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bonanz

Led display Digital Voltmeter question

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This circuit came up at the perfect time! couldnt have asked for a more perfect design for what i need right now, only problem is i don't have a -5 volt supply.  What would happen if i grounded that pin rather than using -5.  would that simply not allow for negative voltages to be measured? because i only need positive for my application. or is there a simple way to make a negative supply from only a +5 being available?

schematic_th.gif

thanks
bonanz

Project Link: http://www.electronics-lab.com/projects/test/001/index.html

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If you ground -5V pin, and apply +5V to the pin 3 then it will work nice :D for measuring positive voltages (i already tried it). I am not sure what will happen with negative voltages, i haven't tried it.

Hope that helps a bit.

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:)hi bonanz,

you said you would be measuring only positive voltages, now what would happen if you accidentally reverse your probe. wont your voltmeter get burned? i believe one of the reson why this device was designed to have a dual supply is bcoz of that. if you reverse the probe your display will have a negative sign.

since you said you only have a 5v positive supply, try using LM7905 to get a 5v negative supply.

hope this solves your problem.

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:)hi bonanz,

sorry i was not able to complete my reply, one way to solve your -5v swupply source is to use a 10v transfo. this is how you do it,
1. series 2 resistors
2. shunt the resistors across the rectified output
3. from the junction of the 2 resistors, this is where you tap your ground
4. connect the usual filters
5. connect your 7805 for +5v and 7905 for -5v

suggested resistance value is 47K 2w

hope you can benifit from this :)

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Okay i've done some playing around with this circuit and i cannot get it to work for my application. I'm not worried about switching terminals because i am not using it as a typical dmm i'm using it to display a voltage in a circuit that will always be positive. I am using the LM2907 frequency to voltage converter and have it convigured so that the output voltage is linearly proportional to the input frequency (ie 200mv ~ 200hz) the max freqency i'd measure would be about 1200Hz which would correspond to 1.2v work perfectly on the 2v scale and i would just leave the decimal points unconnected. I have hooked up the project circuit, and did a test on a voltage divider to see if it was measuring correctly and it did measure the voltage, but i only had to connect terminal one to the voltage point and did not connect terminal 2 to anything and the voltage displayed correctly. (this is btw with the negative supply grounded). The LM2907 freq to voltage converter circuit i have completely working and outputing the corresponding voltage when i use a regular multimeter, but i would like to integrate this circuit with it to have a permanent display. any help would be appreciated.

bonanz

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I would really appreciate if someone could tell me a way to change the default 0-2,0-20,0-200,0-2000 scales to a custom one, for example

0-30 V (or 0-40, it doesn't matter)

because I want to integrate this circuit to the Stabilized 0-30V 0-3 A Power Supply that appears on the entry page of this site. I am about
to design a new PCB with a complete solution for a lab PSU.

Thanks ! :)

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I use this digital voltmeter with this

3-30V power supply: http://www.electronics-lab.com/projects/power/032/index.html

If you configure it at 200V range (R3 = 12K) then it will measure from 0-30V with one decimal point precision.

for 4,1V will display: X04.1
for 10,1V will display: X10.1

where X indicates that the first display is OFF

I think this configuration is ideal for your power supply.

If you finally draw a PCB for a complete bench PSU solution i will be happy to host it under the power supply's page ;) .

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I tried to build the L.D.D.V. yesterday, using a breadboard and
I came up with peculiar readings on the LEDs. ??? I had followed the
schematic from smartkit.

I thought that I had a false ICL7107 and I dismounted the
components. :-[

Today I thought to check the data sheets of ICL7107 and MAN6960 in order to cross-check the connections. Found [glow=red,2,300]inconsistencies[/glow] in the
article schematic ! >:(

For example for the 1's display :

Article : ICL(6) -> MAN(10)
Should be : ICL(6) -> MAN(9)

another example :

Article : ICL(34)
Should be ICL(25) -> MAN(1) and so on... there are many there.

Moreover, the R1 should be 100K, not 180K, I refer to the datasheet
and the "Digital Voltmeter" project

Finally, the LCD dot, is pin number 5, emitted in the schematic.

I am about to search all the errors now and report back later,
after I will rebuild the voltmeter.

I attach the data sheets for further reference. 8)

ICL_and_MAN.zip

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Billy, I look forward to your findings when you are finished. I had planned to use this circuit as part of a larger project. Glad you found the bugs in it. I had also downloaded the data sheets, but I had not yet compared them with the project schematic. Good find!

MP

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Great Work Billy. Thanks for doing that :D .

Sometime ago an internet friend told me that the kit companies are making some small faults (not by chance) on schematics to prevend them from copying :o ! If you relly on the PCB, everything will work nice, becuase it's a copy of the original (already worked) board. So you can use it to debug the schematic.

Please do the debuging and make the changes to the schematic... i will be very happy to see it here!

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I have done serious work on the volt meter, I have really had problems getting a -5v supply out of a 9v battery. I tried using the replacement for the cd4009 as indicated in the data sheet from intersil, cd 4049 all to no avail. I actually wanetd to run the unit from a 9v battery for portability sake, now i have had to revert to the good old transformer and using an LM 317T and LM 7905 in conjuction wit 2 polar capacitors of differnt capacitances I have been able to get the 2 voltages (-5v and +5v) now my problem is calibration, am using a 1k instead of a 1.2k to get the 0-2000v scale,(That is the only scale am using) I shall find out 2 nite if it will work. :'(

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By the way, there are some differences between the scheme and the PCB photo-layout

Is P1 a TrimPot, like the scheme says, or a Trimmer, as the photo and article describes?
Is the ICL34 pin correct in the scheme? Going out to the Display?
Where are the C6 and C7 on the scheme? They are not there, just in the PCB layout you can find and try to discover...

Finally, if someone corrected the scheme, I would apreciate to see it.

p.s.: I tryed to assembly it on the ProtoBoard (breadboard?).

Thanks very much,
Regards.
Gustavo Kowalski

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Gustavo, if you scroll up to page 1 of this thread, Billy has posted some corrections to the circuit. I have also found a board layout file with schematic both in Eagle format on the Eagle site that uses these parts and is very similar. It was donated by a user. If you do not have the Eagle Layout program, you can get a free version from their website at the same time.
http://www.cadsoftusa.com/freeware.htm

Billy: did you find any other corrections needed after your previous post?

MP

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Hello! Thanks for you reply.
I read Billy´s corrections, but I still do not know if its a TrimPot or a Trimmer, and I don´t know where the C6 and C7 goes.
It would be nice to make the corrections on the main site of the project, because I linked that site to many friends that looked for this circuit.

I´m getting this program you said, but where can I found the schematic? Thanks again and contratulations for the great site!

Regards.

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Having gone through the circuits, datasheets, replacement guides (ECG master replacement guide from Philips) and after the alarm about the inconsistencies I came out with these corrections and I believe they are correct as far as am concerned. Also I have included a modified circuit diagram for the DVM which can measure both ac and dc voltages. The maximum ac voltage that can be measured depends on the type of rectifier used; I recommend an IC rectifier which has a peak reverse voltage of 1000V and maximum forward current greater than 3A.So as not blow up the rectifier leave a safe limit of about 100V i.e for a rectifier of PRV of 1000 V don

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Hello Gusk4x4

If you use the PCB, this voltmeter will work very nice. There are some errors at the schem and maestrodamuz has poster here the corrections along with some modifications to measure AC exept from DC voltages:

http://www.electronics-lab.com/forum/index.php?board=4;action=display;threadid=353&start=0

Hope this clarify things

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