The 555 is an integrated circuit (chip) implementing a variety of timer and multivibrator applications. The IC was designed and invented by Hans R. Camenzind. It was designed in 1970 and introduced in 1971 by Signetics (later acquired by Philips). The original name was the SE555/NE555 and was called "The IC Time Machine". The 555 gets its name from the three 5-kOhm resistors used in typical early implementations (Reference 1). It is still in wide use, thanks to its ease of use, low price and good stability. , 1 billion units are manufactured every year.
The 555 timer is one of the most popular and versatile integrated circuits ever produced. It includes 23 transistors, 2 diodes and 16 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8). The 556 is a 14-pin DIP that combines two 555s on a single chip. The 558 is a 16-pin DIP that combines four slightly modified 555s on a single chip (DIS & THR are connected internally, TR is falling edge sensitive instead of level sensitive). Also available are ultra-low power versions of the 555 such as the 7555 and TLC555. The 7555 requires slightly different wiring using fewer external components and less power.
The 555 has three operating modes:
|1||GND||Ground, low level (OV ) ZERO VOLTS.|
|2||TR||A short pulse high → low on the trigger starts the timer|
|3||Q||During a timing interval, the output stays at +VCC|
|4||R||A timing interval can be interrupted by applying a reset pulse to low (0V)|
|5||CV||Control voltage allows access to the internal voltage divider (2/3 VCC)|
|6||THR||The threshold at which the interval ends (it ends if U.thr → 2/3 VCC)|
|7||DIS||Connected to a capacitor whose discharge time will influence the timing interval|
|8||V+, VCC||The positive supply voltage which must be between 3 and 15 V|
Using simply a capacitor and a resistor, the timing interval, i.e. the time during which the output stays low, can be adjusted to the need of the specific application. Thus, the 555 operates in monostable mode.
The interval time t is given by
which is the time it takes to charge C to 63% of the applied voltage (exact figure: (1-1/e)V). See RC circuit for an explanation of this effect.
When in astable mode, a resistor (call it R1) is connected between Vcc and the discharge pin (pin 7) and another (R2) is connected between the discharge pin (pin 7) and the trigger (pin 2) and threshold (pin 6) pins that share a common node. Hence the capacitor is charged through R1 and R2, and discharged only through R2, since pin 7 has low impedance to ground during output low intervals of the cycle, therefore discharging the capacitor. The use of R2 is mandatory, since without it the high current spikes from the capacitor may damage the internal discharge transistor.
In the astable mode, the high time from each pulse is given by
and the low time from each pulse is given by
It has a voltage varying gate. The gate is open when the output voltage is low, and the gate is closed when output voltage is high.
|Supply voltage (VCC)||4.5 to 15 V|
|Supply current (VCC = +5 V)||3 to 6 mA|
|Supply current (VCC = +15 V)||10 to 15 mA|
|Output current (maximum)||200 mA|
|Power dissipation||600 mW|
|Operating temperature||0 to 70 °C|
Many pin-compatible variants, including CMOS versions, have been built by various companies. There also exists bigger packages with two or four timers on the same chip. The 555 is also known under the following type numbers:
|IK Semicon||ILC555||CMOS from 2V|
|Maxim||ICM7555||CMOS from 2V|
|National Semiconductor||LMC555||CMOS from 1.5V|
|Texas Instruments||SN52555/SN72555; TLC555||latter: CMOS from 2V|
|Zetex||ZSCT1555||down to 0.9V|
The dual version is called 556. It features two complete 555:s in a 14 pin DIL package.
The quad version is called 558 and has 16 pins. To fit four 555:s into a 16 pin package the control voltage and reset lines are shared by all four modules. Also for each module the discharge and threshold are internally wired together and called timing.