About
This one is a nifty utility module that basically creates an imaginary voltage 'window' with voltage-controlled position (between -12 and 12 volts) and size (between 0 and 12 volts), and then its three gate outputs show if an input signal falls below, above or inside this window. Sounds boring, right? Well, in right hands this utility can have many uses - from the obvious boring comparator and PWM-ed pulse extractor, to less obvious - rattling gates out of LFOs, rhythmic syncopation, and so on, and so forth. This design is nowhere near being a fancy module that does cool sounds out of the box (unlike some terrible VCFs and digital generators that i made), but it is a great versatile utility that allows for creative patching, analog computer style. Setting the Size dial to minimum makes it a simple "more/less than" comparator, and driving it negative with an external CV makes the Above and Below inputs do a funny overlap.
Schematic
This is one of those 'Aubery gets crazy and does math with op amps' circuit - hence, almost all resistors are 100K. If you have a bunch of 200K resistors - feel free to use them instead; but the 'main' resistor value should fall between ~50K and ~500K. This circuit is nothing fancy - just some addition, subtraction and compatison. I found it a very entertaining engineering task to design and assemble - and it does play well with the rest of the system, too!
I set the task as - a circuit that has voltage-controlled variables P (position) and S (size), and a direct input I. It should show with gates if 1. I>P+S, or 2. I<P-S, or 3. P-SI>I>P+S - above, inside and below the window respectively. I then broke the task down to parts, and made circuits, but so that i also save a few op-amps on my way.
First, IC1A and IC1B inverting-sum the position and size knobs and CVs. Their outputs are -P and -S respectively. IC1C below inverts the input signal I: this is done for two reasons. First, it buffers the input, and then also it is easier to invert both P and I and work in the 'mirrored' voltage range. Output of IC1C is -I. IC2A and IC2B do a non-inverting two input unity gain mix of -P and -S. The top one, IC2A, subtracts -S from -P:its output is -P+S. IC2B adds -S to -P, outputs -P-S. This way, we get the two voltgaes that define our window: -P+S and -P-S.
IC2C and IC2D are comparators that detect if the input signal is above or below the window. Both comparators have a tiny bit of hysteresis going on to prevent noisy bouncing (the 1K/1M network). IC2C detects if the inverted input is above IC2A output, or -I>-P+S. This can be multipled by -1 altogether, and we get I<P-S - the 'below window' condition. Similarly, IC2D detects if -I<-P-S, or I>P+S - the 'above window' condition.
As comparators do, these output near +12V if the condtion is met, and -12V otherwise. I decided to derive the 'inside window' gate by simply diode ORing the two comparator outputs (high whenever it is above or below the window), and using the last op-amp, IC1D, as another comparator that checks if the OR of above/below is less than 6 volts. This only happens if neither of the above/below comparators are true, which means the input is neither above, nor below the window - naturally, it is then inside the window.
The outputs have LEDs that turn on on positive voltage, so the three comparator outputs will have insignificant voltage drop when high. This should not be an issue when they go low. If one wishes to, they can do simple "diode and then 100K to ground" signal rectifiers after all the comparators to cut off the negative part and have 0..appr.12v gates.
