555 timer

NE555 from Signetics in dual-in-line package

File:555-schem.png

Schematic symbol of the 555 timer

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". It is still in wide use, thanks to its ease of use, low price and good stability. Even today, Samsung in Korea manufactures over 1 billion units per year (2003).

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). 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. The 7555 requires slightly different wiring using fewer external components and less power.

The 555 has three operating modes:

• Monostable mode: in this mode, the 555 functions as a "one-shot". Applications include timers, missing pulse detection, bouncefree switches, touch switches, etc.
• Astable mode: the 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, etc.
• Bistable mode: the 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bouncefree latched switches, etc.

Usage

The connection of the pins is as follows:

Nr.	Name	Purpose
1	GND	Ground, low level
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 5 and 15 V, high level

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. An example configuration is shown below:

Example 555 schematic
Example 555 schematic

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.

In the astable mode, the high time from each pulse is given by

<math>high = 0.69 \cdot (R1 + R2) \cdot C</math>

and the low time from each pulse is given by

<math>low = 0.69 \cdot R2 \cdot C</math>

where R1 and R2 are the values of the resistor in ohms and C is the value of the capacitor in farads.

Specifications

These specifications apply to the NE555. Other 555 timers can have better specifications depending on the grade (military, medical, etc).

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

Derivatives

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

Manufacturer	Model	Remark
ECG Philips	ECG955M
Exar	XR-555
Fairchild	NE555/KA555
Harris	HA555
Intersil	SE555/NE555/ICM7555
Lithic Systems	LC555
Maxim	ICM7555
Motorola	MC1455/MC1555
National	LM1455/LM555C
NTE Sylvania	NTE955M
Raytheon	RM555/RC555
RCA	CA555/CA555C
Sanyo	LC7555
Texas Instruments	SN52555/SN72555; TLC555
USSR	K1006ВИ1
Zetex	ZSCT1555	down to 0.9V

External links

• 555 Timer Tutorial
• Data Sheet (Fairchild) (PDF)
• Java simulation of 555 oscillator circuit
• 1972 Signetics NE555 datasheet (PDF)