An introduction of the 555 Timer

The 555 Integrated Circuit (IC) is an simple to make use of timer that has plenty of applications. It is widely used in electronic circuits and this popularity means it is also cheap to buy, usually costing around 30p. A 'dual' version called the 556 is also available which includes 2 independent 555 ICs in package.

The following illustration shows the 555 (8-pin) and the 556 (14-pin).

In a circuit diagram the 555 timer chip is frequently drawn like the illustration below. Notice how the pins are not in the same order as the actual chip, this is because it is much simpler to recognize the function of each pin, & makes drawing circuit diagrams much simpler.

For the 555 to function it depends on analogue & digital electronic techniques, but if they think about its output only, it can be thought of as a digital tool. The output can be in of states at any time, the first state is the 'low' state, which is 0v. The second state is the 'high' state, which is the voltage Vs (The voltage of your power supply which can be anything from four.5 to 15v. 18v absolute maximum). The most common types of outputs can be categorized by the following (their names give you a clue as to their functions):

  * Monostable mode: in this mode, the 555 functions as a "one-shot". Applications include timers, missing pulse detection, bouncefree switches, touch switches, frequency divider, capacitance measurement, pulse-width modulation (PWM) etc

  * Astable - free jogging mode: the 555 can operate as an oscillator. Makes use of include LED & lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation, etc.

  * Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS pin is not connected & no capacitor is used. Makes use of include bouncefree latched switches, etc.

Inside the 555 Timer

You may reflect what is inside the 555 timer chip or what makes it work. Well, the 555 timer chip an Intergrated Circuit (IC) and therefore it contains a small, miniturized circuit surrounded by silicon. Each of the pins is connected to the circuit which consists of over twenty transistors, two diodes and 15 resistors.

The illustration below shows the functional block diagram of the 555 timer IC.

The chip got it's name after the three 5k resistors which are in the image above.

Pin Configuration of the 555 Timer

When drawing a circuit diagram, always draw the 555 as a building block, as shown below with the pins in the following locations. This will help you instantly recognise the function of each pin:

Pin 1 (Ground):
Joins  to the 0v power supply.

Pin 2 (Trigger):
Discovers  1/3 of rail voltage to make output HIGH. Pin 2 has control over pin 6. If pin 2 is LOW, and pin 6 LOW,  output goes and stays HIGH. If pin 6 HIGH, and pin 2 goes LOW, output goes LOW while pin 2 LOW. This pin has a very high impedance (about 10M) and will trigger with about 1uA.

Pin 3 (Output):
(Pins 3 and 7 are "in phase.") Goes HIGH (about 2v less than rail) and LOW (about 0.5v less than 0v) and will deliver up to 200mA.

Pin 4 (Reset):
Internally connected HIGH via 100k. Must be taken below 0.8v to reset the chip.

Pin 5 (Control):
A voltage applied to this pin will vary the timing of the RC network (quite considerably).

Pin 6 (Threshold):
Detects 2/3 of rail voltage to make output LOW only if pin 2 is HIGH. This pin has a very high impedance (about 10M) and will trigger with about 0.2uA.

Pin 7 (Discharge):
Goes LOW when pin 6 detects 2/3 rail voltage but pin 2 must be HIGH. If pin 2 is HIGH, pin 6 can be HIGH or LOW and pin 7 remains LOW. Goes OPEN (HIGH) and stays HIGH when pin 2 detects 1/3 rail voltage (even as a LOW pulse) when pin 6 is LOW.  (Pins 7 and 3 are "in phase.") Pin 7 is equal to pin 3 but pin 7 does not go high - it goes OPEN.  But it goes LOW and will sink about 200mA.

Pin 8 (Supply):
Connects to the positive power supply (Vs). This can be any voltage between 4.5V and 15V DC, but is commonly 5V DC when working with digital ICs. 


555 Timer Operating Modes

The 555 has main operating modes, Monostable, Astable, and Bistable. Each mode represents a different type of circuit that has a specific output.

Astable mode

An astable circuit has no stable state - thus the name "astable". The output continually switches state between high and low without without any intervention from the user, called a 'square' wave. This type of circuit could be used to give a mechanism intermittent motion by switching a motor on and off at regular intervals. It may even be used to flash lamps and LEDs, and is useful as a 'clock' pulse for other digital ICs and circuits.

  

Monostable mode

A monostable circuit produces pulse of a set length in response to a trigger input such as a push button. The output of the circuit stays in the low state until there is a trigger input, hence the name "monostable" meaning "one stable state". his type of circuit is ideal for use in a "push to operate" technique for a model displayed at exhibitions. A visitor can push a button to start a model's mechanism moving, & the mechanism will automatically switch off after a set time.

Bistable Mode (or Schmitt Trigger)

A bistable mode or what is sometimes called a Schmitt Trigger, has stable states, high & low. Taking the Trigger input low makes the output of the circuit go in to the high state. Taking the Reset input low makes the output of the circuit go in to the low state. This type of circuit is ideal for use in an automatic model railway technique where the train is necessary to run back & forth over the same piece of track. A push button (or reed switch with a magnet on the underside of the train) would be placed at each finish of the track so that when is hit by the train, it will either trigger or reset the bistable. The output of the 555 would control a DPDT relay which would be wired as a reversing switch to reverse the direction of current to the track, thereby reversing the direction of the train.

 

Using the Output of a 555 Timer

 

The output (Pin three) of the 555 can be in of states at any time, which means it is a digital output. It can be connected directly to the inputs of other digital ICs, or it can control other devices with the help of a few additional parts. The first state is the 'low' state, which is the voltage 0V at the power supply. The second state is the 'high' state, which is the voltage Vcc at the power supply.

Sinking & Sourcing

When the Output goes high, current will flow through the tool & switch it on. This is called 'sourcing' current because the current is sourced from the 555 & flows through the tool to 0V.

When the Output goes low, current will flow through the tool & switch it on. This is called 'sinking' current because the current is sourced from Vs & flows through the tool & the 555 to 0V.

Sinking & sourcing may even be used together so that devices can be alternately switched on & off.

The device(s) could be anything that can be switched on and off, such as LEDs, lamps, relays, motors or electromagnets. Regrettably, these devices must be connected to the Output in different ways because the Output of the 555 can only source or sink a current of up to 200mA. Make positive your power supply can provide current for both the device and the 555, otherwise the timing of the 555 will be affected.