Comparator boards

// Three comparator boards and three lamps. No board.
//
// Each sensor module is the same LM393 board under a different sensor. DO is
// an open collector: it pulls LOW when the sensor sees something, and it can
// sink a lamp's few milliamps on its own. So each lamp runs from 5 V through
// 330 ohm into a DO pin.
//
//   MQ-2      DO low when AO rises past the trimmer: gas, and the warm-up
//   flame     DO low when a flame is close enough
//   TCRT5000  DO low over white, high over the black line
//
// Things to try: watch the MQ-2's lamp go out as it warms up; drag the flame
// in; click the TCRT5000's swatch to swap white for black.