input hardware

The input module offers extensive support for measuring and converting voltages.

• pin limitations (ADC2 and WiFi)
• averaging

Conversions

Voltage Divider

The input pins of the ESP32 can only measure up $3.3 \, \mathrm{V}$, which means that higher voltages need to be "broken down" before they can be measured correctly. For this purpose a voltage divider with two resistors of known resistance, $R_1$ and $R_2$, can be used to bring down the wanted voltage $U_0$ to the actually measured voltage $U$.
$U_0$ can then be calculated via $U_0 = \frac{R_1 + R_2}{R_2} U = \left( 1 + \frac{R_1}{R_2} \right) U$.

$$\begin{align} U_0 \hspace{20pt} & \circ \\\ & \, | \\\ R_1 \hspace{20pt} & █ \\\ U \hspace{20pt} & ├\text{-}\text{-}\text{-}\,\circ \mathrm{\:\:\: input \: pin}\\\ R_2 \hspace{20pt} & █ \\\ \mathrm{GND} \hspace{20pt} & ⏚ \end{align}$$

It is a good idea to limit the current to about $10 \, \mathrm{mA}$ and the maximum pin voltage measured to $3 \, \mathrm{V}$, which yields a value of about $300 \, \Omega$ for $R_2$. $R_1$ can then be calculated using the maximum expected voltage $U_{\mathrm{max}}$ with $\left[U_\mathrm{max}\right] = \mathrm{V}$ via $R_1 = 100\cdot U_\mathrm{max} - 300 \, \Omega$

Measuring Resistance

As the input is limited to measuring voltages, a variable resistance can only be evaluated indirectly, by using both a reference voltage and a reference resistor. Up to a few $\mathrm{k\Omega}$ this can conveniently be done using a voltage divider with the reference voltage

$$U_0 = 3.3 \, \mathrm{V}$$

and the reference resistor

$$R_1 = R_\mathrm{ref}.$$

By using the resistance under test $R$ instead of $R_2$, the resistance can be calculated as

$$R = \frac{U_0 - U}{U} R_\mathrm{ref} = \left(\frac{U_0}{U} - 1 \right) R_\mathrm{ref}.$$

For maximum accuracy the reference resistor should be chosen considering the maximum expected resistance $R_\mathrm{max}$ and minimum expected resistance $R_\mathrm{min}$ so that

$$R_\mathrm{ref} = \sqrt{0.5 \cdot R_\mathrm{max} \cdot R_\mathrm{min}}.$$

Furthermore the maximum current of $10 \, \mathrm{mA}$ should never be exceeded,

Measuring Electrical Current

An electrical current cannot be measured directly, instead the voltage drop across a known resistor $R_\mathrm{ref}$ caused by the current can be measured. Using the relation

$$I = U \cdot \frac{1}{R_\mathrm{ref}}$$

simply multiply the voltage with $1 / R_\mathrm{ref}$, i.e. use the inverse resistance as multiplicator. Note that the expected value for $U$ should not exceed $3,\mathrm{V}$.

Thermistor

Thermistors are cheap components and therefore quite common ways to measure temperatures. Their resistance significantly depends on the temperature, which is why it can be used as a proxy to measure temperature.

To carry out a measurement, first the resistance needs to be evaluated. This resitance can then be converted to a temperature, if the normal temperature, normal resistance and temperature coefficient are provided. Lastly, if a measurement in an offset temperature scale like Celsius is favored, the absolute temperature can be converted by subtracting the corresponding offset.