One of the most versatile linear integrated circuits is the 555 Timer. Signetics Corporation first introduced this device as the SE/NE 555 in early 1970. Since its debut, the device has been used in a number of novel and useful applications. A sample of these applications includes monostable and astable multivibrators, dc-dc converters, digital logic probes, waveform generators, analog frequency meters and tachometers, temperature measurement and control, infrared transmitters, burglar and toxic alarms, voltage regulators, electric eyes, and many others. The 555 Timer is a monolithic timing circuit that can produce accurate and highly stable time delays or oscillation. The timer basically operates in one of the two modes: either as a monostable (one-shot) multivibrator or as an astable (free running) multivibrator. The device is available as an 8-pin metal can, an 8-pin mini DIP, or a 14-pin DIP. Figure below shows the connection diagram of the SE/NE 555 timer;
The SE555 is designed for the operating temperature range of -550 to +1250 C, while the NE555 operates over a temperature range of 00 to 700 C. The important features of the 555 timer are these: it operates on +5 to +18 V supply voltage in both free-running (astable) and one-shot (monostable) modes; it has an adjustable duty cycle; timing is from microseconds through hours; it has a high current output; it can source or sink 200 mA; the output can drive TTL and has a temperature stability of 50 parts per million (ppm) per degree Celsius change in temperature, or equivalently 0.005%/0C. Like general-purpose op-amps, the 555 timer is reliable, easy to us, and low cost.
Pin Configurations
Pin 1: Ground: All the voltages are measured with respect to its terminal.
Pin 2: Trigger: The output of the timer depends on the amplitude of the external trigger pulse applied to this pin. The output is low if the voltage at this pin is greater than 2/3Vcc. However, when a negative-going pulse of amplitude larger than 1/3Vcc is applied to this pin, the comparator 2 output goes low, which in turn switches the output of the timer high. The output remains high as long as the trigger terminal held at a low voltage.
Pin 3: Output: There are two ways a load can be connected to the output terminal: either between pin 3 and ground (pin 1) or between pin 3 and supply voltage +Vcc (pin 8). When the output is low, the load current flows through the load connected between pin 3 and +Vcc into the output terminal and is called sink current. However, the current through the grounded load is zero when the output is low. For this reason, and that connected between pin 3 and +Vcc is called the normally on load and that connected between pin 3 and ground is called the normally off load. On the other hand, when the output is high, the current through the load connected between pin 3 and +Vcc (normally on load) is zero. However, the output terminal supplies current to the normally off load. This current is called the source current. The maximum value of sink or source current is 200 mA.
Pin 4: Reset: The 555 timer can be reset (disabled) by applying a negative pulse to this pin. When the reset function is not in use, the reset terminal should be connected to +Vcc to avoid any possibility of false triggering.
Pin 5: Control voltage: An external voltage applied to this terminal changes the threshold as well as the trigger voltage. In other words, by imposing a voltage on this pin or by connecting a pot between this pin and ground, the pulse width of the output waveform can be varied. When not used, the control pin should be bypassed to ground with a 0.01 uF capacitor to prevent any noise problems.
Pin 6: Threshold: This is the non-inverting input terminal of comparator 1, which monitors the voltage across the external capacitor. When the voltage at this pin is greater than or equal to threshold voltage 2/3Vcc, the output of comparator 1 goes high, which in turn switches the output of the timer low.
Pin 7: Discharge: This pin is connected internally to the collector of transistor Q1, as shown in above figure. When the output is high, Q1 is off and acts as an open circuit the external capacitor C connected across it. On the other hand, when the output is low, Q1 is saturated and acts as a short circuit, shorting out the external capacitor C to ground.
Pin 8: +Vcc: The supply voltage +5 V to +18 V is applied to this pin with respect to ground (pin 1).
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