What is 555 timer IC ?

The 555 timer IC is one of the most versatile chips ever produced. It was introduced by Signetics (now a part of ON Semiconductor) in 1972. It is widely used in many applications in the world of digital pulse generation. The 555 timer IC can be configured in Monostable Mode, Astable Mode or free running and Bistable Mode or Schmitt Trigger mode. Let us explore pin diagram and internal circuits of 555 IC.

555 timer internal circuits

As shown the device comprises two operational amplifiers used as comparators (C1 and C2), RS bistable element (FF), inverting output buffer(B1), transistor switch (Q14).

Inverting Buffer is incorporated so that considerable current can be sourced or sunk to/from a load. A single transistor switch Q14 is provided as means of rapidly discharging the external timing capacitor.

555 ic pin diagram

The basic 555 timer package is an 8-pin DIP (dual in-line package). There are various variations of the 555 timer IC available which include NE555, LM555 and TLC555 with slight differences in specifications and performance.

555 timer pin diagram

The figure-2 above depicts 555 IC pin diagram and the table below describes all these pins.

555 timer pin number Description
1 GND (Ground Connection)
2 Trigger (555 timer triggers when this pin transition occurs from voltage value from Vcc to 33% of Vcc. Output goes high when triggered )
3 Output (This pin delivers output)
4 Reset ( This pin resets 555 timer when low )
5 Control Voltage (Used to change 'threshold' and 'trigger' set point voltages.It is rarely used. )
6 Threshold (used to detect when the capacitor has charged. The output pin goes low when capacitor has been charged to 66.6% of Vcc.)
7 Discharge (It is used to discharge the capacitor. )
8 +VCC (5V to 15V supply input )

Circuit diagram of 555 timer in astable mode

555 timer astable configuration

The figure-3 mentions 555 timer circuit in astable mode. As shown, astable circuit connects trigger pin to capacitor. It adds a resistor between the discharge and threshold pins. As mentioned astable circuit triggers from previous output pulse. Monostable circuit requires an externally applied trigger.

In short, the output pin oscillates from high to low creating a series of pulses as the capacitor charge oscillates from 33.3% to 66.6% Vcc, without any external triggers.

The duration the output pin stays high (tHIGH) is expressed as follows:

The duration the output pin stays low (tLOW) is expressed as below:

The frequency (f) of the series of pulses is expressed as follows:
f = 1/(tHIGH + tLOW)

Applications of 555 timer in astable configuration.
• modulation of ultrasonic and IR transmitters
• Turn ON and OFF an actuator at set time intervals for a fixed duration.
• create an accurate clock signal.

Circuit diagram of 555 timer in monostable mode

555 timer monostable configuration

The figure-4 mentions 555 timer circuit in monostable mode. The Monostable circuit outputs one pulse for each high to low transition of the trigger pin. The output will return to the "stable" state after a period of time that is a function of the attached resistor and capacitor values.

The length of the output pulse depends on when the capacitor reaches 66.6% Vcc. This rate is determined by the charge capacity of the capacitor (C1) and resistance (R1). The length of the output pulse(tp) is expressed as follows.
tp = 1.1 *R1*C1

Applications of 555 timer in monostable configuration.
• Debounce a momentary or push button switch
• Turning on an actuator for a set period of time
• Turn an output from a resistive sensor from analog signal to digital signal

The 555 timer IC is popular due to its ease of use, low cost and wide availability. It's an excellent choice for beginners learning about electronics and for engineers designing various timing and control circuits. It is used to make LED flashers, pulse generators, timers, oscillators and PWM (pulse-width modulation) generators.

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