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22 March 2009
Author: Giorgos Lazaridis
555 Circuits

Darkness detector

This is a simple yet effective darkness activator. It uses an LDR to detect the light and when no light is present, it will sound an alarm from a cheap 8Ohm speaker. This circuit can be easily converted to a light detector by simply adding an inverting transistor driver at the output of the 555.

Power failure alarm

This circuit performs a power failure alarm. It is a 555 oscillator that is off due to the fact that voltage appears at pin 7. When the voltage at pin 7 fails to appear, the alarm will sound from the buzzer.


This is a simple metronome circuit that is useful for the friends of music. It will generate the classic toc-toc sound like the mechanical metronomes. The period of the signs can be adjusted from the 250K potentiometer.

Morse code generator
This is a Continuous Wave (CW)

generator, AKA Morse code generator. The circuit will generate the characteristic 'beep' sound of the Morse code generators, every time the switch is pressed.

Missing pulse detector

This is what you should expect from this circuit, when an input pulse is missing

The missing pulse detector is actually a one-shot device that is continuously re-triggered from the input pulse. The component values for the resistor and the capacitor are to be changed to fit your needs. You should select the parts so that the delay should be about 30-40% bigger than the period of the input pulses. If it is bigger than two input pulses, then in order to detect the input pulse, there must be at least two input pulses missing.

The transistor can be a 2N2222 or BC547 or BC549 or any equivalent NPN transistor.

Touch switch

This is a touch switch version of the 555 switch connected as monostable multivibrator. The 2N2222 shall perform an amplifier able to sense slight loads on the plate such as static electricity from the human body.

Switch debouncer

A switch debouncer is a mid-stage block that will clear and square the signal from a push button or a switch. Those switches are very noisy and the output waveform is filled with noise that will generate false pulses, due to poor contact condition, moisture or dust.

[P]The above circuit will generate a noise-free output pules when the button is pressed. This pulse will remain high according to the RC circuit. If the button is pressed further more than the RC time, then the pulse will latch even more.

Another use of this circuit is turn-off delay. A big capacitor and a bigger resistor may achieve turn off times like 30sec up to some minutes. This can be for example a car turn-off light delay.[/P]

Basic monostable circuit

This is the output of the circuit

When in input 2 of the 555 (trigger) is sent a low pulse, the output will go high. The output will remain high for a period of:

THIGH = 1.1 x R x C (Seconds)

There are several applications that a monostable circuit as the above is used. With very short periods, it could be used as a switch debouncer or a logic driver from fast acting sensors. Medium periods, like 1-10 seconds could be used for indication sound signals or any other general delay like car cabin lights turn off delay. Bigger periods can be used for delay timers.

Basic astable multivibrator

The output of the circuit

A 555 can be used to generate clock pulses in a wide range of frequencies with enough output power to drive several ICs. The oscillation frequency is calculated with the following formula:

F = 1
0.67 (R1 + 2 x R2) x C

The same circuit can be used to control DC loads such as LEDs, lamps and DC motors. The idea is to use this circuit as a PWM signal generator. To do this, you need to replace R2 with a potentiometer. By altering the potentiometer's value, this results in changing the duty cycle output. The duty cycle is calculated as follows:

D = R1 + R2
R1 + 2 x R2

In a first glance, someone can understand that the minimum duty cycle could be no less than 50%. This is true. check the tips and tricks bellow to find out how can you achieve duty cycle less than 50%.

Relative pages
  • 555 theory of operation
  • Basic transistor circuits
  • How to make a PWM fan controller / LED dimmer using a 555
  • Dr.Calculus: 555 Astable multivibrator calculator
  • Dr.Calculus: 555 Monostable calculator
  • International unit converter

  • Comments


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  • At 25 September 2015, 18:18:47 user aaron wrote:   [reply @ aaron]
    • Dear writer,
      All the info is just amazing; short, interesting yet precise to comprehend at max.
      Thanks for all sharing..

      Best luck keep it up..


  • At 22 February 2014, 18:52:07 user Stefanos wrote:   [reply @ Stefanos]
    • Hello Dear Friend

      I'm interesting to build a circuit by using 555 as a switching controller.
      I want to use a single push button switch.
      The state of the circuit I want is as follows.

      First push of the switch - state ON
      Next push - state OFF
      Next push - ON
      Next push - OFF
      Next push - ON
      Next push - OFF
      ...... and so on

      Actually I want the circuit to remains in the state I want for as long as I want and if I press again the switch then I want to change the state.
      Do you have any idea on how to build this?

      Thanks a lot for listening me.
      Best regards.

  • At 15 July 2013, 9:34:21 user kiranvarma wrote:   [reply @ kiranvarma]
    • That's really wonderful list! Thank you

  • At 13 June 2011, 21:57:08 user Hosseini wrote:   [reply @ Hosseini]
    • please draw schematic for touch button using 555+transistor+741as shmitt
      trigger+led as output.

  • At 20 April 2011, 20:21:59 user cryopyro wrote:   [reply @ cryopyro]
    • u know, for the metronome, u can just use a potentiometer (with 3 pins) like this:
      \_/ < potentiometer
      8 7 6

  • At 9 February 2011, 17:53:35 user Kammenos wrote:   [reply @ Kammenos]
    • well, what do you know! I forgot to post them... anyway, see my circuits:

      I have some different ways to make PWM pulses with variable DC. I suppose, you are interested mostly for this;

      But this one is very flexible, as it can change easily the frequency AND the DC from 0 to 100%:

  • At 9 February 2011, 17:37:58 user xxx wrote:   [reply @ xxx]
    • "check the tips and tricks bellow to find out how can you achieve duty cycle less than 50%."

      I'm weary interested in these tips. where are they? I want to build 1kHz and 10% duty cycle, square wave on 12V with 555.

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