Building a DIY multimeter for my school project

[Projecthelp243454]

TL;DR Summary

I need to build a DIY multimeter for my shcool project but i have no clue how to, please help

Hello all,

I am a college student from the UK and as part of my course i have to do a project, I have been given a project to do which is to Design a portable electronics circuit that can be used for training. It must be;
Able to measure current, voltage and resistance.
Operate on 10 Volts DC, but plugged into the mains. (You will need to rectify and transform it down.)
Have a maximum of 5 resistors that can be in series, parallel and complex.
I have a tight budget and any help would be welcome,
Thanks

 

[berkeman]

Summary:: I need to build a DIY multimeter for my shcool project but i have no clue how to, please help

Hello all,

I am a college student from the UK and as part of my course i have to do a project, I have been given a project to do which is to Design a portable electronics circuit that can be used for training. It must be;
Able to measure current, voltage and resistance.
Operate on 10 Volts DC, but plugged into the mains. (You will need to rectify and transform it down.)
Have a maximum of 5 resistors that can be in series, parallel and complex.
I have a tight budget and any help would be welcome,
Thanks

View attachment 264056

Welcome to the PF.

Fun project! Your thread may get moved to the DIY forum or the Schoolwork forums at some point, FYI.

You can't build a DMM with just resistors. Do you have an Arduino or other microcontroller (uC) with an on-board ADC or something to use for digitizing? What voltage, current and resistance ranges are you required to support? What is the required accuracy?

And on the AC Mains power thing, do you have a safety-approved "wall wart" AC transformer that you can use as a building block? If you have no experience with AC Mains power, it is very dangerous to be trying to build something from scratch without close Mentorship and safety supervision.

https://www.howtogeek.com/thumbcach...-content/uploads/gg/up/sshot4da266d2581a8.jpg

 

[tech99]

I don't think it has to be digital. The Ayrton, or Universal, Shunt is an interesting circuit for this sort of thing.
https://en.wikipedia.org/wiki/Ayrton_shunt

 

[jrmichler]

This is my 1 to 12 volt power supply that I built back when. The ammeter is a one milliamp meter with a shunt resistor made from a piece of copper wire to make it a one amp meter, and the "milli" painted over.

A voltmeter is a low current meter with a series resistor. And a resistance meter is a voltage source in series with a current meter, a series resistor to set the range, and the unknown resistance. You should be able to do the necessary calculations to find the correct resistors.

Hint: You can get almost any resistance by connecting two or three resistors in some combination of series and parallel.

And, yes I was aware of the TCR of copper at the time, but felt at the time that a copper shunt would be good enough. And it has been, and still is, good enough.

 

[berkeman]

to make it a one amp meter, and the "milli" painted over.

Love it!

 

[DaveE]

I would suggest that you start with a block diagram of how you will implement these functions. You will want to focus on commonality in the calculations, what is easiest to measure and how can that be used to complete the measurements you need to do. You can worry about the implementation details later, after you have a road map in mind.

 

[Tom.G]

I don't think it has to be digital. The Ayrton, or Universal, Shunt is an interesting circuit for this sort of thing.
https://en.wikipedia.org/wiki/Ayrton_shunt

Watch out for that wikipwdia article, the text description does not match the schematic!

 

[sophiecentaur]

@Projecthelp243454 It would be useful for PF and essential for you to get more idea of what the actual requirements are for this project. It has been very 'under-specified', if what you have posted is the only information you have. You would be perfectly within your rights to send a query to your teacher about this if you have "no clue".

I'd strongly suggest initially avoiding the D in DMM. It adds a lot of necessary extra levels of understanding and extra possibilities of problems.
You need a simple analogue milliAmmeter, a handful of resistors and a couple of rotary switches to select the 'M' options. There is plenty there for you to get started on the basics and you will be able to fault find and identify the way the switches can be arranged. IT WILL WORK, too.

If you really have to do this digitally (is it actually in your brief?) then I presume you will start with a basic digital voltmeter unit. But deal with that, only if necessary.

 

[DaveE]

I'd strongly suggest initially avoiding the D in DMM.

Yes. Or, you can buy low cost digital panel meters too. Like the worlds simplest voltmeter.
For example:
https://www.ebay.com/itm/0-28-inch-Mini-Digital-Voltmeter-Panel-Mount-LED-Voltage-Volt-Meter-DC-2-50-30V/311416527438?hash=item4881df1a4e:m:mmSr36Oz7J-dqIHQUnIcA8Q

 

[Baluncore]

It must be;
Able to measure current, voltage and resistance.

Have a maximum of 5 resistors that can be in series, parallel and complex.

So it must be be a minimum moving coil meter for demonstration of concept. A voltage scale series resistor, a current shunt and a non-linear ohms scale.
No AC range is needed. No dangerous voltages on the meter circuit.