Timing Circuits

• There are two main types of capacitors – polarised and non-

  polarised. Polarised capacitors include electrolytic capacitors.
• The charge storage capability of a capacitor is measured in

  units called farads. The farad (F) is a very large unit so Capacitor values are usually given in micro-farads (μF).
• When a capacitor is charged directly from a voltage supply,

  it very quickly becomes fully charged. It is slowed by using a resistor in series.
• The capacitor will always charge up in a predictable way in a

  fixed length of time until it approximately reaches the power

  supply voltage. The time taken is dependent on the value of

  the capacitor and resistor used.
• The discharging process can also be slowed down by

  discharging the capacitor through a large resistor
• 555 Timer pin layout

  A) 1

  B) 2

  C) 6

  D) 7

  E) 4

  F) 8

  G) 3

  H) 5

  
• There are two main types of 555 timer circuit:

  • Monostable - a single pulse of fixed duration
  • Astable - a continuous on / off pulse
• Monostable circuit

  A) Capacitor

  B) Resistor

  C) Resistor

  D) Switch

  
• The equation used for finding the duration of a monostable circuit is:

  T = 1.1 x R x C

  * DO NOT FORGET * - Capacitance will almost always be in MICRO farads, do not forget to adjust for this.
• The astable is sometimes called a pulse generator. The time

  when the output is on is referred to as the ‘mark’, and the off

  time is usually referred to as the ‘space’.
• The frequency of the output pulse (astable timer) can be calculated using the formula:

  F = 1 / (T1 + T2) OR F = 1 / T
• Astable circuit

  A) Resistor

  B) Resistor

  C) Capacitor

  
• There are other equations used to define the Mark-space ratio, and the frequency, but these are given on the equation sheet.
• In a monostable circuit pins 6 and 7 are connected to the capacitor
• In an astable circuit pins 6 and 2 are connected to the capacitor
• The time constant (TC) of a capacitor is calculated through:

  TC = R x C

  It is the time taken for the capacitor to reach 2 / 3 of its final voltage
• Advantages of polarised capacitors:

  • Typically have a much larger capacitance
  • Tend to be physically smaller
• DIL for ICs stands for Dual in line
• 555 timer pin names:

  Pin 2: This is the trigger and requires ⅓ or less of the supply voltage to turn the 555 timer on. 

  Pin 6: This is the threshold and requires ⅔ or more of the supply voltage to turn the 555 timer off. 

  Pin 7: This is the discharge and connects to pin 1 (ground) when the 555 timer is off allowing the capacitor to discharge. 
• *REMEMBER* You have an extra equation for 555 astable frequency given in the Equation sheet.