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d1030180 23rd January 2013 08:15 PM

ICL7136 thermometer, LED display
 
4 Attachment(s)
I have an ICL7136 3-digit a/d converter chip and I would like use it as a thermometer. The datasheet features a suitable circuit for measuring temperature with a diode-connected npn transistor.

The problem is that the ICL7136 features outputs for driving a 3-character lcd screen, while I would like to use two or three 7-segment led displays (common anode) for displaying the temperature. Intersil's application note mentions that the IC outputs ~3.5V(rms) AC for driving the lcd.

Would it be possible to use three ULN2803 Darlington transistor arrays to drive led displays with this chip, like in the schematic?

Attachment #1 shows the circuit I drew with Eagle. It still needs a power supply and another three current-limit resistor networks...

gmphadte 24th January 2013 03:54 AM

The signals driving the LCD are multiplexed aound the BP output and your design will not work.

Gajanan Phadte

d1030180 24th January 2013 08:26 PM

Quote:

Originally Posted by gmphadte (Post 3339717)
The signals driving the LCD are multiplexed aound the BP output and your design will not work.

Gajanan Phadte

Thanks for your reply. I have realized that it might be very troublesome to interface an led display with lcd drive circuitry. I need to get an lcd display to use with this chip or use another a/d-converter/display driver IC that has 7-segment led output.

Elvee 25th January 2013 07:31 AM

If you can accept a display that is not very bright, there is a possibility: you can drive directly LEDs at low current.

To do that, you need a high brightness LED display, one that gives acceptable luminosity at 1mA, a common cathode one, you dimension the segment resistors to give 3 mA and you buffer the BP common with a PNP emitter follower to drive the common cathode.
The peak current in the segment will be ~2mA, due to the internal resistance of the drivers, and the average current will be half of that due to the LCD type drive, but it will work, I have already used such a configuration.

d1030180 25th January 2013 10:01 PM

Quote:

Originally Posted by Elvee (Post 3341327)
If you can accept a display that is not very bright, there is a possibility: you can drive directly LEDs at low current.

To do that, you need a high brightness LED display, one that gives acceptable luminosity at 1mA, a common cathode one, you dimension the segment resistors to give 3 mA and you buffer the BP common with a PNP emitter follower to drive the common cathode.
The peak current in the segment will be ~2mA, due to the internal resistance of the drivers, and the average current will be half of that due to the LCD type drive, but it will work, I have already used such a configuration.

Thanks for your tip!

gmphadte 30th January 2013 04:40 AM

Use ICL7107

Gajanan Phadte

peranders 30th January 2013 04:43 AM

https://www.elfa.se/elfa3~se_sv/elfa...oc=0&q=icl7107 buy it from ELFA.

d1030180 30th January 2013 03:53 PM

3 Attachment(s)
I assembled the circuit on a breadboard. I used a 2N2907A transistor to buffer the 'BP' pin to drive the led displays. I experimented with a 2N2222A signal transistor and also with an MJ15003 power transistors for sensing the temperature. It works very well, although the display segments are quite dim (as expected).
So I may need to try if the display segments could be buffered with some transistors for increased brightness. Or just get the chip which has led segment drivers built-in.

Elvee 30th January 2013 04:31 PM

Quote:

Originally Posted by d1030180 (Post 3348748)
Or just get the chip which has led segment drivers built-in.

That's the sensible solution if your display is not bright enough to your taste: adding 23 buffers is not reasonable, unless you have no other choice.
Note that some modern displays can be quite bright with only 1mA average current; yours look like standard-brightness types.

Even a 7107 will not be that much brighter: 8mA instead of 1 looks like a lot, but the eye response is logarithmic, and the BP strobing also increases somewhat the perceived brightness.

audiostrat 13th May 2014 12:30 PM

So how accurate is this thermometer? I would like to make a simple one with 1-2 degrees accuracy to monitor transformer temperature. Would this design be good for my requirement? Or are there any other simpler alternatives that might do just as well? :)


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