# Load Resistor Use for Voltage Sensing by Arduino

I connected a piezoelectric film to a Arduino Uno board via analog input. However, as I was getting very high voltage values (reaching 5 volts without much movement). Therefore, I added a 1 megaohm load resistor in the circuit to get lower and readable voltage values on the monitor. Now, I am connecting the films in series and thus the voltage should theoretically increase. However, the voltage values on the serial plotter appear less than before with only one film. Do I need to switch the load resistor to a lower to higher value?

I was thinking that since the voltage value theoretically doubles, I should use a 500k load resistor. However, I am not too sure since I don't know the exact reason why we need to add a load resistor in the first place. All I know is that electrical energy generated by the PVDF films is very low, so current must be very, very, very low.

Thank you!

• google ohm's law to learn the relationship between current, resistance and voltage – jsotola Jun 22 at 4:51
• but that does not answer my question of how to adjust the voltage value frame on the Arduino IDE plotter when my voltage values increase. There are lower values indicated on the graph and I don't know if it is because I have to change the resistor. With just one film, the voltage values spike to 5 volts. With two films, the voltage values spike to only 3 volts. – ARJ Jun 22 at 5:37
• read your own post ... it says nothing about adjusting anything in the IDE plotter .... you asked if you should use a higher or lower value resisor ... hence my comment – jsotola Jun 22 at 6:40
• PVDFs aren't batteries. You can't series them like that. – Majenko Jun 22 at 7:22
• Guys, comments are for clarifying the question, not answering it. You see that nice big "answer" box? Post your answer there. Thanks! – Nick Gammon Jun 22 at 11:10

It is an easy mistake to make to think putting a resistor in series reduces the voltage. It will, but how much will depend a lot on the current flowing. See Voltage Divider - Wikipedia.

The Arduino analog input will draw very little current, thus very little voltage will be dropped. You probably want at least two resistors: one in series with the device, and one to ground.

For example, a 1k resistor in series, and a 1k resistor to ground will halve the measured voltage, assuming very little is drawn by the analog input.

Using a large single resistor (eg. 1M) with very little current drawn will only drop the voltage slightly, as you found.

Your big mistake is equating a piezoelectric film with a battery. If you put two batteries in series the voltage will double, true. However a PE film is not like a battery.

The big difference is that a battery is always providing potential (voltage) but a PE film only provides potential when it's being compressed. At all other times it acts like a capacitance with leakage resistance.

Simply put: one PE film is blocking the current flow from the other, and it's that which is giving your voltage difference.