Electrical Engineering Dictionary
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## 555 Timer IC

555 Timer Introduction.
The 555 Timer is an integrated circuit used as a timer, a pulse generator, tone generator and many other circuit functions, all from one universal IC function. Each pin function is described and design recommendation are given when required. A Monostable Multivibrator setup is used as an example to describe the operation of the 555. The basic block diagram for the 555 timer is shown in the graphic below and referenced by the text.

555 Timer Voltage Supply.
Note the term Vcc is used because the 555 will operate with any Vcc [pin 8] between 4.5 volts and 16 volts. The actual voltage used by the 555 does not effect the timing circuit because both the charging rate of the capacitor and the threshold of the capacitor use the same voltage.
As with any integrated circuit always Bypass the Vcc pin to ground with a 0.1uF ceramic capacitor [not shown in the circuit below]. Pin 8 is Vcc and Pin 1 is ground; however the capacitor should be connector to Vcc and taken to the closest possible ground, which may not necessarily be pin 1 of the IC. A by-pass capacitor is also called a Decoupling Capacitor. The 555 IC may be operated with any voltage between 4.5 volts and 16 volts [see data sheet info below]. Select the Vcc voltage based on the circuit being driven by the 555 timer. The [trigger] input to the 555 is only being used as a reference switch so the actual voltage used is not important as long as the input transitions past the reference point [more below].

555 Operational Description.
The 555 uses two comparators to control the operation of the timer. The lower comparator is set to trigger at 1/3 Vcc while the upper comparator is set to trigger at 2/3 Vcc. Three internal 5k resistors placed in series between Vcc and ground are used to provide voltage references. The 1/3 Vcc voltage is generated between the bottom and middle 5k resistors and the 2/3 Vcc voltage is generated between the middle and upper 5k resistors.

The Trigger input [TRIG] is applied to the first comparator which is referenced to the 1/3 Vcc. So as long as the trigger voltage remains above 1/3 Vcc the 555 will not trigger [operate]. The second comparator is referenced to 2/3 Vcc and is used to discharge the timing capacitor C1 once the voltage across the capacitor reaches 2/3 Vcc. 555 Monostable Multivibrator

555 Timing.
In addition to the external capacitor an external resistor is also required to complete the timing circuit. The resistor [R1] and the capacitor [C1] form an RC circuit which define the length of time it takes the capacitor to charge [defined by this RC Curve]. The discharge of the capacitor is completed through an internal transistor which is controlled by the two comparators.

Before the Trigger pulse occurs the internal transistor is 'ON' [conducting] so the capacitor is effectively shorted to ground and is not allowed to charge.
Once the trigger is applied and falls below 1/3 Vcc the comparator trips and turns the transistor off. When the transistor shuts off the capacitor is allowed to charge toward Vcc. In effect the above text describes the operation of a monostable circuit. As the output will toggle once the negative trigger pulse is applied and will remain high until the capacitor reaches 2/3 Vcc and is again shorted out [discharged] by the internal transistor. The output will switch back to its resting state [low]. 555 Monostable Waveforms

The top waveform is the input trigger pulse applied to pin 2. The center waveform represents the charging voltage on capacitor C1. While the bottom voltage waveform shows the resultant output on pin 2.

555 Output Drive.
Internally the 555 uses a buffer to drive the output pin capable of about 200mA of sink current [depending on the load]. However the output pulse could have a rise or fall time in the range of 100nS. Back in mid 1970s' a transition time of 100nS would be normal, these days 100nS is extremely slow and could cause problems if used as an input to high speed logic devices.
Pin 3 is the output of the circuit. The output will toggle between ground and Vcc each time a trigger is applied to the circuit. With Vcc set at 5 volts the output will switch to a minimum voltage of 2.75 volts or a typical voltage of 3.3 volts. Using a 15 volt Vcc as another example the 555 will typically reach 12.5 volts [200mA source], 13.3 volts [100mA source]. The rise and fall times of the output pulse will be about 100nS.

555 Reset.
During normal operation the reset pin  should be held high [Vcc]. However if a reset is required the reset pin may be taken low to reset the 555. Once the reset is taken low the internal transistor connected to the discharge pin will turn on and immediately short out the timing capacitor [C1]. The discharge transistor remains on as long as the reset pin is held low. Once the reset pin is taken high the 555 returns to normal operation. If the trigger is high the capacitor stays at zero volts, however if the trigger is low the capacitor begins to charge up to 2/3 Vcc as normal. But as long as the reset is low the circuit is inhibited from working.

555 External Control.
The Control pin  is used to change the timing interval of the timing circuit. Under normal conditions the control pin will not be used and may be left unconnected [it's internally connected to the 2/3 Vcc tap of the resistor chain]. However good design practice dictates that this pin should be bypassed to ground via a capacitor [normally 0.01uF is used].
However the control pin may be used to change the 2/3 Vcc set point within the 555. By applying an external voltage to the control pin the upper threshold voltage feeding the capacitor may be changed to something other than 2/3 Vcc. In effect this changes the maximum voltage the timing capacitor can charge to. Applying a voltage to the control pin alters the amount of time the capacitor will be charging. This in turn alters the amount of time the output will be high because the capacitor will be shorted [discharged] when it reaches the new threshold level.
So in effect by simply changing the voltage on the control pin the output pulse width can be varied with out the need to change either the external resistor or external capacitor. The 555 can be made into a variable monostable pulse generator as required by an external circuit feeding a variable voltage to the control pin. So an external circuit could be used to determine the actual pulse width of the 555 and not just the resistor and capacitor. In this circuit example the voltage on the Control pin is used to compensate for timing capacitor tolerance; 555 Capacitor Adjustment Circuit.

555 False Trigger.
Because there could be a possibility that the 555 could false trigger on the rising edge of the trigger pulse two additional components could be added to the circuit. These two components are not required, but only serve to make the circuit more stable. If required the normal filter circuit would consist of a 10k pull-up resistor from the Trigger pin to Vcc, and a 0.001uF capacitor should be added between the trigger signal and the trigger pin.

555 Trigger: Pin 2 is connected to the input trigger voltage for the circuit. Each time a Trigger is applied the output will toggle high for a predetermined amount of time, based on the value of the resistor [R1] and capacitor [C1] used. A high to low transition [low pulse] on the trigger input will cause the output to switch; however, the trigger should be taken high before the output pulse times out. In other words the output pulse width should be longer than the input trigger width.

555 Timing Equations:
Charge Time; t1 = 0.693*(R1 + R2)*C1
Discharge Time; t2 = 0.693*R2*C1
Total period; T = t1 + t2 = 0.693*(R1 + 2*R2)*C1
Oscillation frequency; f = 1/T = 1.44/[(R1 + 2*R2)*C]
Duty cycle = D = R2/(R1 + 2*R2)

Additional passive components can be used with the 555 to enhance or change its operation depending on the device used and its placement within the 555 circuit. Thermistors could be added to detect changes in the circuit temperature. Photocells can be used to detect changes in the light levels, and many other components could be used.