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Led display Digital Voltmeter question


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Hi guys, just registered, have a couple of stupid questions. i am software guy not an electronics major so go easy on me. i would like to use this project to measure voltage of a car battery.

that is, i am looking for something that i can position on top of the battery with double sided tape and connect to the + and - and have it display the voltage. this way i can pop the hood and see at a glance what the voltage is.

will this project be able to do that? what kind of mods do i need to do for it to accept +12v input? are there ready made gadgets that are available cheaply that will do what i am looking for?

thanks for help in advance...

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  • 4 weeks later...

Hi all,

I've just finished my DVM (with 7805/7905 power supply) and for a minute, it worked well (zero reading with no input voltage, correct reading with input voltage). Good! :)

Then I've fiddled around with the wires and, for some reason beyond me, the DVM ceased to work. Now it keeps showing a crippled (but bright) 3 on the first digit (a 3 without the upper and lower bar). When I switch the DVM off and on again, the 2nd, 3rd and 4th digit show a (seemingly random) value for a split second and then the funny sign on the 1st digit comes back. Bad! :'(

I've re-soldered anything. Didn't help (as expected). Anyone has a hint on what to do? It'd be appreciated.


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Did you read the fixes to this circuit that were posted in the "General" area? If this is all good, I would check the voltages to the chip and then check wires to the display. Since you had a working circuit and it stopped working when you moved the wires, I would recommend a real close look at the wires. Look for breaks, frayed wire, anything there.
Hope it helps.


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Dang. I've written quite a lengthy reply, but when I clicked on send, the forum said I wasn't logged in and my posting was gone. Never click on send before pressing Ctrl+A, Ctrl+C. I should've learned that by now. :(

I'll repeat the most important part only:


Thanks for your reply, MP.

Is there a difference between the corrected schematic (posted my Mixos) and the PCB layout on the project page? I haven't checked that yet. I've relied on the assumption that when there's an updated schematic posted, a faulty layout would not remain on the project page. Should I have designed my own PCB layout, referring to the updated schematic? (Actually, I have designed my own layout, but I've referred to the layout on the project page.)

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mixos is correcting these as he gets the information from those who have actually built the device and given feedback. I assume that mixos has already made this correction. He spends a lot of time keeping the site up and running also. Send a private message to admin and he will be able to give you a direct reply. You can also send a private message to billy who has built it. I have not seen billy post lately.
I do not know if mixos has fixed the drawing and board in the projects section or not. I am assuming that it was taken care of when billy and maestro were building the circuit and found the fixes and made a working version.
I do have a board layout and schematic in Eagle which I posted to the forum some time ago. A search will bring it up.


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  • 2 weeks later...

I pieced the voltmeter on a protoboard (bradboard) and it doesn't works well

The measure it's very unstable.....I think that maybe it's my fault but it's very improbable because i used the fixed schematic and i compared it with the datasheet and it was ok!!

Also it's doesn´t mmeasure well.........I tried to measure a 0-18V power suply.....and it wasn't measuring well!!!

I would like that someone help me!!!!!!!!!
Excuse my english!!.....I'm from Argentina (and i would like to write in spanish but i can't in this forum because nobody will understand me!!!)

Bueno Chau!!!.....Espero q alguien pueda ayudarme.!!!!

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hello, my name is Emiliano, and I have a problem with this circuit:
"I've built the project -0 to 30 PSU-, and I decided to built this Voltmeter to conect it directly to the output from the PSU, but when i do that the Voltmeter turns off, I test the Voltmeter with another power and i worked.....I think that it could be because in te PSU project I have 70,02 V AC at max power (30,1 V DC)"...I don't know!!!......please if you can help me......


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Emiliano, the final fixes for the project were posted in the Feedback section of this site. You can find the information here:



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OK thanks
Now I've just finished making the pcb but i have to make the holes so i'll wait to monday to do it in the sochool (I go to an industrial school called PIO IX).
Meanwhile I wonder If there's something else anyone should tell me about the pcb or the components....
Any more fixes apart from the R1?.....(I don't want a link, i want you to tell me if there is anymore fixes I don't know..........just to be sure)

Well that's all.......bye

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You can use any LED you want. Check the data sheet above that I have attached to my post. The A1, A2, etc pins are the segments. Just follow the pin out of the two different types. Be careful, though. Some LEDs are common cathode and some are common anode. If you choose the opposite type, you will need to add a little more hardware to make it right.


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