# Designing weighing scales

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#### Amelia

##### New Member
Hi,

I have a project to design a circuit that takes the analog readings from 4 strain gauges in 4 groups to produce 4 digital output (therefore making 4 scales-is for cars so you can take a reading at each wheel). My issue is that my electronics knowledge is limited (mechanical engineer) and i;m worried ive taken on more than I can chew. The 4 strain gauges are already wired up so that I have on shield ground, an input and output for the analog display and 12V input and output.

How complicated is it to wire an ADC up? I believe I need an ADC0808 that takes 8 analog inputs, but I am really quite confused from that point on.

I think this may be beyond me, but I just wanted to see if anyone could help a little and perhaps tell me just how difficult this is to do.

Thank you

Amelia

#### Externet

Four cheap digital multimeters at $5 each can give you the 4 digital readouts. That is what a digital multimeter is, an analog to digital converter, with some dividing circuitry. If the strain gauges are linear, extra dividing circuitry can make the readouts match grams, or kilos... Removing their housings, selector knob and switches will leave the bare A/D IC on a pc board. Another way is to search on the net for digital voltmeter schematics, find the parts and build the circuit, make the pc board and calibrate if not in kit form. #### ericgibbs ##### Well-Known Member Most Helpful Member Hi, I have a project to design a circuit that takes the analog readings from 4 strain gauges in 4 groups to produce 4 digital output (therefore making 4 scales-is for cars so you can take a reading at each wheel). My issue is that my electronics knowledge is limited (mechanical engineer) and i;m worried ive taken on more than I can chew. The 4 strain gauges are already wired up so that I have on shield ground, an input and output for the analog display and 12V input and output. How complicated is it to wire an ADC up? I believe I need an ADC0808 that takes 8 analog inputs, but I am really quite confused from that point on. I think this may be beyond me, but I just wanted to see if anyone could help a little and perhaps tell me just how difficult this is to do. Thank you Amelia hi, You say you want to weigh each wheel; the analog system you have described has only one output.?? from the 4 load cells. The 4 groups of individual strain gauges are most likely wired as a strain gauge bridge. [Forming a load cell] The 4 [load cell] bridges I believe are connected together to give a combined analog signal output, that would be suitable for the analog display meter. The ADC0808 adc is only 8 bit resolution, so for a vehicle weighing say 1000Kg the resolution will only be 4Kg/bit at best. We need more detail regarding the weight load range, the spec on the load cells/gauges and the resolution/accuracy required. Last edited: #### Optikon ##### New Member Hi, I have a project to design a circuit that takes the analog readings from 4 strain gauges in 4 groups to produce 4 digital output (therefore making 4 scales-is for cars so you can take a reading at each wheel). My issue is that my electronics knowledge is limited (mechanical engineer) and i;m worried ive taken on more than I can chew. The 4 strain gauges are already wired up so that I have on shield ground, an input and output for the analog display and 12V input and output. How complicated is it to wire an ADC up? I believe I need an ADC0808 that takes 8 analog inputs, but I am really quite confused from that point on. I think this may be beyond me, but I just wanted to see if anyone could help a little and perhaps tell me just how difficult this is to do. Thank you Amelia The issue with the voltmeter idea is that it doesnt give you a digital output unless you get one that has an RS-232 port on it so you can transmit the readings back to a PC. That's not a$5 DVM BTW.

If you have the time to learn while doing this project (sometimes you dont!) then I would suggest going for wiring up an ADC to take your strain gauge readings. You can get 90% of the way done by reading the application notes and datasheets of the ADC part. Put together a schematic and post it here so we can all check it.

#### Optikon

##### New Member
Also consider if you really have the correct ADC for the application. As was pointed out, an 8 bit ADC is pretty "coarse" resolution.

Some things to think about:

1) Match full scale of gauge output to full scale input of ADC (you dont want to throw away dynamic range!

2) The process of weighing something is slow. You dont need an ADC that samples at MS/s. If you want high resolution, think delta-sigma.

3) Also too much resolution is bad. It is a difficult task to design electronics that preserve a 24 bit system for example. Your application sounds like 14 or 16 bits might be the way to go.

4) Resolution and accuracy are not the same thing. The adc will have gain error, offset error and linearity errors. The reference will also drift with time and temperature. Decide how much error you can tolerate for your final readings. Can the system be calibrated to remove some of these error sources?

Picking the right ADC at the start is very important.

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#### Amelia

##### New Member
Hi,

I didnt realise i;d had any replies to my message as it was put in my spam!

Since I last wrote I have done some research and realised I can use a ICL7107 to convert the signal to a digital reading. I have 4 strain gauges that are in a bridge system giving out one reading. I then need 4 of these in a box so I can have a reading from each wheel.

I have bought a pre-made circuit with an ICL7107 built into it and when wired up I get a signal, but no reading. I think you;ll be shocked that i;m doing this project when you realise how little i know! I;ve learnt about the ICL7107 circuit and was going to make it when I realised i could just buy one very cheaply. However, which range do I go for. The analogue output from my strain gauges is 0-20mV, with 12V supply. The digital system can only take 5V/-5V so I have used a 7805 and an ICL7660 to convert the voltage supply.

How do I get it to take readings, as at the moment the numbers don;t change at all when it is wired up. The weight on each scale will be maximum 250kg. Does the ICL7107 not work if i need to get this type of reading from it? DO I need to use a different ADC?

Externet, I did try taking apart my voltmeter, but it wouldnt let me. A quick trip to the amazing electronics market made me discover what was there. I live in China, so if only my Chinese was better i might be able to explain what I was doing to someone and i;d have one made in no time.

Still, i want to succeed in my challenge, would be great if I could complete this project.

Look forward to more posts and thanks for your help.

Amelia

#### ericgibbs

##### Well-Known Member
Hi,
The basic range of the 7107 is 200mV.

Your load cell has an output of 20mV at 250Kg, so even with a full load of 250Kg the 7107 would only display 20mV.
You require a amplifier that will raise the 20mV to say 250mV, this will give a direct reading in Kg.
An amplifier often used is the AD623 instrumentation amp,

The amplifier will also require a ZERO control in order to cancel out the weight of the load cell platform.

The load cell will need to be connected to the AD623 in the 'differential' mode.

Do you follow OK.?

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