Monostables – AS Electronics Revision


  • Monostables are systems that have one stable state (and an unstable state)
  • When they receive an input, they enter their unstable state, and remain that way for a certain amount of time, before reverting to their stable state again
  • Basically, they take a short input pulse and produce a longer output pulse
  • They’re one of the things capacitors are used for
  • Any extra input pulses during an output pulse don’t have any effect
  • Some monostables are rising-edge triggered and others are falling-edge triggered
  • The length of the output pulse depends on the values of the resistors and capacitors!
  • They can be used to leave something turned on for a long time when a button is pressed briefly
  • …or you might just want to turn a short pulse into a long pulse for some reason that’s none of my business

Simple Monostables

  • You can make one out of a capacitor timing network and a NOT gate
  • When the capacitor reaches the gate’s switching threshold (either by charging or discharging), the output logic state will change
  • It’ll probably have to be a CMOS NOT gate, so that it can be used at supply voltages other than 5V
  • If you assume the CMOS NOT gate’s switching threshold is half the supply voltage, the duration of the pulse is 0.69RC, because that’s how long it’ll take the capacitor to half charge / half discharge (see capacitors post)
  • This also assumes the gate draws no current, and that its logic level 1 is the supply voltage
  • The problem with some simple monostables is that you can’t produce an output pulse that’s longer than the input one!

The 555 Monostable

English: Diagram of a monostable circuit made ...

English: Diagram of a monostable circuit made using the 555 timer IC. A low pulse on the trigger line starts the monostable. (Photo credit: Wikipedia)


  • The 555 Timer IC can be configured as a monostables, as well as an astable (it’s very versatile…)
  • To set it up as a monostable, you only need a capacitor and a resistor
  • (Well, the ET2 notes say that a second, 10nF capacitor is put between pin 5 and 0V, to “reduce noise at the input of the comparator”, whatever that means
  • The pulse duration is the resistance multiplied by the capacitance, multiplied by 1.1 (being careful of those powers of three)
  • T=1.1RC is given in the INFORMATION FOR THE USE OF CANDIDATES IN ET2 section at the beginning of the paper
  • The trigger on a 555 monostable is active low
  • I don’t know how to draw a 555 monostable. Should probably look that up.

About Matt

I like writing, filmmaking, programming and gaming, and prefer creating media to consuming it. On the topic of consumption, I'm also a big fan of eating.
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