Connect with us

Read digital multimeter via parallel port?

Discussion in 'Electronic Design' started by Si Ballenger, Dec 20, 2003.

Scroll to continue with content
  1. Si Ballenger

    Si Ballenger Guest

    Is there a "simple" way to read a digital multimeter readout via
    the parallel port? I'm wondering if the below harbor freight
    multimeter (below link) can be hacked to get the reading from the
    LCD display. Being as cheap as they are (I've got a couple and
    they actually work), I'd like to see if they could be turned into
    a data supplying gizmo to a pc. I notice that each character in
    the LCD looks like it would be seven seperate on/off circuits,
    and each probably could be read seperately with the appropriate
    interface. Is there a simple amp type chip that could take the
    on/off voltage/signal supplied to the LCD and output to control
    an npn transistor on/off (I suspect the current to the LCD is
    very small)? The parallel port is probably pretty heavy duty ma
    wise compared to the chip in the multimeter. I've made me a
    little gizmo using diodes, resistors, transistors, and qbasic
    that reads external contact status using the parallel port
    (bottom link), and would like to expand this to be able to get
    actual data from the multimeter. Thanks fo any info!
  2. I did something like this with a temp sensor. Basically, clipped onto
    the LCD DIP, latched the output (the LCD ON vs. OFF driven by phase of
    clock, so latching on known phase gives you ON or OFF for each segment)
    and then read the results through the parallel port. The clock to the
    LCD was very slow (something like 50 Hz IIRC) so any latch would do. I
    used a bunch of wire-wrapped latches pulled out of the garbage bin at
    the school lab. If you have too many LCD segments, you may have to get
    fancy and "serialize," but in fact some segments are redundant (I know
    this because I had an open somewhere, and instead of finding it modified
    the software to ignore the missing segment).

    An easy serialization may be to read one 7 segment chunk at a time, and
    then clock (via the parallel port) the next data, etc.

    It was a mess, and people laughed, but while they were laughing I was at
    home sleeping while my experiments on high-temp CMOS were running.

    Jackson Harvey
  3. CFoley1064

    CFoley1064 Guest

    Is there a "simple" way to read a digital multimeter readout via
    Not really. LCD displays are multiplexed/triplexed to turn on individual LCD
    segments on the low end displays (necessary because a DC signal to a segment
    will burn it out). It would be a major pain to try to read these pins and do

    Another pitfall to make your day is that the meter is battery operated and
    floating. The digital chip common is always relative to the input measurement.
    Your PC, and the parallel port, is relative to ground. That gives you the
    serious potential of ground loops which could affect your measurement, and as
    an extra added bonus, could fry your parallel port and/or power supply.

    A good question, but if there were an easy answer, believe me, everyone would
    be doing it. Hacking a $7 DMM Q&D would be a cool trick.

    Low end DMMs usually update their reading/display only a couple of times a
    second, which should be plenty fast for a serial port.

    Even if you're costing your time at a buck an hour, the Radio Shack DMM with
    serial port interface (not optoisolated) is a much better deal, and has
    included software -- not very useful in itself, but you can always write your

    There are any number of relatively inexpensive ways to read analog data into a
    PC. One you might try (I've heard good things about it, but haven't played
    with it) is the GP-3 PC I/O Board Kit. For $40 USD you get 8 independent I/O
    lines, 5 independent 10-bit A/D conersion pins, a PWM output, and other good
    stuff. This PIC-based project will leave you enough spare change to buy a
    couple of optocouplers and protect your PC from the above problems. Also
    included is a disk with programming examples in Visual Basic and other
    programming languages (Qbasic isn't covered, but you should be able to cobble
    something together by looking at the VB code and the manual).

    Good luck.
  4. Si Ballenger

    Si Ballenger Guest

    Thanks for the info on the GP-3 kit. At $40 it is more affordable
    than most of the current IO gizmos out and about. I'll have to
    look at the LCD burnout issue and see if it applys to these cheap
    multimeters and how the LCD is connected to the main chip.
    Ideally the output from the multimeter chip lines (probably3 of
    them) that go out to each LCD display segment would be where the
    digital info would be obtained. 000 thru 111 should be fairly
    easy to handle. With the GP-3 kit I'll have to look at their
    documentation closely on how their activex application
    communicaqting with the serial port is implemented. Almost in
    every instance of similar serial port gizmos, the use of the
    activex components requires you to have the mscomm object
    registered on the machine (aka $$$$). Qbasic can't communicate
    with the serial port on win XP from my experience. Thanks again
    for the info.
  5. Si Ballenger

    Si Ballenger Guest

    The LCD operation is a grey area for me, so I'll have to do more
    looking to see just how they are operated by the main chip. I
    guess the trick will be to duplicate your latch/buffer setup
    using some type of easy to get chip to be able to read the input
    lines to the LCD segments. Do you know of a chip that could do
    this? One that a high input would result in a high output and
    that could supply at least one ma current to operate a transistor
    would be nice.
    Doing something similar, I've made a multicam webcam video
    switcher controlled via the parallel port using the 74HTC259
    chip. In this I use a seperate data line to power each chip, and
    three common data lines that go to all the 259 chips. When I want
    a particular chip in service, I just power up that chip while the
    rest stay deenergized. Maybe the same type thing can be done to
    the latches/buffers that read the data lines going to the LCD
    segments. My contact status gizmo does this type of thing by
    having a data line energize four lines going through contacts
    that return back to four status lines. The status is read, then
    that data line is deenergized and the next data line is energized
    going through another set of four lines with contacts, which are
    connected to the same status lines. The status is then read
    again, yada, yada, yada. Total of 3 data lines to check 12
    This whole thing comes from somebody that wants a way to monitor
    changes in air flow in his home heating system. My idea is to get
    one of the little hand held personal fans at the dollar store and
    mount it in the duct behind the furnace filter. The air flow
    spins the fan blades and the multimeter is attached to the little
    motor wires. The voltage output generated by the little spinning
    fan motor can be read by the multimeter and tracked for changes.
    Then the $64K question appeared "how do I get this data into the
    computer?". Hmmmmm.... (squinting at the little $4 yellow analog
    to digital conversion box and rubbing chin).
  6. Fred Bloggs

