For work I used to have to read and log 7-segment LCD numbers off this tiny (3cm x 1cm) screen tens of times per day. Now I have a webcam setup that reads it for me and does some OCR to automatically log the numbers. However the current setup stands out, and I want to make something more inconspicuous.

My idea is to replace the webcam with a close-up sensor that can be mounted less than a centimeter away from the LCD screen. Then I can put it all in a tiny enclosure with an LED, and make it look like a little magic matchbox.

The problem is I have no idea what this kind of component would be called. I've searched for lots of things like "low resolution photo sensor" but found nothing remotely like what I'm looking for. How can I find something like this? I am willing to do electronic component assembly, programming, pretty much anything necessary to keep me from being forced to read those damn numbers again.

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    \$\begingroup\$ Why don't you just hook your circuit into the LCD and read the data directly? \$\endgroup\$
    – pipe
    Feb 24, 2016 at 1:18
  • \$\begingroup\$ That would likely be your easiest approach. Tap into the comm/seg leads that are running the display, then translate the signals directly \$\endgroup\$ Feb 24, 2016 at 1:49
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    \$\begingroup\$ Although I agree with others that there may be a better way, the sensors they use in optical mice sound like the sort of thing you're describing. Have you searched for "optical mouse sensor"? \$\endgroup\$ Feb 24, 2016 at 5:41
  • \$\begingroup\$ @pipe Yes I have thought of that. Unfortunately the LCD and circuitry are in a tamper-resistant case. I could try to open it but it might get me in a lot of trouble, and I'd probably end up failing anyway and disabling the whole unit. \$\endgroup\$ Feb 24, 2016 at 13:29
  • \$\begingroup\$ @RogerRowland That's a great idea, I love using up old spare parts. I will look into that! Thanks a lot. \$\endgroup\$ Feb 24, 2016 at 13:30

2 Answers 2


You have two choices, one with lot of pixels (because more pixels is all the rage these days) and one pixel. You can get photo diodes or photo transistors that you would have to place very close to the lcd on each segment. You already have a sensor with many pixels. If you go to the one pixel version, you will probably place it so close to the screen you won't trigger the sensor unless the LCD is backlit.

  • \$\begingroup\$ I see, so I'm probably looking for photo diodes/transistors. I'll take a look for those, thank you. Unfortunately the LCD is not backlit so I might have to get creative with lighting. \$\endgroup\$ Feb 24, 2016 at 13:41
  • \$\begingroup\$ I suppose you could use a white led somewhere on the board. \$\endgroup\$
    – Voltage Spike
    Feb 24, 2016 at 18:40

If you're seriously willing to do "whatever you must" in order to avoid squinting @ those numbers...

  1. If possible, tap into the comm/seg leads from each digit on your lcd, then translate that output directly. (this would likely be the easiest route)

  2. Should that not be successful, you could diy a circuit with a phototransistor aligned over each segment of the display.

    • Feed the output of each such transistor to a capacitor to 'store' the data so you can 'read' all segments from the attached capacitors simultaneously.
    • Add a few LEDs in-box to ensure full illumination of your transistors/sensors, for accurate black/clear segment reads.
    • Now, you can build a circuit to 'read' the displayed numerals using basically the same code already used to display numbers on 7-seg. displays.

I know that option #2 isn't exactly child's play, but it is at least feasible.

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    \$\begingroup\$ Just repeating what I just now put in a comment above, unfortunately the LCD and circuitry are in a tamper-resistant case, which means I can't feasibly access them. Thanks for the keyword "phototransistor" though, I did not know that was a thing, and looks like just what I was looking for. FYI I did some diagramming last night and only 5 out of 7 segments are needed to uniquely distinguish the 10 digits (4 would be an optimal encoding). \$\endgroup\$ Feb 24, 2016 at 13:36
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    \$\begingroup\$ Here's one that looks promising for you: digikey.com/product-detail/en/PT15-21C%2FTR8/1080-1380-1-ND/… -- It has a small form-factor, decently low-cost and has a wide range of light that it'll accept, so if 1 LED doesn't work well with your lcd, you don't have to change out sensors too, just LEDs ;) \$\endgroup\$ Feb 24, 2016 at 20:28

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