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Led display Digital Voltmeter question

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The list for R3 in the project is wrong and is backwards:

1) How can the meter measure 0V to 2V when R3 is zero ohms? The input would be shorted.

2) You have R3 as 1.2k and the project shows 1M for R4. So with 12V into the 1M to 1.2k voltage divider, the meter's input gets only 0.01428V.

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Ok then. So what should I do to have the project working in the 0-20V range, displaying the measured value with two decimals? What is the value of R3 if not 1.2K? Is that 12K? Where should I connect the #5 pin for each display?
I'm sorry to bother you but I'm just a hobbyist and I don't know to make the calculations.
Thanks.

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I don't see a calibration adjustment to make the meter read 1.999V max on its lowest range.
The datasheet for the IC shows it with a 199.9mV max lowest range.

For the lowest range to be 1.999V max, then R3 should be disconnected.
For 19.99V max, R3 should be 111,111.11 ohms.
For 199.9V max, R3 should be 10,101.01 ohms.
For 1,999V max, figure it out yourself.

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Skipper,
Mixos is using this voltmeter for his 0-30V power supply. He states earlier in this thread that he uses 12K for R3 to get one decimal point precision on a 0 to 30 volt scale.

http://www.electronics-lab.com/forum/index.php?topic=5629.msg636#msg636

There were a number of corrections made in this project at the beginning of this thread. However, the R3 resistor scaling was probably missed.

There is also another project in the same directory that uses the same ICL7107 chip. It has a scaling switch. You can see that project here:
http://www.electronics-lab.com/projects/test/007/index.html
Perhaps it will also give you some ideas.

Hope that helps.

MP

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If R3 is 12k like Mixos uses then with a 12V input it will show X12.0 but if R3 is 120k then I think it will show 12.00.

I think the project's translation wrongly says, "zero volts adjustment" for its trimpot instead of, "calibration adjustment" because the trimpot makes a reference voltage for the IC. Then the IC makes the zero adjustment and the trimpot sets the calibration so that a 1M to 12K voltage divider (10V input gives 118mV to the IC) shows as 100mV.

I think the input scaling resistors should be in series. Demonstrating the problem of using separate resistors with a 10.0V input:
1) 1M to 12k divider= 118mV.
2) 1M to 120k divider= 1.07V which is not 10 times 118mV.

Now connect the 12k and 120k resistors in series:
1) 1M to 12k divider= 118mV.
2) 1M to 132k divider= 1.16V which is very close to 10 times 118mV.
The 12k resistor should actually be 12k in series with 1.2k and in series with 120 ohms for a closer 10 times scaling.

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Does anyone actually built the 7107 based voltmeter project posted in this section in Eagle format?
I would appreciate if someone could tell me where the test probes should be attached on that board (should be the IC pins 30 and 31 ?).
Also, I would like to know how to set up the measuring range for 0-20v for this project. I guess there is some resistor to be changed like in the other projects.
Many thanks

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• 1 month later...

You may go through the following web link this will help you to select the correct resistor :
http://www.kitsrus.com/pdf/k61.pdf

if you have finished the project, can you share your experience, I want to construct this as a dashboard meter for my car.

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hello vkssingh

I look at that kit too, the values they used didn't work with my set up. I built it on a breadboard. Got it to work, I have mine set at 200 volts for now, when I tried to set it for 30 volts I couldn't get a consistent reading.Breadboards are noisy lot of bad connections, lot of wire and stuff. I put about 10 10uF cabs on it, that seemed to help.
it works just haven't peeked it out yet
have fun
gogo

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• 2 weeks later...

Hello all, Yes Skipper I have built

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Dear Lupe,

Can you elaborate,why the refrence was not proper using 9 V battery?

I want to use this voltmeter as car dashboard meter, In such case the battery voltage will be coverted to +5 V using a 7805 regulator and -5V using IC7660. Hence the ground of the power supply and voltage to be measured will be same As per the circuit given on www.electronics123.com if I connect the IN LO (pin30), IN HI (pin 31) and COM ( pin 32) of IC7107 to ground these pins also be at battery ground. Would this configuration work?

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I really don't see that as a problem, what I did is

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I hooked up my meter to my cigarret lighter and it seemed to be working fine, It was fluctuating a little bit at first but the engine was cold. After a while it worked pretty well and didn't move by more than say .07 volts. I also verified my ref. voltage and it was at .097, I guess that's pretty close to .1 . I drove around all day working while the meter was connected. I even put my digital meter on it to compare values. Towards the afternoon it seemed my meter kit was off by more than .1 volts but still not more than .7 volts and that was while driving, opening and closing doors etc., . Not to mention my work van has 260,000 miles on it. Overall I am please although I would like to in the least understand a bit more about how the meter actually works. This might help explaing some things.  Does anyone know how I can improve the display readings not to fluctuate as much, my digital meter does not fluctate as much as the one in the kit does.

