Circuit building blocks
Multivibrators
Circuits with two states, HIGH and LOW, made from two switching stages that control each other. How they are coupled decides how many states are stable: none (it blinks), one (a timed pulse) or two (a memory bit).
Think of it as: Astable is a metronome, monostable is a stairwell light timer, bistable is a light switch.
The variants
Astable: no stable state
Both sides are coupled through capacitors. Whichever transistor is on holds the other off only until its capacitor recharges, then they swap. It keeps flipping by itself: an oscillator.
Each half: ≈ 0.69 × R × C (326 ms for 47 kΩ, 10 µF) · With a 555: 333 ms on, 326 ms off (1 kΩ, 47 kΩ, 10 µF)
Monostable: one stable state
One side is coupled by a capacitor and the other by a resistor. At rest Q1 is on. A trigger flips it, and it stays flipped only until the capacitor recharges: one pulse of fixed length, however long you hold the button.
Pulse length: ≈ 0.69 × R2 × C2 · With a 555: 1.1 × R × C (1.10 s for 100 kΩ, 10 µF)
Bistable: two stable states
Both sides are coupled by resistors, so nothing ever runs out. Whichever transistor is on keeps the other off for as long as power is applied. A SET or RESET pulse flips it. This is a latch, the simplest one-bit memory.
Timing: None: stays until pushed · In chips: SR latch, D flip-flop (74HC74), memory cells
The three types side by side
| Type | Stable states | Needs a trigger? | Acts like | Typical use |
|---|---|---|---|---|
| Astable | 0 | No | Oscillator | Blinkers, beepers, clock signals |
| Monostable | 1 | Yes, to start a pulse | One-shot timer | Light timers, debouncing, pulse stretching |
| Bistable | 2 | Yes, to change state | Memory bit | Latches, flip-flops, toggle buttons |
Related: the Schmitt trigger
A schmitt trigger is a bistable that switches on its input voltage instead of on pulses: it goes HIGH above an upper threshold and only back LOW below a lower one. The gap (hysteresis) turns a slow or noisy input into one clean edge. The 555 is two Schmitt-style comparators driving a bistable, and chips like the 74HC14 have six built in.
The 555 does all three
The 555 timer datasheet calls its two main modes astable and monostable, and with the threshold pin tied off it works as a bistable latch.
Where you’ll use it
- Blinking LEDs, sirens and simple clock signals (astable).
- Stairwell light timers, watchdogs, stretching a short pulse so it can be seen (monostable).
- Debouncing a button: one clean pulse per press (monostable or Schmitt trigger).
- Push-on, push-off buttons and every bit of memory in a computer (bistable).
Common mistakes
- Leaving out the resistor in series with each LED. The transistor then passes as much current as the supply can give.
- Expecting a long monostable pulse from a small capacitor; 0.69 × R × C with 10 kΩ and 100 nF is under 1 ms.
- Leaving a 555 trigger or reset pin floating, which makes it fire at random.
- Using electrolytic capacitors backwards in the astable: their + side goes to the collector.
Lessons that use it
Debouncing in 4.1 is a monostable job, and every timing here is the RC time constant from 2.2.
- Lesson 4.1: Pull-Up Resistors & Button Debounce Explained
- Lesson 2.2: RC Time Constant: Capacitor Charging Explained
Go deeper
Two-transistor astable: the base recharges from about −U_CC to +0.6 V, which takes ln2 time constants.
555 astable: the capacitor swings between 1/3 and 2/3 of the supply, charging through R1 + R2 and discharging through R2.
555 monostable: the capacitor charges from 0 to 2/3 of the supply.
SR latch: SET forces Q high, RESET forces it low, and with neither Q keeps its old value.