555 TIMER

555 TIMER:
The 555 timer IC is an integrated circuit (chip) used in a variety of timer, pulse generation and
oscillator applications. The 555 can be used to provide time delays, as an oscillator, and as a flip-
flop element. Derivatives provide up to four timing circuits in one package.
CIRCUIT DIAGRAM:

Pinout diagram




The connection of the pins for a DIP package is as follows:
Pin Name Purpose
1 GND Ground, low level (0 V)
2 TRIG OUT rises, and interval starts, when this input falls below 1/3 V
CC
.
3 OUT This output is driven to +V
CC
or GND.
4 RESET A timing interval may be interrupted by driving this input to GND.
5 CTRL "Control" access to the internal voltage divider (by default, 2/3 V
CC
).
6 THR The interval ends when the voltage at THR is greater than at CTRL.
7 DIS Open collector output; may discharge a capacitor between intervals.
8 V+, V
CC
Positive supply voltage is usually between 3 and 15 V.

Inputs of 555/556
Trigger input: when <
1
/
3
Vs ('active low') this makes the output high (+Vs). It monitors the
discharging of the timing capacitor in an astable circuit. It has a high input impedance > 2M .
Threshold input: when >
2
/
3
Vs ('active high') this makes the output low (0V)*. It monitors the
charging of the timing capacitor in astable and monostable circuits. It has a high input impedance
> 10M .
* providing the trigger input is >
1
/
3
Vs, otherwise the trigger input will override the threshold
input and hold the output high (+Vs).
Reset input: when less than about 0.7V ('active low') this makes the output low (0V), overriding
other inputs. When not required it should be connected to +Vs. It has an input impedance of
about 10k .
Control input: this can be used to adjust the threshold voltage which is set internally to be
2
/
3
Vs. Usually this function is not required and the control input is connected to 0V with a
0.01µF capacitor to eliminate electrical noise. It can be left unconnected if noise is not a
problem.
The discharge pin is not an input, but it is listed here for convenience. It is connected to 0V
when the timer output is low and is used to discharge the timing capacitor in astable and
monostable circuits.


Output of 555/556
The output of a standard 555 or 556 can sink and source up to 200mA. This is more than most
ICs and it is sufficient to supply many output transducers directly, including LEDs (with a
resistor in series), low current lamps, piezo transducers, loudspeakers (with a capacitor in series),
relay coils (with diode protection) and some motors (with diode protection). The output voltage
does not quite reach 0V and +Vs, especially if a large current is flowing.
To switch larger currents you can connect a transistor.
The ability to both sink and source current means that two devices can be connected to the output
so that one is on when the output is low and the other is on when the output is high. The top
diagram shows two LEDs connected in this way. This arrangement is used in the Level Crossing
project to make the red LEDs flash alternately.

Modes
The 555 has three operating modes:
 Monostable mode: in this mode, the 555 functions as a "one-shot" pulse generator.
Applications include timers, missing pulse detection, bouncefree switches, touch
switches, frequency divider, capacitance measurement, pulse-width modulation (PWM)
and so on.
 Astable: free running mode: the 555 can operate as an oscillator. Uses include LED and
lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse
position modulation and so on. Selecting a thermistor as timing resistor allows the use of
the 555 in a temperature sensor: the period of the output pulse is determined by the
temperature. The use of a microprocessor based circuit can then convert the pulse period
to temperature, linearize it and even provide calibration means.
 Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS pin is not
connected and no capacitor is used. Uses include bounce-free latched switches