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lupe,
A bypass cap on the supply to the voltmeter and/or a bypass capacitor on the input work well. I.m not sure how you layed this out in your design, but some bypass in one or both of these places should smooth out the voltage fluctuations. Use an electrolytic or tantalum polarized as a bypass to ground. Try something in the 4 to 10 uf range to see if this is enough. A 0.1 uf tantalum will also help if there are some high frequencies riding on the voltage. The fluctuations in the meter are caused by variations or spiking in the voltage. The voltage on automotive systems is very noisy. If you have a scope, you can usually see some noise on the DC supply.

MP

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Thanks MP, I will try that, the board has a 100uf electrolytic across the 5 volt supply. I will try to smooth out the 12 volt regulator as well. Does anyone have a source for meter pcb's based on the 7107 IC. I have several  7101's from way back that I would like to use. My meter has been hooked up in my van for a few days now and is working well, I covered  up the 4th Led only because looking at a Led that is moving frequently is not cool. So I now have a good reading of 14.1 volts on my display and looks to be within .1 volt from a digital  multimeter.

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• 2 years later...

Hi, I have finished the project, it works, but I have one problem.
The digits are not still, goes up and down little but the voltage is correct.
My English is bad, but I understand very well.
Here is a video of my Voltmeter in work:

And picture:

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Hello RumbaKing

I made one to with the same problem and I think it is because at the time I made it  I didn't have the right resisters. I used 5% instead of 1%. Maybe next time I go on a buying frenzy I'll stock up on 1% resistors.
gogo

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• 3 months later...

My project didn't work. :(

Is there any changes in this circuit?

I think your input (not 5v power supply) ground reference is not stable.
u can ground pin no.30 by using 0.01uf.

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I'm building this digital voltmeter with an ICL7107. http://electronics-diy.com/ICL7107_volt_meter.php

I left away the negative voltage supply and just connected the ground to pin 26. I thought the negative voltage isn't needed when you only measure positive voltages. However, my display only shows zero's now. When i measure a voltage the display show a few numbers fast and then goes back to zero's. I'm guessing this is because i don't have the negative supply? Could someone verify that? And i heard that you need 2 seperate supply's wich are galvanic isolated from each other and the measuring voltage if you use 2 ICL7107 meters, is this correct?
I wanted to search this thread but all the posts have this written in it : "Message is not available".

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This diagram have at least one error: the 10uF capacitor in the -5V supply is inverted!

View the ICL7107 datasheet of Intersil for more details.

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• 3 weeks later...

No, i think that is correct. I'm going to buy an ICL7660 ic soon and test it out. I've been searching on the internet and it turns out that the ICL7107 does need a negative supply.

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• 8 months later...

Hi, I have finished the project, it works, but I have one problem.
The digits are not still, goes up and down little but the voltage is correct.
My English is bad, but I understand very well.
Here is a video of my Voltmeter in work:

And picture:

isn't the display voltage unstable because the resistor 5%-1% relation?any pro answer it ?
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• 2 months later...

Hi Bonanz,

Grounding the -5V will not work on this project or on any other that uses the ICL7107. Believe me, I've tried it, it just causes a blank display. The Maxim website specifies that only the ICL7106 can be used like this if you only want to measure positive voltages.

The ICL7106 by the way is the twin brother to the 7107 but is used for driving a LCD display.

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• 1 month later...

Hello everyone, i've got a problem with my voltmeter. I just finished it and it shows 18.4V when i don't measure anything. It shows 1xxx when i measure a 1,5V battery and when i switch the wires it shows -1xxx. So it looks like it can detect when you switch + and -.

When i hold the measuring points together it shows 00.0. So it looks like i have a problem with my scale, although i used a 12k resister and i only measure a 1,5V battery.

I used the exact same components as mentioned on the website but i use standard resistors instead of the 1% accuracy ones. Does someone know what might be the problem?

Bart

edit: i use a 180k resistor at pin39 instead of a 100k resistor. I see pin39 has something to do with the oscillator. Could this be the problem?

edit2: i think i broke it :p. I saw 2 errors in my voltmeter. I use an experiment pcb with pre drilled holes because i don't have the equipment to etch and stuff. And it also has to fit into a 70mm x 39mm case so i'm making it super compact. I used to work on it during school about a year ago and now during the vacation i wanted to finish it. Some wires got loose and i soldered it onto the wrong place.

Errors : +5V was directly connected to pin 31 of the icl7107 ic. The 1M resistor was connected between pin 31 and 30 instead of in series. The elco connected to pin 5 of icl7660 was inverted because it's wrong in the schematic.

I did some measurements after the corrections: Between pin 3 and 5 of the icl7660 i measure +2,57V and between pin 3 and 5 i measure -7,45. So i guess the icl7660 is broken now? I did new measurements, i get the correct +5V and -5V from the icl7660. So i guess the ICL7107 is broken.

My leds now only show 00.0 and it doesn't even see if i switch + and - anymore. I'm going to check tomorrow for more errors, going to bed now.

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I took out the damaged icl7107 and replaced it but for some reason everything i measure is more or less half of the actual value. My battery is 1,3V and my meter shows 0,6V. When i couple two battery's it shows 1,2V. So it's measuring now, but it measures half of the actual value. Playing with the trimmer didn't help. I'm going to change the oscilator resistor from 180k to 100k tomorrow.

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Did you use the negative supply in the end? If not maybe that's your problem.

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