Introduction

The 555 timer IC is a simple 8 pin DIL package IC.

It can be used as a monostable, be used as an astable, source or sink current of at least 100mA which is significantly more than the logic gates or other digital ICs and use supply voltages of 5v to 15v making it very versatile. However, it can also disrupt the power supply when the output changes state and so it is sometimes necessary to use a decoupling capacitor.

Using the 555 as a buffer

A buffer circuit allows an input circuit to be connected to an output circuit, it is like an interface between one circuit and another. The buffer circuit requires very little input current can supply a significant output current.

The 555 can supply in excess of 100mA of current and so can be used as a convenient buffer for logic gates which cannot supply much current. The 555 can also 'sink' a similar amount of current.

As shown in the diagram, The circuit acts like an inverter or NOT gate. When the input is held LOW, the output is HIGH and will provide (source) current. When the input is held HIGH, the output is LOW and will sink current.

Using the 555 as a monostable

The 555 can be used as a monostable using the circuit shown.

The output is normally LOW but will go HIGH for a short length of time depending on the values of the other components. R and C determine the time period of the output pulse. The input is normally HIGH and goes LOW to trigger the output (falling edge triggered). The length of the input pulse must be less than the length of the output pulse

The equation to work out the time period is: T = 1.1xRxC where T is in seconds, R is in ohms and C is in Farads

The minimum value of R should be about 1kΩ to avoid too much current flowing into the 555. The maximum value of R should be about 1MΩ so that enough current can flow into the input of the 555 and there is also current to allow for the electrolytic capacitors leakage current. The minimum value of C should be about 100pF to avoid the timing equation being too far off and the maximum value of C should be about 1000µF as any bigger and the capacitors will discharge too much current through the chip

These maximum and minimum values give a minimum period of 0.1 µs and a maximum period of 1000s

The 47µF capacitor shown decouples the power supply which means it helps to stop the 555 IC affecting other parts of the same circuit when the output changes.

Using the 555 as an astable

The 555 can be used as an astable using the circuit shown

The output will oscillate between HIGH and LOW continuously - the circuit is not stable in any state. R_a, R_b and C determine the time period of the output. The reset, pin 4, must be held HIGH for the circuit to oscillate. If pin 4 is held LOW then the output remains LOW. Pin 4 can be used to turn the astable 'on' and 'off' in effect.

The equation to work out the time period is T = 0.7( R_a + 2R_b)C where T is in seconds, R is in ohms and C is in Farads

As with the monostable the minimum value of R_a should be about 1kΩ to avoid too much current flowing into the 555. The maximum value of R_a or R_b should be about 1MΩ so that enough current can flow into the input of the 555 and there is also current to allow for the electrolytic capacitors leakage current. The minimum value of C should be around 100pF to avoid the timing equation being too far off. The maximum value of C should be about 1000µF as any bigger capacitors will discharge too much current through the chip. These maximum and minimum values give a minimum frequency of 0.001Hz and a maximum frequency of 4.8MHz (in reality it would not be able to attain these frequencies).

Considering the oscillations in more detail, the output is controlled by the charging and discharging of the capacitor. The capacitor charges through R_a and R_b but discharges through the discharge pin (pin 7) and thus only through R_b. The time that the capacitor takes to charge or discharge is given as T = 0.7RC. Thus the charge time is 0.7(R_a + R_b)C and the discharge time is 0.7R_bC giving a total time of 0.7(R_a + R_b)C + 0.7R_bC = 0.7(R_a + 2R_b)C. The time the output is high (mark) is thus always longer than the time the output is low (space). The 555 astable cannot produce a square wave!

Operation of the 555

It is not necessary to know how the 555 works. In fact a systems approach to electronics would never consider how any such sub-block works. However, a knowledge of how the 555 functions is useful. A much simplified block diagram of the 555 timer is shown.

• The resistors are arranged across the power supply to form a potential divider. The voltages at the junctions of the potential divider are 2/3 Vcc and 1/3 Vcc. The are connected to the inputs to a pair of comparators
• One comparator, switching at 2/3 Vcc is controlled via the threshold input
• The voltage at which the threshold comparator switches can be changed from 2/3 Vcc by applying a voltage to the control pin. This pin is usually decoupled to ground via a 10nF capacitor and, in this case, the comparator switches at 2/3 Vcc as expected
• One comparator, switching at 1/3 Vcc is controlled via the trigger input
• The outputs from the two comparators control a set-reset flip flop (bistable)
• The reset pin of the 555 (not of the bistable) is usually held HIGH. Taking this pin momentarily LOW applies a voltage to the reset pin of the flip flop and the output falls to zero
• The output of the flip flop is connected to the output pin via a power amplifier circuit which includes short circuit protection etc
• The output goes HIGH when the trigger input is less than 1/3 Vcc ...
• The output then remains HIGH until the threshold input rises above 2/3 Vcc
• When the output is LOW, the discharge pin is connected to ground via a transistor. The capacitor can be organised to discharge through this pin but the value of the capacitor should be less than 1000µF to avoid damaging the transistor