    Fred Bloggs Guest

    That would be a cheap buy-in and a lot of headache thereafter. Here is a
    more straightforward way- power it off the serial control lines:
  7. Si Ballenger

    Si Ballenger Guest

    I'd be interesting on knowing what the headache part will be and
    see if there is a workaround for it. I've tinkered some with the
    serial port a little in the past, but with XP machines it seems
    that VB 6.0 professional ($$$$) is required to write any programs
    that can interface with the port. Also, my experience with
    programming AVRs and PICs has not been very successful. Right now
    I think the hard part may be getting the interface that can read
    the output from the multimeter chip to the LCD segments.
  8. mike

    mike Guest

    Don't do this.
    A slip of the probe and you've busted your computer.
    Radio Shack sells (sold at one time) a dmm with serial interface
    and software. It's isolated so you don't blow anything up.
    Seems like mine was under $50.

    Return address is VALID.
    Bunch of stuff For Sale and Wanted at the link below.
    Toshiba & Compaq LiIon Batteries, Test Equipment
    Honda CB-125S $800 in PDX
    TEK Sampling Sweep Plugin and RM564
    Tek 2465 $800, ham radio, 30pS pulser
    Tektronix Concept Books, spot welding head...
  9. Si Ballenger

    Si Ballenger Guest

    There have been some good comments on this, in particular
    connecting the multimeter to something in common with the PC. I
    can see that sticking the multimeter probes into the wall outlet
    to measure the voltage could cause a suprise depending on the
    connected wiring. I'll have to stick to isolated components and
    low voltage DC circuits to stay safe. I'd like to be able to do
    the same to read the display on the cheap battery operated tire
    pressure gages, digital thermometers, and other digital gizmos.
  10. mike

    mike Guest

    Doesn't make much sense to read a tire gauge unless it's portable
    --> laptop parallel port.
    Shall I set up a pool on how long it takes you to blow up your laptop?
    $1 a chance. Post again when it's blown so I can tell who wins.

    If you're serious about this, use a pic, or other similar processor
    (I have to say that cause the AVR guys go ballistic if I don't)
    to read the display and use isolated serial port. It's also easier
    to program the serial port on your laptop.


    Return address is VALID.
    Bunch of stuff For Sale and Wanted at the link below.
    Toshiba & Compaq LiIon Batteries, Test Equipment
    Honda CB-125S $800 in PDX
    TEK Sampling Sweep Plugin and RM564
    Tek 2465 $800, ham radio, 30pS pulser
    Tektronix Concept Books, spot welding head...
  11. Si Ballenger

    Si Ballenger Guest

    Don't limit yourself with your assumptions. I don't have a lap
    top, and I won't be using the pressure sensor to measure tire
    pressure. If you want to bet, I'll bet you are smarter than the
    "blowup pool" makes you seem. Do I win or lose??? ;-)
    It seems the issues would be the same weather using an AVR/PIC
    chip, or the chip that is in the cheap multimeter (I've got AVR
    ISP and Futurlec chip programmers if I need to go that way).
    Circuit isolation or preventing catistrophic current loops is a
    prime key. I'll have to find out if I can actually read the data
    lines feeding an LCD segment first before getting into the
    "blowup" territory.
  12. Fred Bloggs

    Fred Bloggs Guest

    The headache will be attaching leads to the nearly inaccessible LCD
    inputs and decoding the display- that cheap meter may have the lines
    globbed with epoxy. The Visual XXXX programs are just a bunch of
    unnecessary glitz. Use the Rentron single PIC solution, maybe an add-on
    LM324 to rectify and condition your fan voltage for the PIC, and
    periodically, or when reading changes significantly, send an ASCII
    string for time-of-day/date and voltage reading/ or conversion to
    feet-per-minute or whatever, and the COM port can be read by a dumb
    Hyperterminal program running in the background- there are plenty of
    freeware programs that integrate Hyperterminal into a data logger. And
    like I say- power the PIC from the RS-232 control lines- you can fit the
    whole thing in one of those $2.50 Radio Shack boxes with experimenter
    board included- use a DB-9 or RJ connector to suit- you can run this for
    100's of feet at 1200 baud , or less, on cheap plenum cable-4
    conductor,-everything is sealed up and solid construction-may have to
    put distance between your box and a big blower motor. There are a lot of
    people on this NG who are real energetic and full of ideas as long as
    some else is doing the work- with little capacity for thinking anything
  13. I have had great luck with using Microchip PICs. Many of them have
    mulitplexed (mutli-channel) 10-bit A/D converters, on-board
    oscillators and serial ports - for under $3. The development tools
    are free. The only downside is that programmers cost in the $50 range.
  14. Si Ballenger

    Si Ballenger Guest

    I've got some 16F84 PIC chips and am going to try one of the
    below simple programmers to see if they actually work. I've tried
    the simple parallel port programmers with AVR chips, but didn't
    have any luck. These use the serial port and may work better.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day