Topic 11 555 Timer Circuit

The 555 timer is a popular circuit used for timing purposes.  It can be used in:

• Monostable mode in which the output is a high for a time determined by the external circuit.
• Astable mode in which the output changes continually.

  The 555 timer is a small 8-pin integrated circuit, and we can add resistors and capacitors to an external circuit to make it act as a monostable or an astable.

  The pin arrangement (pin-out) is shown below:

555 timer in Monostable Mode

The 555 timer in monostable mode is constructed using the circuit shown in this diagram.

The circuit will give a single output pulse like this:

The components R and C determine the time period T of the output pulse.

• When the push switch S is closed and released, the voltage at pin 2 goes from high to low to high again.
• This triggers the output to go to high. Pin 7 is also disconnected from zero.
• When the voltage across C gets to about 2/3 of the supply voltage, the output goes low.
• The period of the pulse is given by a simple relationship:

T » 1.1 RC.

• Once triggered, the circuit cannot be re-triggered to extend the period T.

 Click HERE for a worked example.

In using a 555-timer we need to be aware that the circuit has one or two little quirks:

• The trigger period must be less than the output pulse.
• The 5 nF capacitor connected to pin 5 is needed to prevent false triggering.
• The circuit can produce brief dips in the voltage of the supply.  This can be countered by placing a large value capacitor across the supply rails.  This eliminates the voltage change (called decoupling).
• If electrolytic capacitors are used in the RC circuit, leakage currents and poor tolerances can result in the output pulse being greatly at variance with the value predicted by the formula.

555 timer in Astable Mode

The 555 timer can be wired up to produce a train of pulses by ensuring that the circuit is astable, which means that it is not in a stable state.  We can make astable circuits from other components, but the 555 timer gives a train of digital pulses. The diagram shows the circuit.

The output of the circuit is a square wave, as shown.

We need to consider some definitions:

• The mark time [t(H] is the time at which the output is a 1. t(H)= 0.7(RA + RB)C
• The space time [t(L)] is the time at which the output is a 0. t(L) = 0.7 RBC
• The mark to space ratio = mark time ÷ space time.
• The astable period T is the time taken for one complete cycle, the mark and the space times added together.  T = mark + space = t(L) + t(H).
• The frequency = 1 ÷ period.

                        f =  ____1.4_____

                               (R1 + 2R2)C

  The time t(H) will be longer than t(L), unless R1 is very small compared to R2.  If this is the case, then t(H) will be approximately equal to t(L), but not quite equal.  We can say to a first approximation that the mark to space ratio is 1.  This will result in a square wave output.