IR Detector with two indicating leds
I have drawn a schematic for a simple circuit that will do what you want. When enough light shines on the photo diode, PD1, Q1 turns on enough to hold Q2 off and current flows through R5, D2 to light up LED2. When light is blocked from reaching PD1 , Q1 will turn off, and current from R4 will turn on Q2, sending the current from R5 in to LED1. Since photo diodes produce some current even when not fully illuminated, R3 must be adjusted to make Q1 turn off. R1 and R2 limit the current in their respective diodes.
I have assumed that PD1 only produces a few microamps. If it is more like 100 uA, it can be put in the place of Q1 and R2,3 can be omitted. It is usually necessary to shield the photo diode from ambient light; a short peice of black tubing slipped over the diode is the easiest method. Transistors are any general purpose type: 3904, 2222, BC547, etc.