555 circuits

Martin McBride, 2020-06-03
Tags 555 timer
Categories electronics analogue electronics analogue electronic circuits

In this section we will look at several typical 555 timer circuits. Refer to the 555 timer article for an overview of the device.

You will often see 555 circuits described as astable or monostable. Astable means that the circuit has no stable state - it continuously switches between on and off states. It is just a fancy term for an oscillator. Note that astable doesn't mean random or unstable, a 555 will oscillate at a fairly precise frequency depending on the resistors and capacitors you use in the circuit.

Monostable means that the circuit has one stable state - its off state. When you trigger the chip, it enters the unstable on state. It will remain in this state for a fixed time, and then revert to the off state. It will remain in the off state until you trigger it again. A monostable circuit is a timer/

We will also have a quick look at bistable circuits. In that case both the on and off states are stable. When you trigger the circuit it turns on, and will stay that way until you reset it,. When it is reset the circuit turns off and will remain in that state until you trigger it again. A bistable circuit never changes of its own accord, it always needs a signal. Another name for a bistable circuit is a flip flop.

We will look at:

  • A 555 bistable circuit - it isn't a very useful circuit in itself, but it is key to understanding how the 555 works.
  • 555 astable circuits - a 555 oscillator, including how to control the frequency and duty cycle, how to obtain a triangle wave, and how to make the frequency voltage controlled.
  • 555 monostable circuit - a 555 one shot timer, including how to control the time period, how to get a riding or falling voltage, and how to create envelopes from multiple 555 chips.

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