Led display Digital Voltmeter question

[bonanz]

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?



thanks
bonanz

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

[gsmaster]

You can use the TC7660 (from microchip I think ) which is for this job. It takes gnd and +5V and gives you your -5V.

[mixos]

If you ground -5V pin, and apply +5V to the pin 3 then it will work nice    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.

[bonanz]

okay great, because i only need the circuit to measure positive voltages, the input will never go negative. when you say pin 3 do you mean pin 3 on the pcb? not on the chip right?

[mixos]

Right... the pin 3 that is indicated with +5V at schematic.

[mozikluv]

 :)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.

[mozikluv]

 :)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

[bonanz]

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

[billy]

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 !  

[mixos]

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   .

[MP]

I have not yet tried this, but one should be able to have auto ranging with comparators and a 4066 cmos switch.

MP

[billy]

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 inconsistencies 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.  

[MP]

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

[mixos]

Great Work Billy. Thanks for doing that   .

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   ! 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!