555 timer · Exterior
  1. 1Pin 1 notch
  2. 2NE555P timer
  3. 3Pin 1 dot
  4. 4Epoxy body (DIP-8) · 9.6 × 6.35 mm
  5. 5Pins (2.54 mm pitch)
Black DIP-8 body, notch at one end and a pin 1 dot, “NE555P” text on top
DIP-8: body ≈ 9.8 × 6.4 mm, pin pitch 2.54 mm, row spacing 7.62 mm
Integrated circuits

555 timer

In simple terms
What is this?

A tiny metronome: you set the speed of its ticking with the resistor and capacitor you connect from outside.

What is it for?

Makes an LED blink, produces a beep tone, or switches something on and off with a small delay.

Where do you see it?

In blinking decorative lights and simple alarms.

How do you spot it?

A black IC with eight legs (two rows of four), with 555 printed on top; the notched end points to the first leg.

Caution:

Place it the right way round using the notched end; do not reverse the supply and ground pins.

Technical definition

A timer IC containing two comparators, a flip-flop and a discharge transistor. You set the times with an external resistor and capacitors. In astable mode it produces a square wave on its own; in monostable mode it produces a single pulse when triggered.

Power supply4.5–16 V
Output≤ 200 mA
PackageDIP-8
Schematic symbol Same in IEC and ANSI
555VCCRSTGNDCVDISTHRTRIGOUT

IEC 60617: the drawing style used in Türkiye and Europe.

Pins 8
1GNDTop left when the notch is up; numbers increase counterclockwiseGND
2TRIG (trigger)When it drops below 1/3 VCC the output goes highTRIG
3OUT (output)Sources or sinks up to 200 mA of currentOUT
4RESETResets when low; tie to VCC if unusedRST
5CTRL (control)Access to the threshold voltage; 10 nF to GNDCTRL
6THR (threshold)When it reaches 2/3 VCC the output goes lowTHR
7DIS (discharge)Discharges the timing capacitor while the output is lowDIS
8VCCPower supply, 4.5–16 VVCC
Features typical
Power supply
4.5–16 V
Output current
≤ 200 mA (source or sink)
Power supply current
≈3–6 mA (5 V)
Thresholds
1/3 VCC and 2/3 VCC
Astable frequency
f ≈ 1.44 / ((R1 + 2·R2) · C)
Monostable time
t ≈ 1.1 · R · C
CMOS versions
TLC555 · LMC555 (low voltage and current)
Example part
NE555P (DIP-8)
Common mistakes
Leaving RESET floatingIf pin 4 is left floating, the IC can reset randomly. If you don't use it, connect it to VCC.
Counting pins backwardWith the notch up, pin 1 is at the top left and the numbers increase counterclockwise. A 555 fitted backwards can heat up and fail.
Choosing R1 too smallIn astable mode, too small an R1 lets the discharge transistor almost short circuit VCC. Keep R1 above 1 kΩ.

Test with a multimeter

Test whether this part is good with the multimeter on the measurement screen.

Take a measurement
Where it is used
LED flasherTone and siren generatorDelayed turn-onBrightness or speed control with PWM
Related lessons
Blinking LED with 555
Module 3 · Capacitor and timesoonRC charge and discharge
Module 3 · Capacitor and timesoonSound with a buzzer
Module 5 · Sensors and outputssoon
Try it in the Lab