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Friday 10 February 2012

555 Timer

Pins

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 VCC.
3 OUT This output is driven to +VCC 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 VCC).
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+, VCC Positive supply voltage is usually between 3 and 15 V.

[edit] 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.

[edit] Monostable

Schematic of a 555 in monostable mode
The relationships of the trigger signal, the voltage on C and the pulse width in monostable mode
In the monostable mode, the 555 timer acts as a "one-shot" pulse generator. The pulse begins when the 555 timer receives a signal at the trigger input that falls below a third of the voltage supply. The width of the output pulse is determined by the time constant of an RC network, which consists of a capacitor (C) and a resistor (R). The output pulse ends when the voltage on the capacitor equals 2/3 of the supply voltage. The output pulse width can be lengthened or shortened to the need of the specific application by adjusting the values of R and C.[5]
The output pulse width of time t, which is the time it takes to charge C to 2/3 of the supply voltage, is given by
t = RC\ln(3) \approx 1.1
 RC
where t is in seconds, R is in ohms and C is in farads.
While using the timer IC in monostable mode, the main disadvantage is that the time span between the two triggering pulses must be greater than the RC time constant.[6]

[edit] Bistable

In bistable mode, the 555 timer acts as a basic flip-flop. The trigger and reset inputs (pins 2 and 4 respectively on a 555) are held high via Pull-up resistors while the threshold input (pin 6) is simply grounded. Thus configured, pulling the trigger momentarily to ground acts as a 'set' and transitions the output pin (pin 3) to Vcc (high state). Pulling the reset input to ground acts as a 'reset' and transitions the output pin to ground (low state). No capacitors are required in a bistable configuration. Pin 5 (control) is connected to ground via a small-value capacitor (usually 0.01 to 0.1 uF); pin 7 (discharge) is left floating.

[edit] Astable

Standard 555 astable circuit
In astable mode, the 555 timer puts out a continuous stream of rectangular pulses having a specified frequency. Resistor R1 is connected between VCC and the discharge pin (pin 7) and another resistor (R2) is connected between the discharge pin (pin 7), and the trigger (pin 2) and threshold (pin 6) pins that share a common node. Hence the capacitor is charged through R1 and R2, and discharged only through R2, since pin 7 has low impedance to ground during output low intervals of the cycle, therefore discharging the capacitor.
In the astable mode, the frequency of the pulse stream depends on the values of R1, R2 and C:
f = \frac{1}{\ln(2) \cdot C \cdot (R_1 + 2R_2)}[7]
The high time from each pulse is given by:
\mathrm{high} = \ln(2) \cdot (R_1 + R_2) \cdot C
and the low time from each pulse is given by:
\mathrm{low} = \ln(2) \cdot R_2 \cdot C
where R1 and R2 are the values of the resistors in ohms and C is the value of the capacitor in farads.
The power capability of R1 must be greater than \frac{V_{cc}^{2}}{R_1}.
Particularly with bipolar 555s, low values of R1 must be avoided so that the output stays saturated near zero volts during discharge, as assumed by the above equation. Otherwise the output low time will be greater than calculated above.
To achieve a duty cycle of less than 50% a diode can be added in parallel with R2 towards the capacitor. This bypasses R2 during the high part of the cycle so that the high interval depends only on R1 and C.

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