Scaling (down) voltmeter 0 - 0.5volt

Hi, I want to measute small voltages. 0 - 0.500 volt. The arduino analog ports can by default read 0-5 volts. And I want to scale down this resolution to 0 - 0.5 volts. How do I do this? I know how to scale up, with a voltage divider, but how do I scale down?

Thanks.

You can use a non inverting op amp.

Or you can change the Vref of the analog port to 0.5V. I'm not sure of the arduino is sensitive enough at this range though and you might need the op amp solution mentioned earlier.

Using an external 0.5 volt reference as suggested above would give the best accuracy (assuming a sufficiently accurate external reference) but you could also use the internal 1.1 volt reference. This should still give you much better than 1% accuracy.

What accuracy do you need?

I want an accurency of 0.001volts. I did get some op amps today, but are just learing how to use them. I have manage to bump the voltage to around 4.5 volts but I got a new problem. No matter my in signal I allways get like 4.5 volts out? 0.2 gives same as 0.3? I assume Im using the op amp in the wrong way.

How should I use the op amp? I want to measure from 0.5 - 1.5volts. I want to use this input do some magic on it and then put it on a pwm pin to control the speed of a motor. Best possible resolution.

No matter my in signal I allways get like 4.5 volts out? 0.2 gives same as 0.3?

You need some negative feedback, how are you wiring them up?

http://www.play-hookey.com/analog/non-inverting_amplifier.html

Tried the negtive feedback now, still the same, wiered em as in the link u sent. my pwn pulse evens out after the op amp. That is no pulse left only steady voltage ..

Its whey I apply my supplt voltage my signal goes away?

Well if you stick to your original 0-0.500vdc desired signal range, just do the:
analogReference(INTERNAL);
in your setup code and the measurement range of the analog input range will be 0-1.1vdc with about a .001vdc resolution.

Lefty

Tried the negtive feedback now, still the same,

So you have not wired it up correctly. If you wire the output to the -ve input the output will be the same as is on the +ve input. If not you have something wrong.

What op amp are you using? What power rails are you using.

my pwn pulse evens out after the op amp.

What does that mean? Are you feeding PWM into this op amp? You shouldn't be.

I shouldn't?

I have a PWM signal of which I want to increase the amplitude of. Maybe I have been unclear.

I have a PWM signal that has the amplitude of 0.5 volts. I want to increase this amplitude to say 5 volts. I almost got it working with the op amps, I got the amplitude to 5 volts but the period when the PWM signal suppose to be 0 volt it also increased to 4.5 volts :-/ How to I pull it down to 0 volts?

And its actually not a PWM signal either :stuck_out_tongue: its a PPM signal from a RC car receiver and I want to measure the duty cycle but I need to bump the amplitude since its so low that I cant catch it with pulseIn() :-/

Maybe I have been unclear.

Yes you have been. Everyone thought you wanted to measure a voltage.
When you say things like:-

I want an accurency of 0.001volts.

Everyone thought you had a voltage that you wanted to measure with an accuracy of 1mV.

So you just want to make a 5V signal from your 0.5V PWM signal. An op amp is not the best way to do this, it's a bit too complex. All you need is a transistor.
When you say:-

I got the amplitude to 5 volts but the period when the PWM signal suppose to be 0 volt it also increased to 4.5 volts

Do you mean that you are getting a signal between 5V and 4.5V?
If so you just want to run the op amp as a comparator. Have the signal fed into the +ve input and make the -ve input to a potential divider set at 0.25V.

As I said what op amp are you using. Cheap op amps can't operate within 1 volt of either rail.

Hi, sorry for being unclear, I guess Im unclear since I'm still learning and don't always know what or how to do things. But I'm getting there, thank you for your patients.

I', using a "741" op amp, I guess it wont work with 0.25 volts for pulling the voltage down. Just tested as you said and no reaction.

But you say all I need is a transistor? I currently only have MOSFETs will the do they job? Will the really open at 0.5 volts? Have to check that.

But if I understand your transistor idea you mean that I have a transistor opening at the 0.5V amplitude PWM pulse? and open say 5 volts to the Arduino. Have I understood this right? Then I only need to find a transistor that opens at 0.5 volts without a problem then.

I currently only have MOSFETs will the do they job? Will the really open at 0.5 volts?

No 0.5V is not enough to switch it on.

I', using a "741" op amp,

Then it can't operate close to the rails.

Then I only need to find a transistor that opens at 0.5 volts

If you ignore a germanium transistor which are very rare nowadays all transistors are made from silicon.
You need 0.7V to turn a transistor on so a lot depends on the input circuit of the thing you are trying to read.

Simplest is to try is a transistor with:-
base to your PWM input.
Base to a 47K (or there about) resistor with other end to +5V
collector to Arduino input pin (with internal pull up resistors enabled or a 10K resistor from the input to +5V)
emitter to ground.

Hi Grump_Mike,

Just reading your last reply here. I have just posted a similar problem on the Forum.

Simplest is to try is a transistor with:-
base to your PWM input.
Base to a 47K (or there about) resistor with other end to +5V
collector to Arduino input pin (with internal pull up resistors enabled or a 10K resistor from the input to +5V)
emitter to ground.

Where you say PWN to base with 47K resistor to base and +5V. You are using a pull up resistor to help the PWM get above the 0.7V required to switch the transistor?

Cheers