Photodioda as removing the transmitter beam infra red with the pulsation frequency 5KHz, then the pulsation will be reflection by reflector in front of it. and received by the phototransistor as the receiver. The detected object is located between the transmitter-receiver with a reflector.
Circuit on the left is the transmitter modulation circuit that produces infrared 5KHz throbbing. On the right side is the receiver circuit, there is a resonance tuned amplifier. Band width it can control so the receiver circuit can catch the beat of the transmitter modulation 5Khz. Bandwidth control is also working to reduce the sensitivity of the effect of light in the surrounding areas.