Timers and Integrated Circuits
Whole circuits shrunk onto a chip, and the famous timer chip that has been blinking LEDs since 1972.
The question
How does a turn signal blink at a steady rate, all by itself?
What you’ll discover: what integrated circuits are, how to read a chip’s pins, and how a timer chip makes steady pulses.
In simple terms
BeginnerA circuit on a chip
An integrated circuit (IC), or chip, is a complete circuit, from a few transistors to billions, built on one tiny piece of silicon and sealed in a plastic package with metal legs.
Beginner chips usually come in a DIP package (dual in-line package): a small black rectangle with two rows of legs, made to sit across the centre gap of a breadboard.
Finding pin 1: look for a notch or a dot at one end. With the notch pointing up, pin 1 is at the top left, and the pins are numbered down that side and back up the other side, counter-clockwise.
The 555 timer
The 555 timer, designed by Hans Camenzind and introduced in 1972, is one of the most widely produced chips ever made. With a capacitor and two resistors, it produces steady on-off pulses: blinking LEDs, beeping alarms, timing delays.
It works a bit like the Fading Light: a capacitor charges through a resistor. When it reaches two thirds of the supply voltage, the chip switches and starts emptying it; at one third, it switches back. Charge, empty, repeat, and the output flips between high and low each time.
A tip for battery projects: the original NE555 needs at least about 4.5 V, right at the edge of a 3 × AA pack. The CMOS versions, such as the TLC555, work down to about 2 V and are a better choice for low-voltage battery circuits.
Go deeper
IntermediateHow fast does it blink?
In its usual blinking (astable) circuit, with resistors R1 and R2 and capacitor C, the 555 runs at about:
f ≈ 1.44 ÷ ((R1 + 2R2) × C)
Example: R1 = 10 kΩ, R2 = 68 kΩ, C = 10 µF: f ≈ 1.44 ÷ (146,000 × 0.00001) ≈ 1 blink per second.
AdvancedDatasheets
Every chip has a datasheet: its manufacturer’s document listing what each pin does, the supply voltage range, the maximum currents, timing and typical circuits. Reading datasheets is a core engineering skill. Always check two things before connecting a chip: its supply voltage range, and which pins are power and ground. Reversing a chip’s power can destroy it instantly.
The 555’s output is rated up to 200 mA in the bipolar NE555 (an absolute maximum; keep well below 100 mA in practice, and much less for CMOS versions), and its timing depends on capacitor tolerance, which for electrolytics can be ±20% or more. That’s why a 555 blinker’s rate is approximate.
Quick check
How do you find pin 1 on a DIP chip?
Look for the notch or dot. With the notch at the top, pin 1 is top left.
What does a 555 timer’s capacitor do?
It charges and discharges between one third and two thirds of the supply voltage, setting the timing of the pulses.
Why is a TLC555 better than an NE555 for a 3 × AA battery pack?
The NE555 needs at least about 4.5 V; the CMOS TLC555 works down to about 2 V.
Words to know
- Integrated circuit (IC)
- A complete circuit built on one piece of silicon.
- DIP
- Dual in-line package: a chip with two rows of legs.
- Astable
- A circuit that switches back and forth continuously.
- Datasheet
- The manufacturer’s technical document for a part.
Learn more
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.