The final step is compiling all this into circuitry. The actual electronic schematic for the creature is shown on the next page. The 11 neurons of our original design are implemented in a number of different ways. Some are modelled with voltage comparators, some with diode logic, and some with electro-mechanical relays. This part of the article presents the details of the implementation; if you are not an electronics hacker, feel free to skip to the next section.
Of all the circuitry, the oscillator for the infrared emitter (lower left corner) corresponds most directly to the neural model. Here, one section (triangle) of a quad analog comparator generates a square wave which is amplified by an MPS2907 transistor to drive the IR LED. The feedback from the chip's output to its positive input provides the "hysteresis" needed by our dual threshold neural model, while the resistor and capacitor on the negative input form the required integrator. The 10K resistor to +12V mimics the action of the always-on neuron.
The high level SEEK behavior is built from similar oscillator (center of page). The 220K resistors make the comparator into a Schmitt trigger, while the 1K resistor to +12V serves as the always-on neuron. This oscillator has an active low output: when pin 1 is at zero volts, the robot should turn. Notice that the oscillator's output is connected back to the integrating capacitor (C1) through two diodes. These diodes cause the top potentiometer to control the charging time when the oscillator's output is high, and the lower potentiometer to control the discharge rate when the output goes low. Thus, we can independently vary the excitatory and inhibitory input strengths (referred to as kf and kt in the neural diagram) using these potentiometers. The last part of the SEEK circuit is the diode descending from the obstacle detection LED. Since the diode has negligible resistance, it can instantly discharge capacitor C2 to synchronize the oscillator. This connection models the weight 100 input in our neural design.
Obstacle detection forms the basis for the AVOID behavior. The circuitry for this consists of the pair of comparators in the upper third of the diagram. Starting at the far left, the reflected infrared is received by the TIL414 photo-transistor which generates a small voltage change across the 1M resistor connected to it. This signal is AC-coupled via the 470pf capacitor into a simple threshold unit formed by one section of the LM339 quad comparator. The capacitors on voltage divider feeding into the positive input of this comparator help stabilize the reference voltage against transients caused by switching the IR LED and normal operation of the motor. An inverted version of the detected signal then enters a low pass filter formed by the 47K resistor and capacitor C2. Since the LM339 has open-collector outputs, the decay of the voltage across C2 is governed solely by the 1M resistor. Finally, the filtered voltage enters another section of the LM339 which again acts as a simple comparator. The trigger level for this threshold unit is obtained from the same voltage divider as used for the first stage. The result is a clean digital signal indicating when an obstacle is present. This, in turn, causes the red LED to light up when the robot sees something.
The basic EXPLORE behavior is incorporated directly into the relay circuitry. Normally the robot's motor is connected so it runs forward (the extra diode, D1, is inserted to slow the creature down, if necessary). However, whenever the relay is energized, the voltage applied to the motor is reversed and the creature turns instead. Thus, the functions of the EXPLORE neuron, the first suppressor, and the turn and forward neurons are all included in this piece of circuitry. The driver transistor for the relay can then be considered the equivalent of the second suppressor node in the neural diagram, with the two diodes in the dotted box modelling the two excitatory connections to this interneuron. Because both the obstacle detector and central oscillator have active low outputs, these diodes form a logical OR gate. Thus, if the creature either sees something (red light turns on) or the turn timer kicks in (green light turns on), the relay will be activated and the creature will turn in place. The diodes around the motor and across the relay's coil, however, serve no behavioral function, they just clamp inductive spikes to the power supply rails.
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