Adding RS232 to a multimeter the hard way

You might want to store information from a multimeter to be graphed over time. This comes with pretty much all of the high-end professional models. But if you buy a super cheap meter you can bet this isn’t an option. [Jazzzzzz] has found a way to pull the data from a $4 meter via RS232. It’s not impossible, but we definitely think he’s doing it the hard way. That’s because he’s not just tapping into a dormant feature. He’s actually adding a microcontroller to sample the data and push it via the RS232 protocol.

On the bright side, this is easier than building a multimeter from scratch. The sampling circuits are still being used, with a PIC 16F688 intercepting the signals as they enter the stock microcontroller. The signal he was after comes into the chip on just one pin, but to get the readings right on the PIC he had to use an OpAmp. That’s only part of the puzzle as he also needed a way to tell what the selector switch was set at. In the end, adding a potentiometer and reading its value let him calculate the position.

[Thanks Karl]

Comments

  1. steaky says:

    Surely the hard way would be to reverse engineer the LCD display protocol and use that to get the data out

    • rasz says:

      you mean the easy way

      • saimhe says:

        LCDs are dynamically updated. The voltage between backplane and a particular segment periodically changes direction (Vbp=”1″, Vseg=”0″ is reversed to Vbp=”0″, Vseg=”1″ etc). Typical refresh rate is about 30 Hz. That, at least for me, makes sniffing LCD contents rather hard. On the other hand, I didn’t try that hard.. yet.

      • tim says:

        I have one of those meters sitting in pieces from my attempt at adding an atmega8 to it. Kudos to you. That function selector dial was what put that project on the I’ll probably never see it again back burner. I like how you thought your way around the problem with the potentiometer instead of shelving the project like me.

    • Orpheus says:

      Don’t laugh! I actually DID read the LED readouts from a digital VOM, hygrometer and a few other meters back in the 70s/80s. Back then, just having a digital display carried a sizable premium [I only dared mess with the innards of flea market finds], and units with a digital interface -if even available- cost as much as the computer I’d be recording with. It was easier then (low sample rates, simple common cathode multiplexing) so 74xx-like chips sufficed, but each was still a custom job. I’ve always intended to make a more general MCU-based solution, just to help me master MCUs, but it always seemed like it’d be obsolete before I even finished, and display multiplexing protocols seem more complex today — but I bet a decent MCU hacker could cook one up in a long weekend

    • AussieTech says:

      Elektor published an LCD DVM to RS-232 converter using mostly CMOS shift registers some time back, fully opto-isolated, self-clocked using a 4060, applicable to just about any type of display on anything. As well as the individual segments the backplane was also sensed and the logging computer was used to sort out which segments were in or out of phase.

    • Jazzzzzz says:

      I was originally going to use the LCD since it is not multiplexed

      but that WOULD have been the hard way

      When I compared the reading to my fluke it was only .01 v off

  2. mjrippe says:

    Radio Shack 22-812 DMM – I bought mine for $10 used. It not only has RS-232, but also does dBm, capacitance, Hz, logic, autoranging and more. Probably more accurate as well. Of course “Buy It” is Not A Hack ;-)

    • SuperNurd says:

      Every time I get anything from radioshack it eighter is already broken or brakes in a day, and the radioshack I live by has not updated and gotten the paralax gear.

      • DS says:

        Most of their self-branded gear is unreliable or dangerous.
        I bought a (RadioShack branded) soldering iron from RadioShack once.

        Within 30 seconds on being plugged in there was a faint stream of smoke coming from the handle were the cord goes in accompanied by the VERY familiar smell of burning wire insulation.

        Took it back to be swapped out with another.
        The replacement was on for no more than a minute when visible white smoke started coming out of the heating element.

        The only way it would burn up is if it was run at half power (series diode).

    • MikeK says:

      I have the 22-812. It’s OK. I wouldn’t brag about its capacitance measuring since it only goes up to 10uF. $10 is a great deal on it. For data logging I made it more useful by adding a jack for a wallwart. Now I can log hours and hours of data without buying new batteries.

  3. kmmankad says:

    RS232? So 1990.

  4. Be wary of those meters. I used one to check an outlet for voltage. Popped in my face. It was made so cheaply that the ground and test lines were touching inside the case.

  5. porkpie says:

    that’s what i would call a dirty hack.
    w/o galvanic isolation it’s definitely nothing for the faint of the heart.

  6. bothersaidpooh says:

    Bluetooth DMM anyone?

  7. Stu says:

    You don’t only pay more for cool features such as USB connectivity but also for accuracy, chances are the $4 cheapo model wont be particularly accurate to bother logging the values!

    • Haku says:

      One of my favourite multimeters is a Micronta slimline thing about 9mm thick and runs off 3x watch batteries, it’s surpisingly accurate for what it is and what it cost me (pennies 2nd hand), because quite often I just want to know if a voltage is present and approximately what it is (ie 3.3v, 5v, 12v) to determine wether something should be working.

      But when I want very accurate measurements I turn to my two Metrix MTX328x multimeters that originally cost a small fortune but I got em cheap, though I’ve yet to get their comms working, the opto-isolated lead is £60 (I don’t need to log data that badly yet), one of them has Bluetooth but I’ve not yet managed to get that working despite it showing up on the PC.

    • tim says:

      I see those meters every where including engineers benches and used as expedient panel meters. They would be hard to beat at twice the price.

  8. Haku says:

    If he’s taken that much trouble to get an RS232 port installed, it wouldn’t be much of a leap to get it Bluetooth enabled using those dirt cheap transceivers off eBay, which actually work suprisingly well for their price, get the right pair and you can also get the modules talking to each other rather than only communicating with a BT host (PC/Mac/smartphone).

  9. ftorama says:

    And what will happen if he tries to measure the wall socket voltage….say “goodbye” to your PC

  10. Agent420 says:

    I agree this was the hard way to accomplish the task, but cudos for getting it to work.

    Every dmm interface I have seen implements optoisolated signaling as a safety measure – you really don’t want your pc potentially connect to dangerous voltages or currents.

    And RS232 data continues to be a lot easier to access than usb. Any real geek appreciates RS232.

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