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Led display Digital Voltmeter question


bonanz
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Hey gogo,
Sorry for the late reply. I was on a business trip in Asia for the week and just got back this morning.
Here are the eagle files for both schematic and board. Feel free to make any changes you need. This zip has a parts list for the LEDs used in the design if you would like to use them. Otherwise, you might have to make changes to the schematic for the one you want to use. Also, you might be able to use one of the edit commands to just use a different package in the board layout.

MP

icl7107-7106.zip

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  • 3 weeks later...
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Hi,

I'm a newbie here. I just want to share some experience in doing this project since many problems encountered are very similar to other big brothers.
I have done this project with Smart Kit version and created the PCB with information in this webpage. Mine is configured in 20v DC range. In the very beginning, after I have finished, I found that the measured voltage cannot be locked with fluctuation of about 0.6v (my measured voltage should be regulated 4.5v, fluctuation range = 4.2v to 4.8v). I have measured the target voltage with another multimeter and confirmed that this voltage is stable. After the troubleshooting, I found that the following steps can help to mitigate the problems:

(1) flux residues must be clear after soldering.

(2) my voltmeter sourced by LT1085 regulated in +5v and attain -5v from ICL7660. But, my transformer's secondary output shared command ground with another power supply(9v-0-9v). The fluctation problem is mainly come from here. After separated the transformer, the fluctation solved in a great extent with only 0.05v (acceptable range for me).

(3) proper grounding and shielding for the voltmeter circuit.

Just my two cents. Hopefully these can help.

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  • 2 weeks later...

Hey wassup people! Man tonnes of work has gone into this circuit, I'm very impressed with all these radical ideas about thermometers and ammeter adaptations! Who'd have thought an IC first manufactured in 1977 (7106, 7107 is the sister IC) would still be so very useful today!

Anyway I just wanted to clear up a few things:

a) http://www.electronics-lab.com/projects/test/007/index.html
b) http://www.electronics-lab.com/projects/test/014/index.html

^ Are those two circuits exactly the same, if not do they accomplish the exact same thing?

I know that this project can be converted into an ammeter, I'd like to know if any of the above circuits (a & b) will measure current from 0 to 200 mA? Or better yet, 0 to 120 mA only. I have a circuit that supplies 0 to 120 mA to a 100 Ohm load and I want to know if this project can measure the current in the load to .1 decimal places perhaps.

Is it a simple case of using a 10 Ohm resistor for R3 according to circuit b? Will that give me a 0 - 200 mA display if I hooked this up as an ammeter?

Appreciate any replies.

v5 out.

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Hi Vicious,
You want the current-measuring resistance to be as low as possible. You don't want to lose too much voltage across it nor ruin voltage regulation.

The IC is capable of providing a full-scale voltage of only 200mV but the Smart Kit version has 2.0V. Then if you use the Smart Kit version with a 10 ohm current-measuring resistor for 200mA full-scale, the circuit you are measuring will have up to a 2.0V loss and its voltage regulation will be terrible.

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V5,
This thread was actually started from schematic (b). It is also the most common one used, since the LEDs are more common.
There is not a significant difference on these two projects other than the dual LEDs. They both get the same job done.  Schematic (b) should be a little more stable since it has a smoothing capacitor across the input. Other members have reported that schematic (a) shows some instability in displaying the numbers. Too sensitive without the cap.
Also, go back to reply # 26 in this discussion. This is where discussion of using this chip for an ammeter starts. You will most likely find some information there that will be helpful.
I highly recommend that you read all of the replies in this thread. I compiled three different conversations into one thread so that all of the discussions were in the same area.

MP

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  • 2 months later...

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

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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.

post-1706-14279143219083_thumb.png

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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?

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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
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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

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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.

post-1706-14279143219551_thumb.png

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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

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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

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