I read some articles about using an LED as a light sensor. I chose an IR LED and checked the voltage levels produced in different light situations resulting in about 200-300mV in full artificial light and around 30 mV in the dark, which seemed to be fine enough for an analog Arduino/AtTiny pin. The IR sensor LED also worked sufficient on the breadboard, but unfortunately the values seem to get messed up in my soldered setting which is an AtTiny dimming a 10W LED when a PIR gets activated and the light sensor value is low. In this setting I get much less sensitive analog readings from the sensor LED. The main difference are longer connecting cables as I want to place the sensor LED in some distance from the rest of the circuit.

* Can it be that the current of the LED is too low and therefore the reading gets disturbed by the circuit in some way?
* May I need a very high pull-down resistor for the sensor because without it the sensor pin floats in the dark?
* IR is supposed to be the most sensitive but actually I want to measure daylight which has a lower IR part than warm electric light. So maybe another LED would make more sense?

I know there should also be a resistor between the PWM and the MOSFET, but it seemed to work fine on the breadboard without it. 

![schematic](https://i.stack.imgur.com/JwFFT.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fJwFFT.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)