Vth = Voltage given to Control Voltage when it is using. Vth = 2/3 Vcc when the Control Voltage is not using. This pin resets the timer (makes output low) when the voltage given to it becomes greater than Vth provided the Trigger input remains HIGH. This pin can be used to change the reference voltages of the comparators through this we can vary the timing. So in normal operations this pin will be tied to Vcc. When the voltage at this pin becomes less than 0.7 V, the IC will reset and the output will be 0V. Vt = 1/2 of the voltage at pin 5 when it is used. Vt = 1/3 Vcc, When the Control Voltage (pin 5) is not used. Precisely when the trigger voltage becomes less than Vt. A negative trigger at this pin sets the flip flop. As the name suggests it is used to trigger the timer. It is the pin connected to the inverting terminal of the lower comparator. It should be connected to the negative terminal of the DC Power Supply. This is the ground pin or common pin of the IC. Pin Diagram 555 and 556 Timer Pin Diagram Pin Functions of 555 Timer 1. Thus the output of the SR Flip Flop will be set according to the outputs of the comparator which in turn depends on the Trigger and Threshold inputs. The output of comparators are directly given to the inputs of SR Flip Flop. These reference voltages can be change using the Control Voltage Input (pin 5). The trigger input of 555 (pin 2) is connected to the comparator with reference voltage 1/3 Vcc and the threshold input (pin 6) is connected to the comparator with reference voltage 2/3 Vcc. Voltage at the junctions of these resistors will be 2/3 Vcc and 1/3 Vcc as all resistors are of same value which are used as the reference voltages of the comparators.Ī comparator is a device which compares two voltage and indicate which one is larger. It is hypothesized that 555 got its name from these three 5KΩ resistors. Then we can find that a voltage divider is formed between Vcc and Gnd using three 5KΩ resistor. ![]() Internal Diagram of 555 Timer 555 Timer Internal Diagramįrom the above diagram we can find that the power supply to the timer can be given through pin 8 ( Vcc ) and pin 1 ( Gnd ). It can be used for various timing applications from microseconds to hours range and can source or sink up to 200 mA current.Ī dual version of 555 named as 556 is also available which has two independent 555 in one IC. It is widely used in electronics circuits as it is very simple and cheap method to produce accurate and highly stable time delays. Now a days it is manufactured by many companies in bipolar and in low power CMOS. In 1972, Signetics Corporation first introduced this device as the SE/NE 555. The 555 Timer is the one of the most versatile linear hybrid Integrated Circuit (IC) which is used in variety of pulse generation, timer and oscillator applications.
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