“Over the years there have been many electronic toys that let you play with and learn about electricity. Jay Silver’s Drawdio (Figure A) from 2008 combines a simple 555 timer circuit with a pencil to make squeals, beeps, and musical tones. When you draw, the graphite (carbon!) in the pencil conducts an electrical signal — you’re literally drawing a circuit! The longer and skinnier the pencil lines, the higher the electrical resistance and the lower the audio tone created by the oscillator. Short, fat lines have more conductive carbon, giving a lower resistance and making higher-pitched tones. Adafruit sells a Drawdio kit with a clever 2-sided circuit board — one side for wired components, the other side for surface-mount! Thirty years earlier Mattel produced The Electronic Connection toy (Figure B), which included game cards printed with conductive ink: drawing with the electronic pencil let you solve squealing mazes, play beeping games, and create music! The silk-screened and heat-cured conductive inks on the game cards were formulated with real silver, expensive even then!”