555 Timer IC
Integrated Circuit
A classic, versatile timer chip used to make delays, pulses, and oscillators (square waves). The best 'first IC' for beginners.
Schematic symbol
Integrated circuit (DIP)
How 555 timer ic appears in a circuit diagram.
Types & variants
Key specs
Supply voltage
VNE555: 4.5-16V; CMOS 7555: 2-18V.
Output current
mACan source/sink ~200mA - enough to drive a small relay or LED directly.
Timing
-Set by external R and C: t depends on R x C.
Laws that govern it
- Law of capacitance — capacitance formula Q = CVQ = C × V, i = C·dv/dt, E = ½CV²
The timing is charge and discharge of C through R: t = 0.693 × R × C comes straight from the capacitor's exponential law.
- Kirchhoff's voltage law (KVL)ΣV(loop) = 0
The internal three-resistor chain divides the supply into the ⅓ and ⅔ thresholds the comparators watch — a divider, so the loop must sum.
- Ohm's lawV = I × R
Timing resistors set the charging current; too small and you exceed the discharge pin rating, too large and leakage ruins the accuracy.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
Markings
Part number on top (NE555P etc). Pin 1 marked by a dot/notch.
Standard values
Astable frequency f = 1.44 / ((R1 + 2R2) x C).
How to choose
Use NE555 for general 5-15V projects; ICM7555 for low-power/battery. Pick R and C with the 555 calculator for your timing.
Pinout & package
DIP-8 (through-hole) — top view, notch up
8 pins: GND, TRIG, OUT, RESET, CTRL, THRES, DISCH, VCC. DIP-8 or SOIC-8.
Example circuits
- Astable LED blinker
- Monostable one-shot timer (button -> timed output)
- PWM generator
Common failures
Latch-up / overheating from output overload; noise if CTRL pin not bypassed with a small cap.
How to test
Build the standard astable blinker - if the LED blinks at the calculated rate, it's good.
Substitutes
NE555 / LM555 / TLC555 are mostly pin-compatible; CMOS versions for lower current.