555 blinker

// A 555 astable: a square wave with nothing but a chip, two resistors
// and a capacitor.
//
// No microcontroller and no clock part. The timing comes out of the RC
// network. C1 charges towards 5 V through R1 and the selected timing resistor;
// when it reaches 2/3 of the supply the threshold comparator trips, the output
// goes low and pin 7 sinks the capacitor back down through the timing resistor
// alone; at 1/3 of the supply the trigger comparator flips it back.
//
//   f  =  1.44 / ((R1 + 2 R2) C)
//
//   slow:  10k + 2 x 47k  = 104k  with 10 uF  ->  1.4 Hz
//   fast:  10k + 2 x 4.7k = 19.4k with 10 uF  ->  7.4 Hz
//
// Click the slide switch while it runs to swap R2. Change C1 in the properties
// panel and the whole thing scales: ten times the capacitance is a tenth of
// the frequency.