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 $U_0 = \frac{R_1 + R_2}{R_2} U = \left( 1 + \frac{R_1}{R_2} \right) U$.

     U0  o
         |
     R1  █
     U   ├----o input pin
     R2  █
    GND  ⏚

It is a good idea to limit the current to about 10 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.

• U_0 = 3.3 V
• R_2 = wanted
• R_1 = R_ref
• $R_\mathrm{ref} = \sqrt{R_\mathrm{max} - R_\mathrm{min}}$