The 555 Timer
Twenty-five transistors, eight pins, fifty years of blinking, beeping and timing — meet the chip you'll build with.
Builds on: 2.3 Capacitors & the RC Time Constant2.2 Voltage Dividers3.2 Transistors
A chip you can actually understand
Designed by Hans Camenzind in 1971, the 555 timer is the best-selling chip of all time — billions made every year, half a century on. Inside are just ~25 transistors implementing three things you already know:
- A voltage divider — three internal 5 kΩ resistors (often said, a little apocryphally, to be the origin of the name) — creating two reference levels: ⅓ Vcc and ⅔ Vcc.
- Two comparators watching pins 2 and 6 against those references, flipping an internal memory (a flip-flop) that drives the output pin 3.
- A discharge transistor on pin 7 that can drain an external capacitor on command.
Astable mode: the electronic heartbeat
Wire R1 from Vcc to pin 7, R2 from pin 7 down to the capacitor, and the capacitor to ground, with pins 2 and 6 watching the capacitor. Now trace one cycle:
- Charge: output HIGH, discharge transistor off. C charges through R1 + R2, its voltage climbing your Lesson 2.3 curve.
- Trip high: C reaches ⅔ Vcc → the threshold comparator flips the flip-flop. Output snaps LOW, pin 7 switches on.
- Discharge: C drains through R2 only into pin 7, sliding back down the curve.
- Trip low: C hits ⅓ Vcc → the trigger comparator flips everything back. Output HIGH, and the cycle repeats — forever.
Every piece of this you have already mastered: the RC curve sets the pace (Lesson 2.3), a divider defines the thresholds (Lesson 2.2), a transistor does the draining (Lesson 3.2), and Ohm’s law picks the part values (Lesson 1.2). The 555 just packages the loop.
Slow it to ~1.5 Hz and you have your capstone blinker. Speed it to 440 Hz on a piezo and it plays an A. One-shot (“monostable”) wiring gives timed pulses — toasters, game-show buzzers, windshield-wiper delays. All from R, C and two thresholds.
⚡ Lab — The Astable Heartbeat
A full 555 astable on the scope: capacitor voltage in cyan ping-ponging between the ⅓ and ⅔ lines, output in amber, LED blinking along.
- Start with R1 = 1 k, R2 = 47 k, C = 10 µF — your exact capstone values. Note f ≈ 1.5 Hz.
- Grow C to 100 µF: ten times slower. Shrink R2: faster — but the duty cycle heads toward 100%. Grow R2 to approach 50%… why?
- At faster settings the sim slows the display so you can still see the shape (your eye couldn’t).