# Help - Great idea, minimal ability! Test meter design

Discussion in 'General Electronics Discussion' started by RaceIt, Apr 9, 2012.

1. ### RaceIt

3
0
Apr 9, 2012
I am looking to create an analogue dc voltmeter as a item of test equipment be used to fine tune a mechanical linkage (attached to a pot) to achieve exactly 2.5v. Hence I would like 2.5vdc at the centre point and simple instructions to adjust left on adjust right on either side. The ideal would be approx 1.5vdc at the min point, 2.5vdc at Centre and approx 3.5vdc at the max , i.e. giving a relatively fine reading of values deviating either side of the critical value of 2.5v.

I had thought this would be easy (even with my limited electronic skills) using an off the shelf analogue multimeter with a 2.5vdc option (not quite the resolution I ideally wanted but close enough). I set about soldering a voltage divider with the intention of halving the signal voltage the hence achieving 2.5v signal = actual reading 1.25v = needle at centre. The problem here is, having tried various resistor values the divider circuit appears to pull down the supply voltage. It is obviously essential that the measuring device does not affect the supply voltage.

My questions therefore are:

Is there perhaps a neater and cleverer way of achieving this with some form a comparator chip based circuit perhaps driving Led's?

Is there any way specifying the optimum resister spec for such a divider?

Are diodes my answer, in which case how do I integrate and or spec the right component?

2. ### GreenGiant

841
6
Feb 9, 2012
if you have an exact reading of the supply voltage, and exact readings of resistor values then you can easily set up a voltage divider to give you 2.5 volts +/- .05 volts

first off though, have you tried using a digital multimeter I know that mine has 3 digit resolution

3. ### RaceIt

3
0
Apr 9, 2012
Greengiant,
Digital multimeter obviously achieves the end goal but doesn't have any of the elegance and simplicity of an analogue reading. This is all about achieving the simplicity of the user interface. A simple visual display of how much you need to move and which direction - hence to my mind analogue is king.

Obviously the supply voltage varies (either side of 2.5v) from 0 to 5v but i'm really only interested in the 1.5-4.5 bit.. I thought the voltage divider was simple but as I say the problem is the effect it has on the supply?
user

4. ### GreenGiant

841
6
Feb 9, 2012
is the whole purpose to get the meter dead center?

or is the supply going to be used for something else?

5. ### RaceIt

3
0
Apr 9, 2012
Yes, basically a sprung lever where the centre and natural resting point is controlled by mechanical stops. The meter is to assist in setting/checking/maintaining the centre point at the 2.5v position - this can be lost as mechanical components, move/wear/bend etc. In normal life the lever is then operated to move the signal voltage up or down.

A user could be checking 30 of these at a time and mechanical adjustment is relatively difficult. Hence I'm trying to create the simplest possible, intuitive setting assistance device.

6. ### GreenGiant

841
6
Feb 9, 2012
You will probably have to use some diodes.

Using resistors you will have to do the math differently every time, depending on the voltage of the source, but using a diode (zener diode rated at 2.5 volts) that would guarantee that you get a 2.5 volt reading every time, plus or minus a small amount