An LDR has a resistance of only 15 ohms at a certain very high light level.  What value of protection resistor is needed if a current of no more than 10 mA is to flow when the supply voltage is 9.0 V?                                        

Current through LDR = 0.01 A (P)

Voltage across LDR = 0.01 A ´ 15 V = 0.15 V (P)

Voltage across protection resistor = 9 – 0.15 = 8.85 V (P)

Resistance = 8.85 V ¸ 0.01 A = 885 ohms (P)