Decoder chaser

// Count, then decode: a 74HC161 and a 74HC138 light one LED of eight.
//
// 1. The clock. Gate 6 of the 74HC14 has a 100k resistor from its output back
//    to its input and a 4.7 uF capacitor from its input to ground. A Schmitt
//    input switches at two different voltages (about 2.8 V going up, 1.8 V
//    going down), so the capacitor chases the output up and down between them
//    and it oscillates: about 0.8 x R x C, a little under 3 Hz.
//
// 2. The count. The 74HC161 adds one on every rising edge of CP. All four
//    outputs change on the same edge: a synchronous counter.
//
// 3. The decode. Q0, Q1 and Q2 go to the 138's address pins A0, A1, A2, and
//    the 138 pulls exactly one of Y0..Y7 low: Y3 when the address is 3. Each
//    LED runs from 5 V through 330 ohms into one output, so the low one lights.
//
// Q3 is not used, so the light runs along the eight LEDs twice for every
// sixteen counts. Change C1 in the properties panel to change the speed.