I was recently diagnosed with Type 1 diabetes. With the need to test my own blood upwards of 8 times a day I have a ton of data to keep track of. My testing unit has the ability to upload its data to a mobile (OTG) or PC (USB).

Sadly, the company who makes the unit does not ship either cable. They are an expensive accessory.

I have enough parts, and should be able to make this, however it's proving difficult.

The USB cable of theirs has windows drivers built into the cable. I was able to get a helpful person at the manufacturer to send me those. That person also told me that OTG wouldn't require drivers.

The photo of both cables on their website show the unit end of the cable as being a 2.5mm (3 ringed) audio jack.

So far I cut open a male USB, I also did the same with the end of an old 3.5mm headphone jack. I then purchased a 3.5mm to 2.5mm adapter. But I'm struggling with the wiring, and trial and error is not producing any results for OTG or USB. I'm also a little concerned that I'll damage my phone, laptop or blood glucose unit. Although all seems to be well after my tinkering.

  • My USB has Green (Data), White (Data), Red (5v), Black (Earth) and some sheilding by the looks of it.
  • My 3.5mm headphone cable has Red (Normally right audio), Yellow (Normally left audio) and White (Earth)
  • I also have a USB to OTG adapter

Note that my adapter has 4 rings, like an ipod shuffle cable or headphones with a mic. I'm assuming that's not introducing any crossed wires as using it for audio still delivers full stereo.

Here are some pics

Am I missing something in my approach? If not, how can I figure out which wires connect to which? I would be happy with either a working OTG or USB cable. My tests have shown no response in the mobile app, and only generated USB malfunction errors in Windows (as expected).

Edit: Thanks for all the really good info. I have contacted CareSens for a NFC unit. I will update here when I find out more. This was going to be a fun project during my time off, but it looks like it's a bit more complicated than I first thought.

  • \$\begingroup\$ How is the device powered? Do you have access to a known ground on the monitor? If so you should be able to determine which pin on the 2.5mm jack is ground (i assume you have a multimeter). I don't expect you'll need the +5v. Once you know ground, there are only two possibilities left for the other two wires (D+ and D-). \$\endgroup\$
    – RJR
    Commented Oct 23, 2014 at 8:31
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    \$\begingroup\$ By the way - did the support guys confirm the device's interface is actually USB and not serial (RS232)? It is possible the expensive cable contains a serial <--> USB interface in the plug. Otherwise you might use something like this: sparkfun.com/products/12977 \$\endgroup\$
    – RJR
    Commented Oct 23, 2014 at 9:32
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    \$\begingroup\$ If the cable "contains windows drivers" - did he mean it came bundled with a CD with the drivers on it, or you plug it in and it emulates a CD drive on the computer (as many mobile dongles do) for you to install the drivers from? If the latter, then there will be a circuit in the cable which will bridge between the computer and the monitor - a simple cable will not suffice. That could be why they're so expensive... \$\endgroup\$
    – Majenko
    Commented Oct 23, 2014 at 9:45
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    \$\begingroup\$ I bet those drivers are FTDI drivers - serial <--> USB interface. \$\endgroup\$
    – RJR
    Commented Oct 23, 2014 at 10:52
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    \$\begingroup\$ Look, you need to test 8 times a day, that's 250 times a month or so, so 250 test strips? Find a sales rep for the company and tell them you test that often, and if they can send you the cable for free (or give you the CareSens NFC model for wireless pairing with a NFC compatible cell phone) or you will switch to another company's meter. \$\endgroup\$
    – Passerby
    Commented Oct 23, 2014 at 14:19

3 Answers 3


Based on the manufacturer's youtube video for the app compatible with the cable:

This is a video guide for SmartLog(Blood Glucose Management Software) App represensted by i-SENS. SmartLog App is a smart phone application which helps patients with Diabetes to monitor their health conveniently anywhere anytime. This app works with CareSens N NFC meter. CareSens N and CareSens N POP meters can also be used when using FTDI cable.

A FTDI cable typically refers to a USB to RS232 (TTL level Serial) IC FT232 (or other generations of the FT232 chip) created by FTDI. They also make other USB bridge ICs with similar functions.

Connecting a USB cable straight through to a 2.5mm plug will most likely cause a problem. as you have already seen.

If it's a simple straight through connector, it will have Ground, TX and RX. FTDI's official 3.5MM cable uses Tx {To device from PC}, Rx {From Device to PC}, Gnd (Tip, Ring, Sleeve, respectfully). With your multimeter, you can confirm the 2.5mm pinout by doing a continuity test between each section of the adaptor and your 3.5mm cable wires, then confirm the signal by checking for voltage between the three wires. The voltage it runs at is a concern because using a 5v signal on a 3.3V port might be bad.

Of course it could be more complex. TI calculators used a 2.5mm port for their Graphlink cables. It was able to connect to a serial port, but required 6 pins, resistors and diodes between.

If you had a cable to hack up, or even a meter to hack up, it would be easier. They occasionally pretty much aways give the device away for free, check with your doctor or the manufacturer's local sales rep.

Update: Based on the two links below, the pinout is more likely to be Tx from Device to PC, Rx From Pc to Device, Ground (Tip, Ring, Sleeve). Like a defacto standard amongst Diabetes Meter manufacturers. You need the USB to serial IC for the OTG cable, but you could use a serial port for the PC instead (I am not liable if you fry your meter).

Reading data from a glucose meter

  • \$\begingroup\$ Put it this way, a while back, Bayer was PAYING people (combining coupons and sale price, I was making 5 dollars a box) to get meters in a retail store (walgreens). I had a large stack of Contour 2, Contour USB (acted like a flash drive and had a high quality usb charger), and Contour for Kids that had a Nintendo DS game. At the time, they all came with 20 test strips, which cost more than some of the meters. Seriously, DON'T PAY FOR A METER. \$\endgroup\$
    – Passerby
    Commented Oct 23, 2014 at 14:16

There is no standard conversion between a 2.5mm audio jack and USB. It's not like USB A and USB B, where these are simply different connectors doing the same thing. Audio jacks usually carry audio. USB is USB. These are different things. It makes about as much sense as looking for a 2.5mm audio jack to 110V wall outlet adapter.

While it's entirely possible that the meter's manufacturer simply used a 2.5mm audio jack for a USB compatible interface (maybe they hadn't heard of micro USB connectors?) this is only one of many possibilities. It could also be that the electrical protocol at the audio jack is a simple serial protocol, like RS-232. Or, it could actually be audio, somehow modulated to convey the relevant data.

Point being, the cable accessory likely has the necessary electronics to make this conversion. Unfortunately, without specifications from the manufacturer, or at least a working cable to reverse engineer, we can only guess at how it might work.


I have run into the same issue as I have also been diagnosed with diabetes. It becomes pretty clear after doing some research that there is some electronics involved. FTDI, for example, makes SSOP devices(less than 1/4 in. square) that contain a microcontroller and programmable memory that can make the conversion between usb protocol to a serial interface. In other words, it's not a matter of how to wire one end of the cable to the other; you need a cable that has the chip built into it that's been programmed to provide the interface between the usb port and the glucose meter.


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