I am building a very simple thermometer circuit on breadboard, based around the ICL7107. The 7-segment LED display I am using is made up of 3x TOD5261BE-N displays.

My intention was to make something nice and easy, just as an example circuit to show some kids at a local fair to try and get some interest in electronics. I have built circuits based on the ICL7107 and made a PCB for them and they work fine. Now I have built it on breadboard (I do not have time to design a PCB and order it/build it before the fair) and it will not illuminate the 7-segment LED displays.

The circuit is pretty much this:

enter image description here

The only difference is my input comes from the TMP35GT9Z which is similar to the LM35, and I am using a 0-5V supply. The V-, RefLo, InLo and Comm pins are tied to GND.

The oscillator pins are using a 100k resistor and a 10nF capacitor.

I have measured 100mV at he RefHi pin, and 212mV at InHi. Unfortunately, I do not have an oscilloscope at home so cannot measure the frequency of the oscillator pins.

Unfortunately, my laptop won't upload the pictures of my breadboard setup, or the pictures of me taking the measurements, so you'll have to take my word for the wiring being correct and the measurements accurate. I have double and triple checked the pinout and my wiring.

Is there some strange phenomena that means this IC won't work on breadboard or something? I have changed the oscillator components, tried a different IC and even tied the InHi to GND to try and force an output of all 0's but still the LED displays show nothing. If anyone can offer assistance, that would be great.

If pictures are needed, I can upload some tomorrow.


Now able to upload pictures! First one is the breadboard set-up (I tried to keep it neat, so apologies for the mess of it! I'll tidy it up once its working!):

enter image description here

The next image is when I have turned it on (The last 2 digits are connected directly to GND via 470 Ohm resistors. This does 2 things. Firstly, proves the displays are working and secondly it shows it is a Thermometer as it will read °C:

enter image description here

With this, I find it strange that the -1 is being displayed, which are connected to pins 19 and 20.

These are pictures taken of measurements (directly on the IC pin). The first os of the reference (100mV) and the second is the actual input (212mV)

enter image description here

enter image description here

Just in case it was the clock line (as that is the only thing I cannot measure as I don't have a scope at home) I made a little 555 timer circuit and put it straight on pin 40 which I believe would be correct if you had an external clock. This still didn't do anything though.

  • 2
    \$\begingroup\$ "Is there some strange phenomena that means this IC won't work on breadboard or something?" - if you are sure the circuit is correct then perhaps the problem is the breadboard! Check every connection for continuity, even the ones you think must be OK. \$\endgroup\$ Oct 2, 2019 at 0:19
  • \$\begingroup\$ Breadboards tend to be very unreliable. I once spent 6 hours trying to figured out why half of the channels on an I2C GPIO expander weren't working, only to discover that some of the IC pins had managed to wrap partially around the IC strip without touching the contacts. \$\endgroup\$ Oct 2, 2019 at 2:28
  • \$\begingroup\$ @BruceAbbott I did think it could be the breadboard. But I have checked for continuity, and took my measurements at the pins of the IC. If it were just a few pins, I would still expect at least some segments to light up when the output is set to 000 \$\endgroup\$
    – MCG
    Oct 2, 2019 at 7:38
  • \$\begingroup\$ @CalebReister indeed, I have known breadboards to be unreliable also. But I took my measurements from the IC pins themselves, rather than somewhere else on the strip. It is also a brand new breadboard (I bought a few especially for this as I have a few other working circuits on them for demo) and the holes are still good tight fits \$\endgroup\$
    – MCG
    Oct 2, 2019 at 7:40
  • 1
    \$\begingroup\$ @SillyInventor unfortunately not. I stripped down and rebuilt the entire thing and it did work eventually, but I am still unsure as to what exactly it was \$\endgroup\$
    – MCG
    Jun 29, 2020 at 7:33

1 Answer 1


I can't tell for certain, but it looks like you have IN-HI connected to the +5V line (red wire) which matches up to the schematic, and the COMM pin (32) is going to the ground side of the supply. The COMM on these chips floats below the positive supply, and tying it down to the negative side will put the amplifier out of its common mode range. What that will do to the reading obtained isn't clear.

The GND pin of the 7107 also isn't supposed to be connected to the ground side of the supply - the 7107 is supposed to have a split supply, but you can use a single supply if you generate the ground level with a regulator - I'm guessing that's what you're doing there, since the charge pump approach in the schematic isn't on the board. Then you can connect the 9V between V+ and V- directly, and use the +5V regulator to feed the GND pin.

enter image description here

I fount this one here

What does happen when these go over-range is also not spelled out well, but there's a line in the datasheet -

This is because over-range is a low dissipation mode, with the three least significant digits blanked.

page 7 of the datasheet

so that may be why you are seeing what you do on the display.

There's a test function built in - pull the TEST pin high and all the segments will light up. First I'd suggest that you get the supply sorted out so that the input isn't out of its common range.

  • \$\begingroup\$ Thank you for the answer. The IN-HI pin is definitely connected to the output of my temperature sensor, which is where the 212mV was read. Unfortunately, I only have a breadboard wire kit, which has pre-cut wires so I wasn't able to colour co-ordinate as well as I would have liked. I'll be getting some wire soon though! It has been a while since I have done breadboard stuff! You are correct that I have COMM tied to GND, along with the GND pin, REF-LO, and IN-LO. I can't make out what is on the V- pin.... I'll check when I am home from work. I'll check the TEST function too. \$\endgroup\$
    – MCG
    Oct 9, 2019 at 7:15
  • \$\begingroup\$ +1 for the answer so far, I will need to check a few things though. I thought a 0-5V should work fine though. Your datasheet is a bit easier to read than the one I used, which was from maxim, and has a bit more information, so I think I'll follow that one now! Interesting about the over-range thing.... I have updated the schematic in the question to clear up confusion. Found one online that is pretty much exactly the same one I came up with \$\endgroup\$
    – MCG
    Oct 9, 2019 at 7:30
  • \$\begingroup\$ Microchip's datasheet says that TEST works only on 7106. \$\endgroup\$ Oct 9, 2019 at 11:29
  • \$\begingroup\$ @thebusybee damn, ow many datasheets are there for this? Lol. I can only assume there isn't much difference between the TC7107 and the ICL7107. A quick read through both datasheets looks pretty much the same \$\endgroup\$
    – MCG
    Oct 9, 2019 at 12:30
  • \$\begingroup\$ It's a well-copied product. I'm amazed that you can still buy a device that I used to build a multimeter around in 1984. Still works, though. \$\endgroup\$
    – Phil G
    Oct 9, 2019 at 13:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.