Switching Temperature Sensors to Single Display Using PIC ADC and Digital Potentiometer Limitations

Hi,

I am looking for advice on how to design the following system, I've run out of ideas and don't really understand why my attempts didn't work. If you guys would be so kind as to chime in...

I have a temperature display with its sensor. I've measured both the voltage and amperage that is proportional to the temperature displayed. I used a potentiometer instead of the temperature sensor to do so. Valid values are displayed from 7.8 to 1.8 V and 0.1 to 2.3 mA. The goal is to be able to read in a second sensor and have the option to switch between sensors. To do so, I have a pic microcontroller reading in the sensors by ADC as the sensors have different scaling and I can't just switch between them.

I was hoping to control the display by sinking it into a digital potentiometer. The problem with this method is that voltages above 5.5v are cut off. Another problem is the size of the steps are too large leading it to be imprecise.

Next attemp was to run it through a FET driven by a PWM signal. This responded much better to varying the frequency on a square wave generator and displayed accurate results. With this method, the display couldn't reach the higher voltages, I think about 5.5v was also the problematic point. Beyond there, varying the frequency wouldn't adjust the display to show values that correspond to the 5.5 to 7.8v range.

Could you suggest a better way to go about this one?

Thank you,
Pete

Good day Pete.

Sorry, I'm not sure if I got this right but you have a PLC with 2 ADCs whose output is a binary digital signal chosen from either of those 2, whichever sensor was picked.
You also want to interface that binary signal into an LED display so that the temperature can be viewed. I don't understand why you'd need a pot. or FET though. Pull-up for the LEDs?

Regards,
Justin

Hi Justin,

The display is fully self contained and has it's scaling preset. It outputs a steady voltage and amperage. Depending on the temperature, the sensor which acts like a resistor varies it.

So in essence, I'm trying to "fake" the original sensor to be able to set the value on the display with an electronic signal.

Peter