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Force sensitive resistor output voltage

shivasage

New Member
Hi guys. I'm attempting to create a simple circuit for obtaining a variable output voltage controlled by a force sensitive resistor which I'm controlling with my finger. The input voltage is +9V DC center negative. The output voltage will connect to a musical device with a "control voltage" input, i.e. an input which is expecting to see somewhere between 0V and +5V and changes a parameter depending on what that voltage is. This input has an impedance of 100k. I've included a 50k pot in the circuit to add further fine-tuning of the effect of the FSR on the output voltage. The FSR ranges from >1M ohms with no pressure to ~68k ohms with maximum pressure.

sensor.png
(the 820k resistor is representing the FSR, the 483mV the output voltage, the 100k resistor the musical device input)

In this circuit simulator, as I increase pressure (i.e. decrease resistance), the output voltage increases. The problem I'm having is that, when I breadboarded the circuit, everything worked more or less, except that instead of starting at minimum voltage and increasing with increased pressure, it did the exact opposite. With no pressure, the voltage is at maximum, and as I increased pressure, the voltage went down. Why is this happening?

breadboard.jpg
 

rjenkinsgb

Well-Known Member
Most Helpful Member
You appear to have your meter connected across the force-sensitive resistor, rather than the 100K load / output resistor?

The FSR is near enough open circuit with no pressure so you are just reading the pot output voltage with 100K in series with the meter.
 

shivasage

New Member
You appear to have your meter connected across the force-sensitive resistor, rather than the 100K load / output resistor?

The FSR is near enough open circuit with no pressure so you are just reading the pot output voltage with 100K in series with the meter.
You're right, thank you!!
 

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