LDR and transistor
Our LDR-brick changes its resistance not mechanical, like the potentiometer, but depending on the light entering the sensor. If the LDR-brick is getting light, the resistance value becomes smaller and the current ow is larger. Its resistance value reaches a very high amount of several 100kΩ at darkness, but a very low value of a few 100Ω with a bright light, the maximum change is about 1000 times. The potentiometer is connected in series with the LDR-brick, that allows to find the threshold of the yellow LED by turning the knob. At the left position the LED does not light up, at the right position the transistor is active and the LED lights up. The optimum for the threshold and a maximum sensitivity can be found between both values.