Know a bit about the popular 555 timer IC!
555 IC was one of the earliest integrated circuits to be marketed, as early as 1971. By 1972, there were five companies manufacturing the 555 IC and it became a best-selling product. In 2017, it was estimated that a billion 555 timers were produced annually, making it the most popular integrated circuit ever! 555 timer was designed by Hans Camenzind. It has the equivalent of 25 transistors, 2 diodes and 15 resistors on a silicon chip and comes in an 8-pin dual in-line package (DIP-8). Important components of 555 timer as can be seen on a block diagram are voltage divider, threshold comparator, trigger comparator, reset latch, output and discharge.
Voltage divider consists of three 5 K resistors between the supply voltage and ground. Earlier it was thought that these resistors were the reason for calling it 555 IC. Supply voltage is given to pin 8 and pin 1 is the ground. Control pin with number 5 is connected between the upper two resistors, allowing an external voltage to control the reference voltages. When the control is not driven, the voltage divider creates an upper reference voltage which is two thirds of the supply voltage and a lower reference voltage which is one third of the supply voltage. When a control voltage is applied to pin 5, the upper reference voltage will be the control voltage and the lower reference voltage will be half of that.
Threshold comparator’s negative input is connected to the upper reference voltage of the voltage divider, which also receives control voltage from pin 5. Positive input of the threshold comparator is from pin 6. If control pin 5 is not being used, it should be connected with a 10 nF decoupling capacitor to the ground pin 1 to ensure that electrical noise does not affect the internal voltage divider.
Positive input of the trigger comparator receives the lower reference voltage from the voltage divider. Negative input is from pin 2. When the trigger voltage falls below half the control voltage, output goes to the high state and a timing interval starts. Output will remain in the high state as long as trigger voltage is kept low. High state is supply voltage minus approximately 1.7 Volts while the low state is at ground voltage.
A set-reset latch circuit stores the state of the time and is controlled by the two comparators. Input from the reset pin 4 overrides the other two inputs and thus the latch and therefore the entire timer can be reset at any time using this input. If pin 4 is not being used in a circuit, it should be connected to the supply voltage pin 8 to prevent electrical noise from accidentally causing a reset.
The output stage following the latch has a push-pull circuit which can supply up to 200 mA output to pin 3.
Output of the latch also controls a transistor which acts as an electronic switch that connects discharge pin 7 to ground pin 1. Typical supply voltage of 555 timer IC to pin 8 ranges from 4.5 to 16 volts.