Square up reads magnetic stripe cards through headphone jack on both Android and iOS. I want to capture fingerprints on an Android phone. Can I develop a hardware to capture fingerprints in the same way using a fingerprint scanner (UART/Serial)?

I want to develop such hardware (like square) which can be used to capture fingerprints through the headphone jack. If it possible?

The fingerprint scanners are available with UART communication protocol. So is there a way to send the fingerprint image through the headphone jack by developing such connector?

  • 3
    \$\begingroup\$ nice job re-posting. I think this fits pretty well. \$\endgroup\$
    – Kortuk
    May 19, 2011 at 14:00
  • \$\begingroup\$ Is bluetooth not an option? There are ready made UART-Bluetooth modules available. \$\endgroup\$
    – jippie
    Sep 18, 2013 at 19:08

2 Answers 2


It sounds like your question is, how do you go from a UART device to something that can be plugged into a microphone jack.

What protocol does a headphone jack use? should help you understand what is being done on the existing device that you are talking about.

There is still a link missing for you though, that is the ability of turning UART into the audio itself. The easiest way to do this is to buy a microcontroller that you will place between the UART device and the Android device. All the microcontroller will do is read in any UART data and then convert it to what ever form you want the headphone jack to receive. You may also need to implement some handshaking between the microcontroller and the phone in order to know that the device you think is plugged in.

There is also the issue of getting power to the device. A headphone jack is in no way designed to be used as a power source. The easiest method would be to just slap a battery on the device. If you wanted to, you could potentially do some clever tricks with playing audio at full volume for some period of time and have your device charge up a capacitor. You could then run your device off of the power stored in the capacitor. This sounds rather tricky to me though and would personally just go with a battery.

  • \$\begingroup\$ If one-way communication is the only requirement perhaps it could be simpler to use some FSK modulator (based on NE555 perhaps?) and demodulate in software? \$\endgroup\$ May 19, 2011 at 12:54
  • \$\begingroup\$ @Czajnik I have found that purely analog based methods tend to be harder to design and get working then digital. It is all where your comfort zone is though. \$\endgroup\$
    – Kellenjb
    May 19, 2011 at 13:05
  • \$\begingroup\$ @Kellenjb Same here, I'm primarily a software developer, feeling much more comfortable in the digital world, yet for this particular thing analog way looks tempting :) \$\endgroup\$ May 19, 2011 at 13:50
  • \$\begingroup\$ I would be a bit concerned about the availability of power. I suspect that the square needs far less current than an image sensor running fast enough to read fingerprint features. \$\endgroup\$
    – Toybuilder
    May 19, 2011 at 20:16
  • \$\begingroup\$ @Toybuilder the scanner I am planning to use consumes just 20mA of power (at 4.5V). Do you think this can be managed from the headphone jack or do I have to provide an external power source? \$\endgroup\$
    – Hitesh
    May 20, 2011 at 6:10

I want to develop such hardware (like square) which can be used to capture fingerprints through the headphone jack. If it possible?

Yes this is possible . The IR sensor or any other sensor will take analog data and you will need to process the data and send it over 3.5 jack to Iphone or Android

Regarding help on the device check this out http://web.eecs.umich.edu/~prabal/projects/hijack/


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