Besides the ordinary resistors we have been looking at so far there are also resistors with quite special properties. We are not going to bury ourselves in their characteristics but merely have them presented.
One which undoubtedly is known to everybody from Danfoss’ products programme is the photo resistor, the little “glass eye” which is placed in the burner tube of an oil burner watching whether the flame is ON or OFF. The official designation of this resistor is LDR = Light Dependable Resistor.
It is made of sintered cadmium sulphide, a material which changes its value of resistance depending on the amount of light to which it is exposed. The resistance value in the dark is usually 10 MOhm and at 1000 Lux it has decreased to between 75 and 300 Ohm.