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Many BEAMbots are based on bicores (2-Nv neuron "loops") in some fashion or another (and there are two main kinds of bicores -- but more on that later). Even if you're not building a bicore-based BEAMbot, studying how bicores work will help you understand more-complex Nv networks. So how do we get there from here?
The grounded bicore
Now hook the output of the second neuron to the input of the first:
This is a basic, "vanilla" bicore. Since both neurons' resistors are connected to ground, it's official full name is the grounded bicore. Thanks to the two inverters, this bicore will happily oscillate (the outputs of the two inverters will always be opposite each other) without further input.
The frequency of oscillation is a function of the values you chose for the resistors and capacitors -- higher values result in slower oscillation frequency. The time that any given Nv neuron is "active" is very nearly equal to R*C (in seconds). "Typical" values for these are 0.1 - 0.33 uF for the capacitors, and 1-4 MOhm for the resistors (so with 0.1 uF capacitors and 1 MOhm resistors, each Nv neuron "fires" for about 0.1 second). Depending on your application, and what's being driven by the bicore, you may want something faster or slower than this.
You'll generally use Schmitt inverters for building grounded bicores -- so these circuits will most-often be built up from 74*14 or 40106 ICs. A Schmitt inverter has built-in hysteresis circuitry -- so, the rising input voltage at which its output toggles "down" is guaranteed to be higher than the falling input voltage at which its output toggles "up." With a slow-rising or slow-falling input in a noisy environment (which describes a grounded bicore's job precisely), a conventional inverter could start to toggle on and off as the input approaches the threshold -- while a Schmitt inverter is guaranteed to toggle very cleanly.
Note that we've gotten rid of one resistor, without really changing much in the way of circuit functionality.
Courtesy of Wilf Rigter, here's an animation of a suspended bicore in action:
A common way to modulate the duty cycle in a suspended bicore uses a pair of photodiodes across the timing resistor. If more light falls on one diode compared to the other, the duty cycle of the bicore output signal will change (e.g., 1:2 ratio). Now the motor will rotate twice as long in one direction than the other. It's a bit like taking one step back and two steps forward, on average you go forward.