A little plastic reflector bundles the light emitted by the SFH484 on the right. Quite reasonable, since the Spider has to stop in order to listen, or else his own noise might block reception of weak chirps. An obstacle has to be touched before avoiding action can be taken. It is more interesting if the Spider actually sees table legs, walls and other objects in its path, and changes course before contact is made. One or more infrared LEDs are used to transmit short bursts of modulated infrared light.
Photo diodes pick up any light reflected by obstacles.
Since 4.5 KHz is a quite acceptable modulation frequency, the ears can double as infrared receivers, if the Spider is able to switch from microphone to photo diode and back at will. One third of a 4053 analog multiplexer switches between infrared and sound. The bias circuit between the anode of the BPW41 and ground makes the photo diode nicely receive the weak infrared reflections even when a fairly strong light source is shining directly into its face. That may well be needed anyway, to make sure it doesn't see any direct light coming from the transmitter. For the same reason it is wise to put the transmitting LED not too close to the receiving BPW41.