Chosing a pull down resistor for an LDR: Axel Benz formula

LDR_circuitIn deciding on the value of the pull down resistor in a voltage divider with an LDR, one needs to take in account the range of light levels one wants to measure.
If you want to have the sensor in a bright area and use a 10KΩ pull-down, it will quickly saturate. That means that the voltage over the pull-down resistor will soon be 5V and you will not be able to differentiate between kinda bright and really bright. In that case, it is better to replace the 10KΩ pull-down with a 1KΩ pull-down, but then it will not be able to detect dark level differences as well but it will be able to detect bright light differences better. This is a trade-off that you will have to decide upon!

You can also use the “Axel Benz” formula by first measuring the minimum and maximum resistance value with a multimeter and then finding the resistor value with: Pull-Down-Resistor = squareroot(Rmin * Rmax), this will give you slightly better range calculations.

axel benz