Game-show "contestant-selection" controllers aren't new. But the designs are complicated, especially as the number of inputs grows. This contestant controller is simple and expands easily. The idea is to keep arbitration to a minimum. The design has a window of opportunity so small (nanoseconds) that if two or more signals get through, you can call it a tie.
Designs frequently require the conversion of a microprocessor's pulse-width-modulation (PWM) signal into an analog voltage. Often a passive single-pole RC filter will satisfy design requirements, but this approach typically suffers from several drawbacks, including slow response time, noisy results, and having an unbuffered output. The circuit uses the ubiquitous LM431 shunt regulator to implement a second-order Sallen-Key low-pass filter together with a level shifter.
Early radio receivers and record players nearly always had a knob marked "tone," which was usually a crude low-pass filter with some form of severity adjustment. At best, these controls could partially compensate for bass loss caused by poor speaker baffling. More sophisticated controls were developed for more modern equipment, including the bass/treble (Baxandall) controls, parametric equalizers, and graphic equalizers. Nevertheless, a single-adjustment tone control often can be useful for fine balance adjustment or where multiple controls are impractical or unnecessary.