Picture of DMM Piggyback (add RS-232)

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.

Step 1: The Theory

Picture of The 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)
harayz6 months ago

i am about to modify my DMM and put CP2102 in it. If you (or anyone else) have any advice thought and/or suggestion, i would like to hear them. Thank you~

nicksteen1 year ago
Very nice to see this is is possible, but afaict, only the voltage is transmitted, right? Or am I missing something? Because I'd like to have all data (resistance, current, voltage, continuity etc) to be transmitted. Is this possible?
jazzzzzz (author)  nicksteen1 year ago
All of the data is able to be transmitted, the pot on the selector dial is there to differentiate the measurements. This way the raw and dial position data is sent. The dial selects the formula used to calculate voltage, amperage, resistance, ect. from the raw data.
Oh that's beautiful, so I can read everything from that single pin? Amazing, thanks a lot! And thanks for that incredibly fast answer
jazzzzzz (author)  nicksteen1 year ago
No problem, let me know if you need any help with anything else.
mario592 years ago
and *SIMPLE* datalogger Modification!
Congratulations! You could never imagine how many times I've thought about this kind of mod and never dare it!
Very nice hint to follow!!!
Bravo! ;-)
jazzzzzz (author) 2 years ago
The Firmware I have written is very simplistic. And is in basic (a free c compiler is a little challenging to find I might have found one and if I get it working will post some C code)

TX_VAL = 83
SerSend 1, TX_VAL
TX_VAL = ReadAD(AN0)
SerSend 1, TX_VAL
TX_VAL = ReadAD(AN2)
SerSend 1, TX_VAL
Wait 100 ms
Goto SampleLoop

After the initialization it is just an infinite sampling loop. The raw values are faster to convert on a computer, and both values are necessary regardless where conversion occurs. Sending the ‘S’ (83) is so the computer knows the next byte is AN0 or the knob.
umbk2 years ago
Very nice. I've been collecting these meters (free with coupon), thinking of making VU meters and maybe trying to add auto-scaling. I would appreciate your source code for interfacing with the chip and the knob.
zapro2 years ago
Cool project. I have one thing to add. The points around the Blob IC isn't test points . They are made for the "Real" IC the instrument is designed for. This way the manufacturer can use what's available on the market at the time of manufacture, a smart decision. Some times you can't get the parts, only the dies.
rharding642 years ago

I am M2M(machine to machine) design specialist on the job, This work is well done here. My congratulations to the author.

Myself, on my day job, I design/build smart terminal emulator applications for various types of devices, OEM test gear, microcontroller, ect.since 2006. I do use RS232 at my current job, but prefer to use D2XX MS .NET library in C# with bitbang i/o, and a mode called 'MPSSE'(Multi-protocol synchronous serial engine)- allows for JTAG, I2C to name a couple of protocols.

consider that many of today's circuits use many protocols like 1 Wire, I2C, JTAG, I2S, CAN, ECT, it becomes important to add these capabilities to as many of our existing toolsets as possible. i am working on designing such applications on both pic device and associated desktop application to provide intuitive, easy to use controls.

So to continue on my own build project i also visited the Hardware sales as well, and i got the bigger brother DMM, actually 2 of them; the CEN-TECH P37772. i am taking bigger steps here; actually taking these two DMMs and creating another DMM with much more built inside; solid state switches, latching relays, microcontroller/FTDI USB using D2XX library,, 200Msps 12 bit A/D converters, larger Color LCD as well.

if folks are interested learning more about my complete package approach, please contact me.


jazzzzzz (author)  rharding642 years ago
Sounds like you do not even need the CEN-TECH's. I would like to build one from scratch ( because it could do anything I think I might need). You should think about a wi-fi modual, that or Ethernet is my next step
jazzzzzz (author)  jazzzzzz2 years ago
kwilliams383 years ago

I have been wanting to get something like this for a long time. This is excellent.
fgugliuzza3 years ago
I strongly recommend optoisolating those RS232 lines... without doing so, if you connect the RS232 to a computer and the COM terminal to a high potential point, all sorts of bad things may happen.
jazzzzzz (author)  fgugliuzza3 years ago
I built an external optoisolator
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)
wstdffrts693 years ago
Please note:

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'.
jazzzzzz (author)  wstdffrts693 years ago
oops it is 11x I forgot the +1
And i always mess up my schematics
jazzzzzz (author)  jazzzzzz3 years ago
fixed them
For some reason the provided Web Site Links were not included in the posted Reply.

(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/
mattbeddow3 years ago
nice, but can i ask, how did you work out which pin on the ic you needed to tap into?
jazzzzzz (author)  mattbeddow3 years ago
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”.

Ok cool, seemes like as agood a way as any to find it :)
815turbo3 years ago
Hey, i have this meter, grabbed at harbor freight for around $5. Excellent mod. I have looked at it and began considering "What else could be done"?

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>
Nerdz3 years ago
Very Awesome Hack. Makes me want to buy one of those cheap meters on ebay and poke around inside.

Ohh! Idea: USB 2.0 Support! But on the comp side things might be a bit more difficult to get to..
jazzzzzz (author)  Nerdz3 years ago
I bought a PIC18F2455 for USB 2.0, I guess it acts as a serial port so the same program should work
c_madsen Nerdz3 years ago
Have a look at dealextreme.com and search for "USB to RS232 Serial Port Adapter". - There is your USB support.
jazzzzzz (author)  c_madsen3 years ago
Yeah I got a SIIG usb to serial converter $20
agis683 years ago
really nice hack....i will try it.....sometime
c_madsen3 years ago
Nice hack, we don’t see these to often.