# Reversing voltage polarity

#### The Spin Doctor

Joined Jan 27, 2018
3
I am trying to breadboard a simple circuit that uses a piezoelectric transducer with an amplifier IC powered by a 5 volt DC powersupply. It all seems straightforward but it requires 5 v to the Ic amp which I have done. Then it needs a variable voltage -1 to -5 volts to the piezoelectric sensor. I am using a voltage divider (variable pot) from the same powersupply to create the range of voltages. It works great but only puts out positive voltages even when I reverse the + and - to the pot. How can I get -1 to -5 volts? Looks like I need a second powersupply? Thanks for the help!

#### MrChips

Joined Oct 2, 2009
19,780
"a simple circuit" is how they always describe it. Then why is there a problem if the circuit is so simple?

Why do you need -1V to -5V to the piezoelectric sensor?

A circuit diagram is the language of electronics.
If you post a circuit schematic that would go a long way in helping to reveal and finding a solution to your problem.

#### WBahn

Joined Mar 31, 2012
24,987
The problem is almost certainly that you can't get a negative voltage from a positive voltage supply using a voltage divider because the positive supply is already referenced to the same ground that you are now trying to get this negative voltage from.

Now, if what you need is a differential voltage between two points that is negative, then this can be accomplished using two positive voltages. But it doesn't sound like that is what you are doing.

We really can't do more than speculate unless you provide the schematic of the circuit you are trying to use along with a clear description of what you are expecting it to do.

#### AnalogKid

Joined Aug 1, 2013
8,232
The interface circuit is an inverting amplifier, similar to what is used with a photodiode. To get an output that is a positively increasing voltage with increasing force, the sensor output *current* is negative. Since the reference for the amp (the + input) is GND, there must be a negative voltage bias to the sensor. There are a lot of cutesy ways to get a few - volts from a +5 V supply, but the simplest way to test your circuit is with a single AAA cell with the anode (+ terminal) connected to GND.

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