Now trying to use TLV271IP.

It looks like a pretty benign sort of thing. Is there anything nasty on the input signal or its power supply?

 

Yes, your messy long wires all over the place are causing the opamp to oscillate at a high frequency and to pickup all kinds of interference.
With the circuit connected, please show the power supply voltage on the oscilloscope set to show 1V per division.

 

Yes, your messy long wires all over the place are causing the opamp to oscillate at a high frequency and to pickup all kinds of interference.
With the circuit connected, please show the power supply voltage on the oscilloscope set to show 1V per division.

Thank you for all your help. I will try to get back to it tomorrow.

 

If your electrolytic capacitor is for decoupling, it may be in the wrong place. Is it connected to pin 6 ? Because pin 7 is +V

Ioannis

 

Does it work with the flow-switch open? That 56 ohm resistor in the transistor base looks rather too low. The op-amp might be struggling to drive it (although it doesn't really explain why it oscillates)
How long is the cable to the flow switch? Op-amps really don't like driving long leads (That DOES make them oscillate). Put 100 ohm between op-amp output and cable to the flow-switch, and change the base resistor to something like 10k.

 

If your electrolytic capacitor is for decoupling, it may be in the wrong place. Is it connected to pin 6 ? Because pin 7 is +V

Ioannis

I was smoothing out the output but I'd rather avoid that component. I will put a cap on pin 7 next test. Thanks.

 

Does it work with the flow-switch open? That 56 ohm resistor in the transistor base looks rather too low. The op-amp might be struggling to drive it (although it doesn't really explain why it oscillates)
How long is the cable to the flow switch? Op-amps really don't like driving long leads (That DOES make them oscillate). Put 100 ohm between op-amp output and cable to the flow-switch, and change the base resistor to something like 10k.

I'll try that. I'm hoping to put the flow switch up away from the OpAmp in series with the relay coil. There is really no need to have it coming to the little PCB. I'll try your suggestions. Thank you.

 

You tried to smooth the output of the opamp with a capacitor.
Every opamp oscillates at a high frequency if a capacitor to ground is at its output. For the TLV271I, its datasheet shows in fig. 17 that it oscillates if the capacitor value is about 600pF or more. 600pF is only 0.0006uF.

 

You tried to smooth the output of the opamp with a capacitor.
Every opamp oscillates at a high frequency if a capacitor to ground is at its output. For the TLV271I, its datasheet shows in fig. 17 that it oscillates if the capacitor value is about 600pF or more. 600pF is only 0.0006uF.

That’s why it doesn’t like long cables on its output!

 

You tried to smooth the output of the opamp with a capacitor.
Every opamp oscillates at a high frequency if a capacitor to ground is at its output. For the TLV271I, its datasheet shows in fig. 17 that it oscillates if the capacitor value is about 600pF or more. 600pF is only 0.0006uF.

Oooops. I'll take that out. Thank you. Heading up now to experiment more.