# Analog voltmeter modification

Discussion in 'The Projects Forum' started by nathomas, Mar 9, 2011.

1. ### nathomas Thread Starter Member

Mar 3, 2011
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0
I have to measure voltage between 0.75v and 3V. The voltmeter should not go down to 0 and I want to scale the whole voltmeter between this voltage. I am using the voltmeter to measure the output of an rtd sensor whose output is scaled from 0-5 V but will be measuring around only at the above given voltage of 0.75-3V. could anybody suggest an idea..thanks.

2. ### blueroomelectronics AAC Fanatic!

Jul 22, 2007
1,758
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Adjust the zero set screw so zero is 0.75V, of course you'll need a new scale.

nathomas likes this.
3. ### nathomas Thread Starter Member

Mar 3, 2011
82
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But how do I scale the voltmeter that reads 0-5V to read 0-2.5v. i.e
1-2.5V ==> 0 - 5V ?
So when we are actually getting 2.5V the voltmeter should be showing 5V.
And when we re getting 1V the voltmeter should show 0V.

4. ### wayneh Expert

Sep 9, 2010
12,396
3,246
Use an op-amp. It will allow setting both an offset and a scale factor. The downside is you'll need to power it and have a little PCB for it.

Why not just use a cheap DVM - they're under \$5 - and use a quick lookup table printed out of Excel. For example, 2.15v = 37°.

5. ### nathomas Thread Starter Member

Mar 3, 2011
82
0
Thanks Wayneh. well I am going to use a pcb as I have more components for the circuit. completely forgot abt using an op amp. i cannot use a dvm as thats the design requirement. how can i offset the 0v to a 1v in an opamp.

6. ### wayneh Expert

Sep 9, 2010
12,396
3,246
One input to the op-amp is established by a voltage divider including a pot, supplied with a voltage reference. The pot can then be used to dial in the offset voltage. Gain is set by a feedback loop from the output.

Feb 5, 2010
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8. ### nathomas Thread Starter Member

Mar 3, 2011
82
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@Kermit:None of the links on the page work.but thanks.now i know what Im looking for.

9. ### nathomas Thread Starter Member

Mar 3, 2011
82
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I saw this article about using two voltage regulators as ESV but I cannot seem to find the circuit diagram. And if I thought of using the zener diode method, are there 1v zener diodes in the market?

10. ### nathomas Thread Starter Member

Mar 3, 2011
82
0
the ESV in all the designs mentioned uses an ammeter rather than a voltmeter. I have an existing voltmeter that I want to use.

Feb 5, 2010
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