ICL7107 Volt meter question / issues

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dacflyer
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ICL7107 Volt meter question / issues

Post by dacflyer » Fri Feb 12, 2016 4:24 pm

i have a hand full of the ICL7107 IC's and led displays. i have followed the schematics for a 2v set up.
the meter functions but the readings are not steady. the readings fluctuate and i cannot figure out what is making it do that.
i am running single 9v battery power
any suggestions..
i am planning to use a LM34 ic for temperature reading..
attached is a image, the one displayed on the left is what i am trying to use. but on pin 21 i read that pin 21 may cause the crazy readings, but if i disconnect this then the led display does not light up. so what to do ?
ICL7107 2v schematic.png
also as far as the set ref voltage (1V) i know my meter probe goes onto the wiper, but where does the other lead go to get the correct reading ?
i read this was supposed to be a very accurate chip and very stable, but it is nothing like this for me.

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jwax
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Re: ICL7107 Volt meter question / issues

Post by jwax » Fri Feb 12, 2016 7:39 pm

The left image is for a + and - voltage supply. If you're just using a 9 volt battery, go with the schematic on the right. (Single supply)
In that case, you set the pot to a 0.100 volt reference (Pin 36 to ground).

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Re: ICL7107 Volt meter question / issues

Post by dacflyer » Tue Feb 16, 2016 7:23 pm

the circuit on the right uses a 5v supply.. but the circuit i am using is 9v, and it does not use the zener.
i have searched the net, can't find a copy on here..i'll scan a picture of the circuit tomorrow and then post it on here..

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Re: ICL7107 Volt meter question / issues

Post by jwax » Wed Feb 17, 2016 4:41 am

Would it be worthwhile to add a 5 volt regulator and a capacitor to use the "right side" circuit with your 9 volt battery?

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Re: ICL7107 Volt meter question / issues

Post by dacflyer » Wed Feb 17, 2016 12:46 pm

well if i did then the display is not as bright..

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Re: ICL7107 Volt meter question / issues

Post by dacflyer » Fri Feb 19, 2016 4:35 am

ok, i finally had time to scan my file to show you.
the posted item is the diagram i am trying to use.

[The extension pdf has been deactivated and can no longer be displayed.]

any suggestions for a more stable reading ??

again, once this is set up i plan to use a LM34 temp sensor to make this a temperature display.
the sensor will be at the end of a 6' cord 3-cond. unshielded.

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Janitor Tzap
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Re: ICL7107 Volt meter question / issues

Post by Janitor Tzap » Fri Feb 19, 2016 8:41 am

Have you put a .1uF or .01uF Ceramic cap on pins 1 +5V, and 21 Gnd?

Many PIC's have this cap mounted as close too the power input pins to help smooth the ripple from a power supply.

Even on battery powered projects I've seen this done.


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Robert Reed
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Re: ICL7107 Volt meter question / issues

Post by Robert Reed » Fri Feb 19, 2016 9:55 am

any suggestions for a more stable reading ??

100k - 1% film resistor
100 pf - 5% COG ceramic capacitor
24K - 1% film resistor
25K Trim pot - Multi turn cermet

1.000V listed shows they want this voltage to 0.1 percent accuracy and/or resolution.
If you know the approx. value that current pot and 24K is set at, you can up the value of 24K and reduce the pot value - 90%/10% ratio
These changes are for stability as well as accuracy and components are dirt cheap.

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Re: ICL7107 Volt meter question / issues

Post by jwax » Fri Feb 19, 2016 2:12 pm

Again, this layout is for a double-ended supply: +, -, and ground. Where are you connecting the 9 volt battery? Is the neg of the battery going to V- in the schematic, or pin 21 (Ground)?
Also, where is the Ground of the LM34 connected?

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Re: ICL7107 Volt meter question / issues

Post by dacflyer » Fri Feb 19, 2016 6:39 pm

i have totally re-breadboarded what i wanted to do and now things seem to be stable.
i decided to abandon the single supply and go with the +5 -5 supply.. the readings were still a little erratic, so i experimented with the osc. cap. to slow the reading down a lot, and now things are stable and my temp. reading are stable..
in the book it did not really explain that i could slow the osc. down. but i tried it anyway.
so now i am not getting 4 readings a second, now i am getting them once about every 15 seconds or so.
i went from a 100pF (101) cap to a 1pF (.001)? i am not too sure myself on cap conversion code markings
but i like how it is working now..
i think i'll order me some of the 10 turn 25K pots.
i do not seem to have any in my junk boxes..

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