*Remember - If Input pin 2 is LOW, Output pin 3 is HIGH*

Component Theory - 555 Timer



Copyright © A.F.Billington 2022. All rights reserved.

Time Period =

1.45 s

A 555 timer is an 8 pin DIL IC that is used to produce a timed digital pulseIt's internal circuit is made up of 25 transistors, 15 resistors and 2 diodes and is encased in a silicon package.

  • PIN 1 - Negative Ground Voltage (0v)
  • PIN 2 - Input pin (trigger), Starts the pulse
  • PIN 3 - Output Pin, Digital Pulse
  • PIN 4 - Reset; if LOW no pulse, if HIGH pulse
  • Pin 5 ​- Control, typically unconnected or to Ground
  • Pin 6 & 7 - Threshold and discharge, control length of pulse
  • Pin 8 -  Positive voltage, 9V DC supply


In monostable mode the 555 timer will produce a single timed digital pulse at Pin 3. The pulse is triggered by the switch on the input pin (2) and the length of the pulse if controlled by the capacitor and resistor on Pins 6 & 7. As discussed in 'capacitors', this combination creates a time delay that is calculated using;   T (s) = C (F)  x   R (Ω) 

​​When powered the voltage at the trigger pin (2) is HIGHmaking pin 3 LOW.When the push to make is activated Pin 2 goes LOW and Pin 3 HIGH for a set period of time depending on the time constant. When the capacitor (C1) reaches 2/3 the supply the pulse will stop


In Astable mode a second resistor is added and the Trigger (2) becomes automatic. When powered it will produce a continuous timed digital pulse at the output pin (3). The pulse will alternate between 0 (off) and 1 (on) at a constant timed rate that is dependant on the resistors and capacitors on pin 2, 6 & 7

When powered C1 is not yet charged and is at less than 1/3 of the supply voltage making pin 2LOW and Pin 3HIGH. As the C1 charges through the resistors it reaches 2/3 the supply voltage causing pin 2 to go HIGH and pin 3 to go LOW. This causes C1 to discharge through R2 until at 1/3 the supply. This will then repeat continuously.


In order to calculate the frequency (Hz) of the astable timer pulse, the equation on the left is used. It is vital that the value for R1 is the top resistor and R2 the bottom. Hertz (Hz) = how many time per second.​

So if you had a 1K resistor (R1), 10K resistor (R2) and a 10uF capacitor -

1,000 (R1) + (10,000 (R2) x 2) = 21,000 x 0.000,01 (C) = 0.21 

1.44 / 0.21 = 6.9Hz   (6.9 times per second)


Mark/Space Ratio is the ratio between HIGH (Mark) and LOW (Space) for a 555 Astable timer. Often this will be 1:1, meaning the pulse is high and low for the same amount of time. However, when the pulse train  is uneven a calculation is required to determine 'Time High' and 'Time Low'.

Time High = 0.693 x (R1 + R2) x C                           Time Low = 0.693 x R2 X C

                      Mark/Space Ratio =  Time High / Time Low

​So... if R1 = 1K,  R2 = 10K,  C= 100uF then;

Time High = 0.693 x (1,000 + 10,000) x 0.000,1           = 0.76s

Time Low= 0.693 x 10,000 x 0.000,1                            = 0.69s

Time High/ Time Low =  0.76 / 0.69                               = 1.10

<- Example circuit shows a second output on pin 3. When LOW  it will act as drain (Negative) with the Positive coming from the source. In this way the component must be connected as a mirror image of the other.