LM 3914

Thanks Mapj1 you`ve made it so simple. Guerss I was having a senior moment trying to complicate it.

All those years ago I did build one for the heating engineers to test gas boiler thermocouple voltage.

I had it to read from zero to good (say 30mV or some such figure) and with a switch to put a dummy load across the thermocouple to represent a gas valve solenoid.

Put magnet feet on so they could attach it to a boiler.


Even made a simple calibrater, 9v PP3 battery dropping the supply across a load resistor and zener diode in series. A preset across the zenner to give a ref output voltage for calibration. Once set using a multimeter the calibrater was left with the tester.


Fasten thermocouple output to croc clips , light boiler, observe bargraph display. Repeat with dummy load switched in. It gave a good indication of how the thermocouple was acting. Not only by voltage attained but rate of rise too. Gave a good visual indication.

I know they used it for some years. I think I`ll ask them if they still got it.


Note this was acting from 0V to full expected reading.

On my application it was the both high AND low references that got me fuddled.

Like I said - senior moment.


Thanks Mapj1 for your help, much appreciated. Thanks AJJ for your reply too.


Mapj1, I`m considering rplacing resistors with presets, I`ll calculate the ideal resistor values then subtitute with presets. That way I could tweek not only the LED current but the high and low parameters. In this application it`s to give an easy visual indication, some metres away, of the output signal from a photo electric external beam detector rather than using a multimeter. But I can envisage other uses too.


I knew my brethern would enlighten me. ?

Thanks Mapj1 you`ve made it so simple. Guerss I was having a senior moment trying to complicate it.

All those years ago I did build one for the heating engineers to test gas boiler thermocouple voltage.

I had it to read from zero to good (say 30mV or some such figure) and with a switch to put a dummy load across the thermocouple to represent a gas valve solenoid.

Put magnet feet on so they could attach it to a boiler.


Even made a simple calibrater, 9v PP3 battery dropping the supply across a load resistor and zener diode in series. A preset across the zenner to give a ref output voltage for calibration. Once set using a multimeter the calibrater was left with the tester.


Fasten thermocouple output to croc clips , light boiler, observe bargraph display. Repeat with dummy load switched in. It gave a good indication of how the thermocouple was acting. Not only by voltage attained but rate of rise too. Gave a good visual indication.

I know they used it for some years. I think I`ll ask them if they still got it.


Note this was acting from 0V to full expected reading.

On my application it was the both high AND low references that got me fuddled.

Like I said - senior moment.


Thanks Mapj1 for your help, much appreciated. Thanks AJJ for your reply too.


Mapj1, I`m considering rplacing resistors with presets, I`ll calculate the ideal resistor values then subtitute with presets. That way I could tweek not only the LED current but the high and low parameters. In this application it`s to give an easy visual indication, some metres away, of the output signal from a photo electric external beam detector rather than using a multimeter. But I can envisage other uses too.


I knew my brethern would enlighten me. ?