an op amp to go high once the voltage has breached a 12Vdc reference voltage.

I have 2 sources for Vin. One source is an 8.2 Vp-p square wave as seen below. The other source is battery voltage of 14.X volts.

When I test this circuit on my bench it works as it should. However, when I take it to my car and connect it, the op amp doesn't respond properly.

Explanation:
When a 14.X Vdc signal is set as Vin, I can adjust the pot to have the output of the op amp trigger and turn on the transistor, activating the relay. When I switch Vin to a square wave (I have a switch to do this not seen in the schematic), the voltage on Pin 3 of the op amp drops to 8.2Vdc square wave, but the op amp will not change state. Although the 8.2 Vp-p signal is lower than the 12 Vdc reference, the op amp seems to stay in a "no-man's" land and will not switch to output low.

The other strange thing is when I adjust the pot to change state on the op amp, the measurement at Pin 3 is approx. 12.1 Vdc - this is fine... however, when I switch to the square wave input, I can only make the op amp change state if I tweak the pot... and I mean by just a hair. It seems the pot adjustment is at or near the same level whether Vin is 14.X Vdc or 8.2 Vdc square wave. You would think that the difference in voltage would mean there is a greater difference in the pot range.

Once again, the circuit works 100% on my test bench, but all laws of electronics seem to go out the window when I connect this to my car.

What might I be overlooking?

Thanks for any replies.

 

Do you have the square wave generator ground connected to circuit (chassis) ground?
Dees the square wave swing from 0V to 8.2V, or -4.1V to +4.1V, or ???

 

Do you have the square wave generator ground connected to circuit (chassis) ground?
Dees the square wave swing from 0V to 8.2V, or -4.1V to +4.1V, or ???

My first thought was that the square wave was floating so I tied the S.W. ground to circuit ground with no difference.
After reading your reply, I am now wondering if the S.W. ground I tied to circuit ground is actually the proper ground.

Essentially, the square wave is coming from the PWM signal from the vehicle's daytime running light circuit. I am tapping the signal coming from the headlight wiring harness and up until now I have made the assumption that the Headlight harness ground is also the PWM ground.

This is probably my problem. If the PWM is truly floating, then I have to figure out how to ground one side of the PWM without causing any damage to the vehicle's wiring system.

When scoping the PWM (with reference to circuit ground) it measures 8.2p-p. The bottom of the square wave appears to be at circuit ground level which is what has thrown me off the track so far.

 

If you want to pursue this, you should post the question on one of the ETO forums. The terms of service here will cause this thread to be locked.

 

If you want to pursue this, you should post the question on one of the ETO forums. The terms of service here will cause this thread to be locked.

Agreed. Thank you. I missed the Automotive mods when I skimmed the TOS earlier.

 

What Ron said. Automotive modifications is a restricted topic here in AAC.

Here are some alternative options: http://forum.allaboutcircuits.com/showthread.php?t=54400