Tuesday, March 25, 2014

UNI-T UT61E RS232 serial interface cable to USB diy mod

I'm using my UT61E a lot. One great feature it has is that it can send data to pc for data logging. And it has interface cable included in the box with no additional cost. But the cable is RS232C compatible using DE-9 connector and +/-12V logic levels.
Do you have DE-9 connector on your laptop? I don't :) You obviously can use that RS232 to USD Adapter cables, but DIY guy can make his own cable, right? Add bluetooth too perhaps?
Let's take a closer look at the supplied RS232C cable:

From the first look it has only one working "eye". Which means there is no connection from PC to the meter, easier to hack then.

What's inside:

Definitely only one RX IR sensor diode used, TX circuitry is not populated.
Here is a schematics I reversed from the pcb:

This thing pulls up RX line when IR diode senses light from the meter. UT61E sends inverted serial signal to its IR led.
There is a bit of space inside this sensor box. And I need to fit a circuit inside, which senses IR light and pulls down RX line from 3.3V to Ground. This is classic UART serial interface.
Unsolder everything from the pcb except IR sensor diode.
This is new schematics using 2 n-channel mosfets, any general purporse mosfets will do. I used 2N7000.

R1 needs to be set lower than original because we are using 3.3V now and there is not enough juice to turn Q1 on fast enough with 4.7k.
OK, then I made this piece of art:
This fits perfectly inside original enclosure. New cable needs only 3 wires: +3.3V, GND and RX, I cut the rest. You may be wondering why I call it RX, well this is how DE-9 was wired. RX means PC side RX pin.

At this stage you already can connect UT61E to the microcontroller or Arduino or anything else which understands 3.3V UART. But I started it as USB mod, so we continue.
I have some CP2102 Serial UART TTL USB converters laying around and this is enough for this mod. Just connect that 3 wires and there we go.
Female header used so I can disconnect CP2102 converter and use it anywhere else.

Raw output from the meter:

Correct serial port settings for this meter are: speed 19200, bits 7, stop 1, parity odd.

That's it.
But, there is more! You can even add a Bluetooth feature to this UT61E multimeter. It's easy as soldering 6 wires to some serial bluetooth module with this mod.
UT61E with Bluetooth, sounds great! Interested?


  1. Nice work Anton!
    Just a question: in your circuit, the value for R3 seem too low. Maybe R3=4.7 k?
    Thank you for sharing!!

  2. very useful article, thanks!

  3. There is no need to rework the cable.
    Connect RTS to ground(Pin 5 and Pin 7), DTR to 5V or 3,3V and invert the output with 74hc14 and you have TTL output.
    This will work at 5V and 3.3V.

    1. This comment has been removed by the author.

  4. I am going to add bluetooth module to Uni-T UT61E to connect it to PC to virtual RS-232.
    I guess to install HM-10 bluetooth module due to low consumption, it is possible to supplay it from internal battery and it can be located internally (I am not sure does it have SPP and does it have to have it). Others need additional battery. Is it good idea?
    So, we do not need optical isolation with bluetooth module due to we will have bluetooth isolation. Can anybody say is it possible to connect bluetooth module to RX by other way?
    Please point the place on the ut61e diagram where in is possible to get RX signal to HM-10 bluetooth module. Or we need transistor invertor due to UT61E sends inverted serial signal to its IR led?

    1. You need to find a pad where power is available only when multimeter is turned on. If you connect bluetooth module directly to battery it will drain it even when multimeter is turned off.
      Also as you said, you need an invertor if you want to connect to ir led.

    2. I did backlit mod with APO, so with bluetooth module will be the same, its OK. http://www.eevblog.com/forum/chat/ut61e-auto-power-off-modification/msg863473/#msg863473
      I have an idea to connect BT module to pin 123 directly (see attached picture, point A) thru inverter (part of diagram - pic.2) with n-channel mosfet 2N7000.
      Is it right? See pictures:

  5. Спасибо за данную статью, очень помог! Могу отчитаться, работает на win 10 в родном приложении версии 4.01, без каких либо изменений в битности com порта. А вот на версии 2.0 (с офсайта, якобы последняя) программа вылетает на первой же секунде с ошибкой, хотя за эту секунду успевает показать правильные показания с мультиметра.

  6. Да, спасибо большое за статью!

  7. Hi,
    Could you give any advice on how to make this cable (I have ut70b - but I guess wiring is the same) work with poor quality usb2serial converters? I guess because they do not provide rs232 +/-12v (mine converter gives about 8v) UNI-T cable does not work at all. Would it be sufficient to change resistors? (I guess this 12v is a problem - since I do not own any motherboard with serial interface - and I can't check if it's why I can't log data from my UNI-T. Last time I've used data logging it was few years ago with real RS232 port)
    Regards - Adrian

    1. No, this cable as it is would not work with converters which not supply proper levels of +/-12v. That is why I've made this mod :) You can follow this article and use any TTL USB to Serial converter.

  8. Very great effort indeed. New cable needs only 3 wires: +3.3V, GND and RX, Cutting the rest. And well this is how DE-9 was wired. RX you means PC side RX pin.The whole thing looks pretty perfect. Thanks for the post.