Photo Diodes

Try something like this.

Parts List:

R1,R4 = 10K R2,R3 = 470 ohm R5 = 1K R6 = 100K - 1M (optional--see text) P1 - 10K, trimmer pot LDR1 = Light Dependent Resistor U1 = LM741, opamp D1 = 1N4001, General Purpose diode Q1 = NTE128, or any other replacement Ry = Relay

The circuit as shown act as a light sensor. Under normal conditions the resistance of the LDR is high, keeping pin 2 low. When light falls onto the LDR the resistance drops to a couple hundred ohms and triggers pin 2 high which biases the base of Q1 via pin 6 and R4 and in turn activates the relay.
Trimmer pot P1 and the two 470 ohm resistors, R2 and R3, are a voltage divider to adjust for sensitivity.
If you want the action reversed (make it a dark sensor), change the positions of the LDR and R1.

If the relay 'chatters', add a bit of hysteresis by adding a 100K to 1Meg-ohm resistor (R6) over pins 6 and 2 of the 741 op-amp, but in most cases 100K to 330K will do the job. The LDR is a regular, general purpose type. D1 serves as a spark-arrestor when the relay contacts open.

However a Light Dependent Resistor (LDR) is used instead of Photo diode as asked, this schematic can be used.

Besides a Photo diode, you can also use a photo transistor.