This circuit gives an audible indication when any object comes in front of the infrared reflecting sensor (containing IR LED and phototransistor). The sound generated by the sensor will be louder if the object close to the reflecting sensor is opaque.
The home-made reflecting sensor contains a 5mm IR LED and a phototransistor. It can detect up to a longer range than commercial sensors like HO-4R. It can be used for indoor IR proximity detection.
The circuit uses an IC 555 wired as astable multivibrator. The 10kHz signal produced by the multivibrator is fed to the base of transistor T1. This signal is further fed to the IR transmitting LED used in the sensor. When an object comes in front of the sensor, modulated light emitted by the transmitting LED of the sensor is reflected back and sensed by the phototransistor of the same sensor.
The signals sensed by the phototransistor of the sensor are amplified by transistor preamplifier T2. These are further fed to a power amplifier based on IC LM386 to drive a speaker. The tone of sound can be varied by changing the value of tone capacitor C3 of IC1 (NE555).
To make a home-made sensor place the IR transmitting LED and the phototransistor in a piece of bakelite, parallel to each other such that the transmitted IR beam, after reflection, can be received by the phototransistor.
The circuit works off a 9V battery. Assemble it on a general-purpose PCB and enclose in a suitable cabinet. Connect the sensor such that it is oriented towards an approaching object.