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Led display Digital Voltmeter question


bonanz
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Did you substitute any of the parts? Did you change any of the values or omit any of the parts? Is the voltage you are reading a true DC voltage (clean?).
Do you have access to a scope? You should look at the regulated voltage to the chip, the reference pins and also at the voltage you are measuring. Note that if the voltage you are measuring is not a clean DC voltage, the value on the display will fluctuate as does the signal. If you need to smooth out DC signals, you could add a bypass cap to the input for added filtering. However, this will slow down your response a bit.

MP

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schematic.gif

i think in this schematic there is a problem...
several times i tried to build this circuit as mentioned but always there is a stable value on displays and i realized the fault situation(that's my idea :-\)

we apply the + voltage on pin "1" but there is always a stable voltage on this pin through R3 resistor(i need 0-2V range and as mentioned i shortcircuited 32-31. pins), that's why i've read always a constant value...

i wonder if this constant value comes from supply voltage +5v and follows R2,P1 and R3...

i think the only way for range adjustment is using voltage divider before input...

i think R3 resistor is unnecessary...

is there anyone to explain this situation?
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Are you connected between pins 30 and 31?
This is project #14. Also look at project #7 to see how it is done. I think you did not connect to the ground pin. The schematic is not very good, showing connections of the lines. You can also look at the board layout to verify.

MP

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in schematic there is no connection point on pin 30...(no dot)

by the way for 0-20 v range it is said "R3-->0 ohm" ,0 ohm means shortcircuit and  if we shortcircuit pin 30 and pin 31 we will read always "0000" i think...

according to the 7107 datasheet (Low Cost Digital Panel Meter Designs and
Complete Instructions for LCD and LED Kits--->AN023) there must be no connection btw pins 30 & 32...this connection is for LCD display design...

i also tried it and with this connection displays show always overrange (1___) but i should say i had grounded pin 26 (-5v) because i dont need negative voltages (it is not possible for my circuit to get any "-" voltage because of my other circuit's output)

as a result, in usual 7107 datasheet for both LCD and LED display circuits there is a connection btw pins 30 & 32 but in the application note of 7107 (AN023) there is no connection btw them on LED display circuit...

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Tokuoni,
Yes, you can use a higher voltage rated capacitor. No problem.

Dmitry,
Yes, it does look like there is something missing in the schematic. I tried to point out to you that the other project shows a resistor connected between pins 30 and 31. Please refer to the board layout on this project as I mentioned. This is a "smart kit" design. They were notorious for making problems in the schematic so newbies would buy their kits when they failed to make it from scratch. The board layout is usually good. Thus, you can compare the board layout to the schematic to find the needed changes. Also look through the past posts. There was a problem in the schematic on one of these two projects and corrections were posted.
Also, I posted a schematic and board layout from Eagle that I found on the Eagle website. Please make changes to your project as needed.

MP

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Hello everyone.
  I would like to have a digital ammeter to add to my recently completed 0-30 volt power supply. Is supposed to have a 30 amp capacity. Have not tested it that hard at this point. Can this meter be easily modified to read current???

Thanks in advance for any help  rfranzk

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Just connect a very low value resistor across its inputs as a shunt. Use Ohm's Law to calculate its value.
Which supply project gives a whopping 30A? The one on this site is 3A.

It is best to make a voltmeter with a 2V max scale reading and use it to measure the voltage across R7 (and therefore show its current) in the 0-30V supply project. Then the resistance of the ammeter doesn't spoil the supply's excellent voltage regulation.
You will need to adjust the meter to read AMPs accurately, since R7 isn't 1.0 ohms.

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Hi auidoguru.
  Thanks for the quick reply. I knew this was the case with analog vein type meters but was unsure about the digital displays.  I will dig up schematics and possible photos of completed stuff.  ps. I made the supply to power a capacitive discharge welder for welding battery packs. works cool!!

Thanks rfranzk

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Hi
I tried to build digital voltmeter but it doesn't work :'( . I tried two ICl 7107 and it still doesn't work. I have never do anything with CMOS ICs so a I don't know if the IC is bad. ??? . I have done it on test board with schematic from http://www.electronics-lab.com/projects/test/014/ .
Everything is feeded by old AT PC voltage supply.
Only change I've done is change of 2K trimmer for 2k5 trimmer because I can't buy 2K trimmer

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Go back to page 1 of this thread and read all of the posts. There were changes made and since there are two different schematics on the site, I am not sure which one was corrected. Click on the links posted. Read the posts and make changes accordingly.

MP


Thanks, I'll try to find something usefull.
So why didn't you move the right schematic to the projects pages ? ???
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...Like I said, I am not sure if this is the one or not. There are two Digital Voltmeter projects in our projects section. One was corrected and the corrections were posted in the projects area by mixos. It is possible this one also has some problems but the other one was corrected only.
If you go back in the posts, you will see a corrected schematic and another schematic which I have posted as well. You should be able to find your problem by comparing the schematics.

It is also possible that you are using a correct schematic and built it wrong.

MP

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Hi again
I've found this -> http://www.jaknato.cz/konstrukce/teplomer.htm . If you say that you can't measure voltage on the same power supply, how does this thermometer work ? There is no other power supply for transistor PN junction. It's everything in one. I just only need to know if it's possible to make circuit which can measure itself? I want to make thermometer with LM35DZ sensor (same ground and voltage from power supply as

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Ok
    I don't understand the language but from the schem  I think if you had two voltage regulators even if  they had the same voltage out put one would go to the led readout and the the other would go to the sensor, should work. The Positive rails would need to have small diodes to prevent feed back. 
                                        gogo





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The project in your link is not measuring the same voltage which is supplying the unit. It is measuring a simple transistor used as a temp sensor. The earlier post about measuring the voltage that you are using for a supply was a reply to a member who wanted to actually measure a varying supply voltage to the voltmeter with the same meter. Problem with this is that your supply voltage to the chip is always changing. Thus, your reference voltage is also always changing. If you continually change your references, you no longer have accuracy.

MP

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  • 1 month later...

HEllo everyone , i would jus like to confirm that if the Eagle Version of the voltmeter    is correct to build , because i need acurracy. i am trying to build a digital voltmeter so i can attach it to a LM2907 Frequency  to Voltmeter Converter , and measure frequencies on the range of 1hZ till 10Khz . although i can also use low pass , band pass and high pass filters for frequency to voltage conversions, then again does the meter has milivolt range and is it that accurate... if this project is complete a higher VHF Filter can be used to measure VHF Frequencies

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