Step 1: Material Required
1.npn transistor ( mine is 2n2222a of cdil)
2. 2 ir LEDs (mine is 940 nm each a wide beam to receive and a narrow beam to transmit)
3. 2 100k ohm trimpots
4. a 10k ohm trimpot.
5. batteries ( 3-4v)
6. ic socket(to change transistors
1. soldering iron
2. screw driver
Step 2: Circuit
It can explain more than words.
use ic socket instead of transistor so that you can change transistor easily to measure other's frequency.
Make two collector pins so that any transistor can be plugged easily.In last picture c,b and e refers to collector, base and emitter respectively.
Tape the two LED in front of each other so that the light emitted by one directly falls on the other as shown in picture.
Step 3: Explanation
Here IR led is used because it generates maximum voltage for low light as it compromises with current. since the transistor amplifies the current it has no effect on our circuit.