The 555 is a integrated circuit (chip) implementing a variety of timer and multivibrator applications. Introduced in 1971 by Signetics (later acquired by Philips), it is still in wide use, thanks to its ease of use, low price and good stability.

The purpose of the 555 is to hold its output line at a high potential for a certain time after it has been triggered. It can also be used as an oscillator, by triggering itself.

Usage

The connection of the pins is as follows:

Nr.	Name	Purpose
1	GND	Ground
2	TR	A short pulse 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
5	CV	Control voltage allows access to the internal voltage divider (2/3 Vcc)
6	THR	The threshold at which the interval ends
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 5 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. A example configuration is shown below:

The interval time t is given by

<math>t = RC<math>

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.

Derivatives

Many pin-compatible variants, including CMOS versions, have been built by various companies. The 555 is also known under the following type numbers:

• CA555, CA555C
• ECG955M
• HA555
• ICM7555
• LC555
• LC7555
• LM1455, LM555C
• MC1455, MC1555
• NE555
• NTE955M
• RC555, RM555
• SE555
• SN52555, SN72555
• XR-555
• KA555

External links

• 555 Timer Tutorial (http://www.uoguelph.ca/~antoon/gadgets/555/555.html)