555 TIMER IC

The 555 timer IC is an integrated circuit (chip) which is used in a variety of timer, delay, pulse generation, and oscillator applications.

555 Signals and Pinout (8 pin DIP)

Figure below shows the input and output signals of the 555 timer as they are arranged around a standard 8 pin dual inline package (DIP).

Pin 1 - Ground (GND)

This pin is connected to ground of the circuit i.e. ground reference voltage.

Pin 2 - Trigger (TRI)
A low voltage (less than 1/3 the supply voltage) applied for a very short time to the trigger input that causes the output (pin 3) go high. The output will remain high until a high voltage is applied to the threshold input (pin number 6).

This pin is responsible for transition of the flip-flop from set to reset. The output of the timer depends upon the amplitude of the external trigger pulse applied to this pin.

Pin 3 – Output (OUT)
In output low state :

The voltage will be close to 0V.

 In output high state:

The voltage will be 1.7V lower than the main supply voltage.

 For example, if the supply voltage is 5V output high voltage will be 3.3 volts and the output can source or sink up to 200 mA (maximum depends on supply voltage).

Pin 4 – Reset (RES)
A low voltage (less than 0.7V) is applied to the reset pin  that will cause the output (pin 3) to go low. This input should remain connected to Vcc when IC is not in used.

Pin 5 – Control voltage (CON)
You can control the threshold voltage (pin 6) of the IC through the control input (which is internally set to 2/3 of the supply voltage). You can vary it from 45% to 90% of the main supply voltage. This enables you to vary the length of the output pulse in monostable mode or the output frequency in the astable mode. When not in use it is recommended that this input should be connected to circuit ground using a 0.01uF capacitor. Pin 5 determines the pulse width of the output waveform. An external voltage applied to this pin can also be used to modulate the waveform output.

Pin 6 – Threshold (TRE)
In both astable and monostable mode the voltage across the timing capacitor is observed through the threshold input. When the voltage at this input rises above the threshold value then the output will go from high to low. It compares the voltage applied to the terminal with a reference voltage of 2/3 of the Vcc. The amplitude of voltage applied to this terminal is responsible for the set state of the flip-flop in the IC.

Pin 7 – Discharge (DIS)
When the voltage across the timing capacitor exceeds threshold value, the timing capacitor is discharged through this input. It toggles the output from high to low till voltage reaches 2/3 of the supply voltage

Pin 8 – Supply voltage (VCC)
This is the positive supply voltage terminal of the IC. The supply voltage range vary between +5V and +15V. The RC timing interval will not much vary over the supply voltage range (approximately 0.1%) in either astable or monostable mode.

555 as Monostable

 Figure shows the schematic diagram of the LM555 that flashes an LED in monostable mode.

Design Requirements

 The main design requirement for this application requires calculating the duration of time for which the output stays high and low. The duration of time is dependent on the values of R(Resistance) and C(Capacitance) and can be calculated by formula:

 t = 1.1 × R × C seconds

Detailed Design Procedure

 To allow the LED to flash on for the amount of time that can be noticed, a 5 second time delay was chosen for this application. By using above equation, RC equals 4.545. If R is selected as 100 kΩ, then       C = 45.4 µF. The values of R = 100 k Ω and C = 47 µF was selected based on standard values of resistors and capacitors. A momentary push button switch connected to ground is connected to the trigger input with a 10-K current limiting pull up resistor to the supply voltage. When the push button is pressed, the trigger pin connects to GND. An LED is connected to the output pin with a current limiting resistor in series from the output of the LM555 and negative terminal to GND. The reset pin is not used and was connected to the supply voltage lane in the breadboard.

555 as Astable

Operation

If the circuit is connected as shown in figure below (pins 2 and 6 connected (shot)) it will trigger itself and free run as a multi vibrator. The external capacitor charges through RA + RB and discharges through RB only. Thus the cycle may be precisely set by the ratio of these RA and RB two resistors. In this mode of operation, the capacitor charges and discharges between 1/3 of VCC and 2/3 of VCC. As in the triggered mode, they charge and discharge, and therefore the frequency is independent of the supply voltage.