555 Timer and 555 Timer Working

555 Timer Integrated Circuit (IC) is a monolithic timing circuit introduced by Signetic Corporation in 1970. This device can be easily configured to produce accurate, highly-stable time delays or oscillations, just by adding a very few extra timing components. 555 timer IC is available as a 8-pin metal can or as 8- or 14-pin or 16-pin DIP. Depending on the manufacturing company, the internal structure of the IC has around 23 transistors, 2 diodes and 16 resistors.

pin configuration of 555 timer ic

The pin configuration of 8-pin DIP and 14-pin DIP are as shown by Figure 1a and 1b, respectively. Apart from these, they are also available these days in their CMOS version like that of Motorola MC1455.
The working of this IC can be understood by analyzing its internal structure which is shown in terms of block diagram (for 8-pin DIP) in Figure 2. From this, the IC is seen to be composed of

  • A resistive network formed by three equal resistors (R)
    It is due to these three 5 KΩ resistors, the IC bears its name as 555 timer. These are arranged in voltage divider configuration and as a result provide the voltage values of 2⁄3 VCC and 1⁄3 VCC along their chain.
  • Two Comparators
    These two devices are meant to compare the user-provided input values with their reference levels and accordingly trigger the output of the flip-flop.

    internal block diagram of 555 timer

  • Two Transistors
    These are associated with discharge and reset operations of the device.
  • One SR flip-flop
    This flip-flop will be either set or reset depending on the output from the comparators.
  • An Inverter
    Inverter is used to obtain the output from the device by complementing the flip-flop’s Q̅ output.

Following is a brief description on the function associated with each of its pins

  1. Ground (Pin 1 of 8-pin and Pin 3 of 14-pin package):
    Used a reference with which each of the voltage is measured.
  2. Trigger (Pin 2 of 8-pin and Pin 4 of 14-pin package):
    This pin is used to provide trigger to the circuit when the device will be configured to behave like a monostable multivibrator. As evident from Figure 2, it is seen that this pin is connected as an input to the comparator C2 which compares it with 1⁄3 VCC, fed as an input to its other terminal. As a result, when the user-provided negative pulse exceeds 1⁄3 VCC (obtained from the resistive network), the output of this comparator goes high. This causes the output Q of the SR flip-flop to become zero, thereby pulling its Q̅ pin high which makes the output of the inverter to go low, thereby resulting in a high output from the IC.
  3. Output (Pin 3 of 8-pin and Pin 5 of 14-pin package):
    This is the pin at which the output of the IC can be obtained. 555 timer IC provides two options for the user to load this pin viz., (i) Normally on load configuration where the load is connected between the Supply and the Output pins and (ii) Normally off load configuration where the load is connected between the Ground and the Output pins.
  4. Reset (Pin 4 of 8-pin and Pin 6 of 14-pin package):
    This pin can be used by the user to reset the IC as the user-provided negative going pulse on this pin switches OFF the associated transistor. This is because, a logic low on this pin causes the output of the flip-flop to go high, turning ON the discharge transistor. However, usually this pin will be connected to +VCC when not in use so as to avoid false triggering.
  5. Control Voltage (Pin 5 of 8-pin and Pin 9 of 14-pin package):
    This pin is used to control the levels of threshold as well as triggering. In addition, this pin can be used to control the pulse width of the output waveform as the voltage applied at this pin decides the condition at which the output of the comparator (C1) switches its state. The same regulation in the output waveform can be even experienced by connecting a potentiometer to this pin. Next, it is to be noted that when this pin is to be left unused, it is to be bypassed to ground via 0.01 μF capacitor in order to get rid of noise issue.
  6. Threshold (Pin 6 of 8-pin and Pin 10 of 14-pin package):
    This pin is connected to the positive terminal of the comparator C1 which compares the applied voltage with 2⁄3 VCC. Next, when the user provided voltage exceeds this reference level of 2⁄3 VCC, the output of C1 goes high, and thus the flip-flop’s output (Q) will be set. Due to this, the complement of its output (Q̅) will go low, resulting in a high output from the inverter, which will be nothing but the output of the IC.
  7. Discharge (Pin 7 of 8-pin and Pin 12 of 14-pin package):
    This pin is connected to the collector terminal of the internal transistor in 555 timer IC. Generally, a capacitor will be connected between this terminal and ground. This capacitor discharges through the transistor when it saturates, a phenomenon experienced when the output of comparator C1 sets the flip-flop indicating that the threshold voltage has increased in comparison with that of the control voltage. On the other hand, if the negative-going trigger pulse exceeds 1⁄3 VCC, then the output of the flip-flop goes low as the lower comparator’s output will go high. This inturn turns OFF the transistor during which the capacitor attached to its terminal starts to charge at a rate decided by the external resistor and the capacitor.
  8. Supply (Pin 8 of 8-pin and Pin 13 of 14-pin package):
    This pin is used to provide a voltage within the range of +5V to +18V wrt ground.

These ICs are most extensively used in electronic industry as they are versatile, compact, cheap and highly reliable. Further, these devices are seen to be used for a wide variety of applications wherein they act as oscillators (astable or monostable or bistable), one-shot or delay timers, pulse generators, LED or lamp flashers, alarm or tone generators, frequency dividers, logic clocks, power suppliers, DC-DC converters, digital logic probes, analog frequency meters, tachometers, temperature measuring devices, control devices, voltage regulators, etc.

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