# Arduino Forum

## Topics => Science and Measurement => Topic started by: tim1365 on May 28, 2018, 05:29 pm

Title: how to measure negative voltage
Post by: tim1365 on May 28, 2018, 05:29 pm
Dear all,
I want to use arduino to read an output of photodector. But the output voltage is on the negative voltage range and depending of incident light is in the range from -100mV to -1V. How to convert the signal from negative to positive region, idealy via 1:1 invertor. I have read about inverting operational amplifier (op), and how a question about connection of op, photodetector and arduino. How do do such a connection and which op it is better to use?
Title: Re: how to measure negative voltage
Post by: wvmarle on May 28, 2018, 05:45 pm
A three-resistor voltage divider (https://electronics.stackexchange.com/questions/266786/arduino-negative-voltage-reading-with-positive-ground) may work for you.

But a much smarter thing for you would be to post more details in your project, as outlined in the sticky (http://forum.arduino.cc/index.php/topic,148850.0.html) that you apparently didn't read or ignored.
Title: Re: how to measure negative voltage
Post by: GolamMostafa on Jun 15, 2018, 10:20 pm
You may try this op-amp circuit. 741 is usually operated on dual (+/- : +5/-5 or +12/-12) power supplies. However, it can also be operated in single supply to get expected result. The ouput of the op-amp could be connected with Channel-A0 of Arduino UNO.
(http://www.krdcbd.com/opa.png)
Title: Re: how to measure negative voltage
Post by: allanhurst on Jun 26, 2018, 12:26 am
The 741 is ancient and won't work with a total 5v supply , and it's input isn't compliant down to the 0v supply.

Try 1/2 a LM358 - that'd be OK.  And set your arduino a/d reference to 1.1v for better resolution.

But in general a photodode detector is much better run as a current source with a transconductance amplifier - I'll see if I can dig out a reference.....

Allan
Title: Re: how to measure negative voltage
Post by: allanhurst on Jun 26, 2018, 12:37 am
Found it....

See NatSemi's introduction to photodiodes....

http://edge.rit.edu/edge/P09051/public/photodiodeamplifers.pdf (http://edge.rit.edu/edge/P09051/public/photodiodeamplifers.pdf)

In general a much better approach.

Allan
Title: Re: how to measure negative voltage
Post by: MarkT on Jun 28, 2018, 12:53 pm
Dear all,
I want to use arduino to read an output of photodector. But the output voltage is on the negative voltage range and depending of incident light is in the range from -100mV to -1V. How to convert the signal from negative to positive region, idealy via 1:1 invertor. I have read about inverting operational amplifier (op), and how a question about connection of op, photodetector and arduino. How do do such a connection and which op it is better to use?
What photodetector?  Give links for all hardware please, we cannot even begin to guess these details!

Also what bandwidth are you looking for? (An LM358 for instance is totallt hopeless for anything high bandwidth,
but then again so is the Arduino ADC).

In case anyone's wondering, the LM358 won't even do audio bandwidth at full signal swing.