A close up of the eye's. The stalks are photodiodes and control the opposite motor to follow lines. Then there are IR fhototransisters on the front and sides. The IR transisters are surrounded by IR emitters or the same frequency, and control same side motors for wall avoidance. As it gets closer to an opject the IR from the emitters is "seen" by the transisters and over powers the photodiodes (in theory). I worked, but as usual, not perfectly.

Close up of the IR assembly.

Naked

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In an effort to advance twddlebots performance, I decided to use the head circuit for its intended purpose.
My initial assemble allowed for much versatility when trying different configurations. The chasse, motor and sensor cables where wired to an accessible plug. The plug acts, sort of as a backbone to which is attached the head circuit and power supply.

For the head circuit I used Wilfs SPH. I took the gearmotor from a two-degree freedom head. I had tried.

The eye sockets were taken from a destroyed Styrofoam airplane kit. The eyes are centered with a rubber band, and at the left and right extreme of travel the head will hit a touch sensor (short). At the left and right extremes the right or left motors are shorted to the battery causing a fast turn. I added some IR emitters (from the previous design) under the eyes to brighten the line to be followed. The two yellow LEDs are just for show. They are on the battery side of the left and right short. I originally intended to use resistors to calibrate the turn rate, but the LEDs facilitated a voltage drop essentially doing the same thing with a bright show to boot :-D The head worked very well, with a dramatic shake at worse, but when locked onto a bright target was almost infallible.

So, all worked well. It’s the day of the competition, and I put in fresh batteries, and the last gear to the head breaks at the start of the race :-( Bitter failure can only be appreciated when occurring in front of fifty people or more. Oh well… next time. Watching this guy chase the light from a flash light on the floor was worth the effort.