the 555 timer
/ five-five-five /
The 555 timer is the most beloved little chip in hobby electronics, an 8-pin part that, with just a resistor or two and a capacitor, makes timing pulses and square waves. Designed in 1971 and still sold by the billion, it is the friendly Swiss-army knife of relaxation oscillators: blink an LED, make a tone, generate a delay, build a simple PWM dimmer. If you learn one timing chip, this is the one.
Inside, the 555 contains two comparators, a flip-flop, a discharge transistor, and a three-resistor divider that sets two thresholds at one-third and two-thirds of the supply voltage. You hang an external resistor-capacitor outside the chip. The capacitor charges up; when it reaches two-thirds of the supply, the upper comparator trips and flips the output low while turning on the discharge transistor; the capacitor drains; when it falls to one-third of the supply, the lower comparator trips and flips the output high again. Those two fixed thresholds make the timing depend only on your external R and C, not on the exact supply voltage, which is a big part of why it is so easy to use. It runs in two main modes: astable (free-running square wave) and monostable (a single timed pulse on demand).
The 555's charm is robustness and simplicity, but be honest about its limits. The output can swing close to the rails but not exactly to them, especially the classic bipolar version, which also injects a noticeable current spike on the supply each time it switches, so it needs a decoupling capacitor nearby. Its timing accuracy is set by ordinary resistor and capacitor tolerances, typically a percent or few, not crystal-grade. For modern low-power or low-voltage work, the CMOS versions (like the 7555) draw far less current, switch more cleanly, and work down to lower supplies.
Because the thresholds are fixed fractions of the supply, a 555 astable runs at nearly the same frequency on 5 V or 9 V: the capacitor always charges between one-third and two-thirds of whatever the supply is, so changing the supply barely shifts the timing.
Two comparators at one-third and two-thirds of the rail turn an external R-C into timing.
The classic bipolar 555 draws a brief current spike at each transition and cannot drive its output fully to the rails; add a decoupling capacitor and prefer the CMOS 7555 for low-power, low-voltage, or cleaner designs. Its accuracy is set by everyday R and C tolerances, not a crystal.