I am working on a project which includes two FSRs (force sensitive resistors) connected to an Arduino Uno. I have each FSR setup as in http://www.instructables.com/id/FSR-Tutorial/?ALLSTEPS, but each FSR is connected to a different analog In pin. My question regards the value of the pull-down resistor in the voltage divider circuit. In any tutorial I have found online a 10k resistor is recommended but I'm not sure why. I have a feeling its to do with the FSR's sensitivity but havnt found an explanation anywhere.
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2 Answers
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The cynic in me says that everyone uses a 10kΩ resistor because under the technical data section of the FSR above, a chart is given with output voltages specified with 5V supply and a 10k pull-down resistor. I have copied the chart below for reference: \$\begin{array}{} Force (lb)& Force (N)& \begin{matrix}FSR\\ Resistance\end{matrix} &(FSR + R) Ω&\begin{matrix}\text{Current thru}\\ FSR+R\end{matrix} &\begin{matrix}Voltage\\ \text{across }R\end{matrix} \\ \hline None& None&\infty&\infty&0 mA&0V\\ 0.04 lb &0.2 N &30kΩ&40 kΩ&0.13 mA&1.3 V\\ 0.22 lb &1 N &6 kΩ&16 kΩ&0.31 mA&3.1 V\\ 2.2 lb &10 N &1 kΩ&11 kΩ&0.45 mA&4.5 V\\ 22 lb &100 N &250 Ω&10.25 kΩ&0.49 mA&4.9 V \end{array}\$
So we can see that a 10kΩ gives us a reasonable amount of range over the operation of the sensor. If we wanted to have better resolution with the smaller force ranges, we could increase the pull-down resistance. We could also decrease the pull-down resistance to improve the resolution at higher force ranges, although that would also come at a cost of greater current draw.
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\$\begingroup\$ \infty \$\rightarrow \infty\$ I created a cheat sheet with these things: meta.electronics.stackexchange.com/a/1245/8627 \$\endgroup\$– jippieCommented Feb 12, 2013 at 16:50
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As the technical details for the AdaFruit Force Sensitive Resistor indicate, the resistance value of the sensor varies from around 250 Ohms to 30 kOhms (or infinite, for no force applied).
Therefore, to get a reasonable spread of voltages from a voltage divider consisting of the FSR and a fixed resistor, the resistor could be of a value anywhere within that range, such that at minimum force applied, the voltage is close to zero, while at maximum expected force, the voltage is close to Vcc i.e. 5 Volts.
Thus, the resistor needs to be greater than 250 Ohms, but less than 30 kOhms. A 10 kOhm resistor is a pretty common value in most experimenter's parts box, since this is also the most commonly used value for digital line pull-up or pull-down resistors. Thus, it is as good a value as any, for initial experimentation.
Should you later want to optimize the voltage range for a specific pressure range of interest, such as really firm pressures, or very light pressures, then the resistor can be replaced with lower or higher values respectively, to bring the voltage divider outputs in the region of interest to spread better across the ADC range (0-5 Volts).
answered Feb 12, 2013 at 15:52
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\$\begingroup\$ Ok thanks very much. I am powering my Arduino with a 9V battery, if there is a high force on the FSR the total resistance will be just over 10kOhms. Is there a possibility of draining the battery with such a low resistance? Thanks again. \$\endgroup\$ Commented Feb 12, 2013 at 19:06
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1\$\begingroup\$ @user18925 With 10 kOhms, the resistive divider will draw 900 microAmperes. Taking an Energizer 522 as an example, that translates to over 666 hours to drain the battery. The rest of your circuit will be a far greater drain, thus the defining factor for battery depletion - not the FSR + resistor part. \$\endgroup\$ Commented Feb 13, 2013 at 2:42