555 on the bench

// A 555 astable, with a scope on it and a meter reading its average.
//
// R1 = 10 k from the supply to pin 7, R2 = 10 k from pin 7 to the timing node,
// C1 = 100 nF from that node to ground. The capacitor charges to 2/3 of the
// supply through R1 and R2 and discharges to 1/3 through R2 alone:
//
//   high   =  0.693 x (R1 + R2) x C  =  1.386 ms
//   low    =  0.693 x R2 x C         =  0.693 ms
//   period =  2.079 ms  ->  481 Hz, high for 2/3 of it
//
// Three readings of one signal, and they have to agree:
//
//   CH1 (yellow)  the output. The strip under the screen says Vpp, the
//                 frequency and the duty, measured off the trace itself:
//                 about 474 Hz at 67%. That is a little under the 481 on
//                 paper, and CH2 is most of the reason: its 1 Mohm probe
//                 sits on the timing capacitor and takes a little of the
//                 current that charges it.
//   CH2 (blue)    the capacitor, sloping up to 2/3 of the supply and back
//                 down to 1/3. That sawtooth is where the timing comes from.
//   the meter     DC volts on the same output. A DC meter averages, so a
//                 square wave that is high for two thirds of every cycle
//                 reads two thirds of its height, 2.2 V.
//
// Two thirds of *its height*, not of the supply. A bipolar 555 cannot pull its
// output all the way up: it tops out around 3.3 V on a 5 V rail, which the
// scope shows and the meter then averages. That is the difference between
// reading a datasheet and measuring the part.
//
// Things to try. Make R2 100 k and the duty goes towards 50%, because the
// charge and discharge paths become nearly equal; the meter follows it down
// towards half the height. Make C1 1 uF and everything slows by ten, so turn
// the scope's time base up with the + button on its face. The circuit keeps
// running while you do, which is the point of having it there.