The SC-Head is the simplest head that can lock. It uses a pair of gates, usually inverters, with different hysteresis. This way, at a point the two will 'disagree' and the motor stops moving. Since it compares two Schmitt based inverters, it got the name Schmitt comparator head.

Here is a little thought example to make it more clear: A light is perpendicular to a pair of photodiodes. One photodiode gets all the light, and the other gets none. Let's say this photodiode bridge outputs 0 v. As it turns toward the light, the bridge voltage increases to 1/2 v. The normal Schmitt trigger still needs to get to 2/3 v, but the reduced Schmitt trigger operates more like a normal inverter, switching around 1/2 v.

There are several ways to achieve two differnt triggers. The simplest is to use a Schmitt gate and a non-Schmitt gate, but this requires two chips. The alternative is to strenghten hysteresis or weaken it. My first version used a resistor to weaken the hysteresis by using negative feedback on a **14.

SC-Head version 1

Wilf Rigter was quick to point out that this head would oscillate instead of locking, but he did like the concept. He corrected my version, and added this about the component values: 1M in series and 4M in parallel with the schmitt trigger and 1000 pf to ground works but is sensitive to light level.

SC-Head version 1 revised

Since this was sensitive to light levels, and the negative feedback is inherently unstable, I made a version that uses positive feedback instead. It uses a **240 with added hysteresis. The resistor should be around 3M.

SC-Head version 2

Wilf's next tweak on the revised SC-Head v1 was to replace the series resistor with a pait of diodes. This was to make it less sensitive to differences in light levels and more reliable.

SC-Head version 3

In making version 3, Wilf found a spot for a huge improvement. By isolating one of the sides with a deadband diode, all of the Schmitt-weakening components could be eliminated! With this, Wilf turned the SC-head into the best head circuit availible.

SC-Head version 4

All that was left was to solarize it. Dave and Wilf came up with a good way. A modified FRED engine powers it.

solar SC-Head version 4

I was planning on adding an SC-Head to a walker. I had two inverters left over from the microcore, and I wanted to use them. I was going to make an SC-Head v4, and hook this up to a **245 motor driver, using cg shifting to control the walker. But why go through the trouble of using a single diode when I could make a new circuit that uses no extra parts? This SC-Head takes an extra chip, but it works very well on a walker.

SC-Head version 5

While the SC-Head can't be made any better, it has been the inspiration for bigger and better thing. The first was Dozer. This beautiful little bot used a similar circuit, but without the deadband diode. While this didn't make the bot any smarter, it did make it easier to build. The SC-head also lead to Wilf Rigter's Carnivore circuit, possibly the simplest vore driver ever made. The PV-Roller is the most advaced circuit based on the SC-Head. It uses an extra set of eyes to give it peripheral vision. Depending on which set of eyes sees light, it will turn gently or sharply.

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Created 6/7/02