DMM Piggyback (add RS-232) using PIC16F688 microcontroller

There are mods that give a multimeter serial connectivity by soldering some pins but, it only works if it already has RS-232 ability. So I decided rather than building a PIC multimeter from scratch, I would Piggyback off a $4 DMM.

I used:
one PIC16F688
one LM358
a button and a 100k potentiometer
3 resistors (1k 10k 20k)
A PCB, piece of thin plastic, and hardware
Despite the low part count assembly and programing can be a little troublesome.

This mod uses all the Voltage, Current, Resistance, and hFE circuitry of the original DMM.
Since the meter is relatively cheap it has bed of nails test points (found on electronics with a "blob" IC)
These points are a circuit benders dream.
I just pulled the input to the original chip (I will call the golden pin) amped it, ran it through the PIC, and sent it through serial. I also used a potentiometer to encode the selector dial.
The golden pin is the middle pin of the left side (circled in red)
DO NOT SOLDER TO THE LARGE PAD EVEN THOUGH ITS CONNECTED TO THE PIN THOUGH A CAPACITOR

Power is supplied by the power pin to the original IC (on bottom 4th from the right) and the common lead (banana jack). It gives 6V and uses the built in power switch.
The op amp is set up as 11x voltage amp from the golden pin. I used an amp because the pin only gives 200mV on full DC voltage signal. The amp feeds AN0 of the pic. The pot goes to AN7 no amp needed and hooks to the same power as the PIC and OP AMP

USE ONLY LM358 OR EQUIVALENT. LM741 DOES NOT WORK!

This was probably the hardest part to figure out, and my solution is far from perfect.I used a PCB mount micro pot (1.69 at radio shack) and curled the leads around to mount it. (this also makes it continues rotation)

To mount it cut a thin plastic sheet (like the DMM's container) to fit to the screw holes.
then cut a hole for the pot (make it a tight fit) I also cut a hole for the diode, lead board solder, and power leads
take out the screws slip the pot in the plastic and screw it down.(I pre drilled screw holes)

I used  a screw and cut the head flat(see last pic keep green part) to couple the dial and pot together.
The screw fits nice in the hollow center of the dial.
The flat head goes into the flat-head screw hole on the pot.
I suggest using red lock tight because if the screw losens the encoder stops working.

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