Pull down resistors are often used with sensors, especially with resistive sensors like the Photoresistor (light sensor) and Force Sensitive Resistor (FSR). An easy way to normally spot a resistive sensor is that it will have two terminals (legs) instead of three.
Three terminal sensors typically have a power, ground and sensor reading terminal. However, a two terminal (resistive) sensor has electricity (voltage) flow through the component, as a result of the sensing elements, the current is impeded. For example in the case of the photoresistor, when bright light is hitting the component, the current will flow unimpeded and the sensor will read a high value. When there is little light hitting the component, the current will be resisted and you will get a low value.
The problem is we still need a complete circuit (where everything can flow to ground) and the ability to read the sensor reliably.
We do this with a Pull-Down Resistor: This is where one terminal is connected to the Power (3V3) and the other to a resistor which connects to ground. The terminal which connects to the resistor also connects to the analogPin. That gives us a complete circuit where the current flows to ground but it is safely and cleanly connected to the analogPin.
Spot the Difference
There is very little difference visually between a 1K and 10K resistor.
Make sure you use the right one.
A 1K resistor is brown, black, RED while a 10K resistor is brown black ORANGE.
Why do we do this?
Pull-down resistors are used to establish a ‘known output impedance’ or in other words give us a signal that we can reliably read from. It helps to reduce noisy and erratic readings in the sensor signal which results from the way resistive sensors work.
There’s a little bit of math behind this which uses Ohm’s law (see earlier)
You can read a more detailed explanation below.