Operating a PIC from a negative supply

[BobW]

I have a little project that involves measuring an analog signal that's negative relative to ground, and then displaying the result on some LEDs. The simplest solution, I think, would be to connect the PIC's VDD to ground and VSS to a –5 volt supply. Then the analog signal would fall within the range of the PIC's analog input. This isn't anything that needs surgical accuracy. Has anyone done this before? Anything that I need to beware of?

Obviously I'll be programming it out of circuit.

 

[ronsimpson]

As long as the LEDs are also working over the 0 to -5V supply.

 

[BobW]

Yeah, it's a multiplexed 4x4 grid of 16 LEDs operating from 8 output lines. So, the only thing that's really linked to the outside world is the analog input signal.

 

[ronsimpson]

0,+5V: In software 0=0 Volts and 255=5V
-5V, 0V: In software 255= 0V and 0=-5V

 

[Nigel Goodwin]

Everything is relative - it makes no difference to the circuit, just consider the negative rail as your ground reference if it makes it easier for you.

 

[BobW]

The only real issue is that I need to run it from a negative voltage regulator so that the positive side of the supply is at true ground potential. Aaaannnd... I suddenly realized that I forgot to order a negative regulator in my parts shipment that arrived today. Arrrrgh! So, I've had to temporarily test things out using a positive supply, which is fine for bench testing. Anyway, I got the whole thing breadboarded and working. I was a bit concerned about the analog input section because I need an input impedance of many megohms. The input signal actually goes down to around –20 volts. I built a source follower with a P-channel JFET, with a 20 Meg / 4.7 meg voltage divider at the input, and it works like a charm.

Now to find a negative regulator.

 

[ChrisP58]

If the power feeding the regulator is floating, you can use a positive regulator and ground it's +5V ouput pin.

 

[ronsimpson]

I almost said: Zener + resistor, then I remembered you have LEDs to light. So 7905!

 

[BobW]

Yes, I've used the zener + resistor method in the past when the PIC wasn't driving any load, but the LEDs are the problem. And to answer Chris's comment, the power comes from a 6.3 V AC supply that has one side grounded. So the 7905 is the only option. Anyway, I did find a 7905 locally and everything is now working perfectly.

BTW, if anyone's interested, the point of this project was to make a solid state replacement for a 6E5 or 6U5 magic eye indicator tube, which was used as a tuning indicator in old radios. I found a circular LED bargraph display on Ebay that was the correct diameter. So that was the motivation for the project. The details are here: **broken link removed**

 

[ronsimpson]

I did find a 7905 locally and............

I spent many years in Alberta. 40+ years ago in a small town there was no locally.
Fill out a order, and a post office money order, put it in the post, wait a month.