Led display Digital Voltmeter question

[STEVE2]

This may already have been answered on previous pages ... but don't have time to read all 19 at the minute!

In the power supply which supplies the +/- 5 V (schematic attached) what are the two 1 K resistors for?

Thanks

View attachment 39681

 

[audioguru2]

what are the two 1 K resistors for?

To make a 0V reference voltage for the circuit. It is supposed to have a positive 5V supply and a negative 5V supply.

 

[vkssingh]

I have gone through all the 19 pages on board but still some questions remained to be answered:
I want to use this VM to measure car battery voltage, what should be the ground point in this case? This has been asked earlier also but no replies.

If I use the car battery (12V) as input to 7805 and 7905 to genrate +5 V and -5V to power the IC will it work ? Will the

 

[audioguru2]

Hi Vk,
A 7905 regulator isn't a generator. It takes a higher negative voltage and regulates its voltage down to -5V. It needs a negative voltage source as its input.
There are a few ways of converting a positive voltage into a negative voltage. The datasheet for the ICL7107 IC in the project shows the circuit for one way. A 555 or an ICL7660 can also make a negative supply.

A 7805 positive regulator needs a minimum input voltage of +7.5v. A 7905 negative regulator needs a minimum input voltage of -7.5V. Therefore their total minimum input voltage is 15V which is not available in a car anyway.

View attachment 40073

 

[vkssingh]

Thanks for reply, I have decided to use IC555 to generate -5V as it is cheap and easily available, In such case should I take +5 V from  output  of    7805    to  make  -5V OR it is better to generate -12V with IC555 and then regualte it to -5 V  using 7905?

Will -ve terminal of battery be the ground point in the circuit?

 

[audioguru2]

It is best to make a +5V with a 7805 and make -12V from a 555 then regulate it with a 7905 to get -5V.
The datasheet shows the ground of the meter connected to its negative input lead. The same ground as the negative terminal of the car battery.

 

[MP1]

I have gone through all the 19 pages on board but still some questions remained to be answered:
I want to use this VM to measure car battery voltage, what should be the ground point in this case? This has been asked earlier also but no replies.

If I use the car battery (12V) as input to 7805 and 7905 to genrate +5 V and -5V to power the IC will it work ? Will the

 

[vkssingh]

Thanks for clarification, can you suggest what capacitor value to be used to supress the alternator noise?

Since I shall be meauring battery voltage can I eliminate the -ve voltage and leave the IC7107 pin unconnected? I will put a diode in the supply line to avoid wrong polarity apllied to IC. Will it work?

VK

 

[audioguru2]

Since I shall be meauring battery voltage can I eliminate the -ve voltage and leave the IC7107 pin unconnected? I will put a diode in the supply line to avoid wrong polarity apllied to IC. Will it work?

Don't you have the datasheet for the ICL7107 that is used in this project?
It says that the inputs don't work near ground (outside the input common-mode range) unless it has a negative supply, and a few other problems must be solved.View attachment 40077

 

[vkssingh]

One may be last question before I start construction: the capacitors in the circuit have to be ceramic or other types may also be used.

 

[MP1]

You will have to experiment a little on the capacitor size for the alternator noise. This is the same problem you might encounter with anything you build and add to your car. You might even need to use a choke and build an alternator noise filter if you have a lot of noise in your system. It will depend a lot on your car make, how old the alternator is, how worn out it is, etc. It is also possible that you will not even need to do anything at all. I would try no filtering at first and then add on to the design as needed.

Ceramic capacitors? I would recommend that you use better capacitors.

MP

 

[vkssingh]

Would you pls suggest the capacitor and resistor types for the project?

Thanks

 

[MP1]

I use 1% tolerance resistors for everything. R3 in this project must have such accuracy, but the others could be 5% if you have trouble getting 1% tolerance in your area. Ceramic caps are cheap, but they are not real good, either. If you use a higher grade material such as poly, mylar and tantalums, you will have better results. There is no problem using Electrolytic capacitors for the supply bypass lines. There might be some guidelines in the data sheet regarding this as well. This is not a big issue. If you use ceramic caps, the project will work. In fact, the Smart Kit probably uses the cheapest components off the shelf. This was only a suggestion to a better product.

MP

 

[SKIPPER1]

Hi guys,

Just finished to build this project for 0-20V range and it looks like I have a problem: instead of 12.00 V, for example, the display shows 00.12V, instead of 07.00 V it shows 00.07V and so on.
What should I do to make it show the correct value with two decimals?
Thanks.

 

[MP1]

What did you use for R3? Also which pin 5 did you jumper on the display?

MP

 

[SKIPPER1]

Hi MP,

Have used R3 = 1.2 Kohm 1% tollerance as described in the project page for 0 - 20 V range.
I have connected the pin #5 of the 2nd display (from left to right) to the R6 resistor.

Thanks for support and also for sharing this very useful project.

 

[audioguru2]

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.

View attachment 40089

 

[SKIPPER1]

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.

 

[audioguru2]

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.

 

[MP1]

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.

His comments are here:
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