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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.
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Signing UpStep 1The Theory
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
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
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I have been wanting to get something like this for a long time. This is excellent.
I use it on everything I make (after I fried my first com adapter)
Running positive to Data Carrier Detect Would power the isolator
(did not show for people that strait connected it to com)
01. National Instruments Literature, (1), and software, (2), do not agree with the Pin assignments used in the Schematic.
LM358 Pins (as per the Files listed below):
1 - Output of OpAmp 1, of 2
2 - Inverting Input of OpAmp 1, of 2
3 - Non-Inverting Input of OpAmp 1, of 2
4 - Ground
5 - Non-Inverting Input of OpAmp 2, of 2
6 - Inverting Input of OpAmp 2, of 2
7 - Output of OpAmp 2, of 2
8 - Vcc
02. That the LM358, in the Article's Schematic, is not configured as a 10X non-inverting amplifier. Instead, it is wired as a Rail to Rail (~Vcc to Ground) zero Voltage cross over Detector (Polarity Switcher).
To be a 10X Non-Inverting Amplifier - the 10 KΩ Resistor should be connected between the Output Pin, Pin 1, and Inverting Input Pin, Pin 2, of the LM358; and not to Ground.
For a true 10X Gain the 1KΩ Resistor should be 910Ω, and the 10KΩ Resistor should be 8.2KΩ.
G = ( Rf / Ri ) + 1 = ( 8.2KΩ / 910Ω ) + 1 = 10.01.
---------------
(1) - , ,
and .
(2) - National Instruments 'NI Multisim 11.0'.
And i always mess up my schematics
(1) - LM358 Specification Sheets: http://www.national.com/ds/LM/LM158.pdf and http://www.farnell.com/datasheets/3634.pdf
(2) - National Instruments 'NI Multisim 11.0'. - http://www.ni.com/multisim/
I noticed most of the top and right side went to the elastomeric connector for the LCD so I ignored them.
I set up a potentiometer as a variable voltage supply and tested voltages on the remaining pins.
Then changed the voltage and tested again. I found a pin that changed so I held my meter on it and adjusted the pot.
Sure enough the voltage on that pin changed in proportion to the pot’s, locating the “golden pin”.
I have a few of these "La Fonera" routers which have RS-232 headers. I am considering taking this mod to the next level and connecting it to the router to have a wifi meter>
Ohh! Idea: USB 2.0 Support! But on the comp side things might be a bit more difficult to get to..