Fish Hook Turbot Circuit Description

In the "BEAM Robotics: Step by Step" articles, published in SERVO Magazine, Tom Gray and I developed a Tumbler Turbot circuit. Fish Hook uses this circuit for its Nervous Net. Here is how it works:

The Bicore sets the heartbeat, either charging or discharging the Nus. The Nus charge thru CdS resistors. Whichever side is receiving more light will have less resistance and, consequently, will fire first. When an Nu fires, it inhibits the other Nu (thus mimicking the inhibitory property of biological neurons).

Now, the way to build a 2-dimensional Turbot is to wire it to be phototropic when right-side up, and photophobic when up-side down. This way, the correct motor spins to move the Turbot toward the light.

This behavior is built into the Nervous Net using a photodiode voltage divider as a “This Side Up” detector. When the Nv(2,18) is active, the “This Side Up” voltage divider’s output will be high when right-side up and low when up-side down. This output signal is fed into the enable pin, making for a Inverting Multiplexor (IMx).

If the robot is right-side up, the Nu output goes directly to the motors. The Turbot tumbles towards the light by running the motor on the same side. When the ‘bot is up-side down, the IMx inverts the Nu output. The Turbot runs the opposite motor, still tumbling toward the light.