CMOS 555 Timer: Structure Explained and Reverse Engineered
This is an amazing walkthrough from Ken Shirriff on the CMOS version of the 555 timer IC. Using a die shot from Zeptobars he examines transistors, gates, ‘how resistors are implemented in silicon,’ and much, much more in the low-power version of this venerable integrated circuit. If you’ve ever tinkered with the 555 – or, for that matter, any IC! – these are all probably points of interest that have further piqued your curiosity – and here they are, explained and reverse engineered!
The diagram below illustrates the internal operation of the 555 timer used as an oscillator. An external capacitor is repeatedly charged and discharged to produce the oscillation. Inside the 555 chip, three resistors form a divider generating reference voltages of 1/3 and 2/3 of the supply voltage. The external capacitor will charge and discharge between these limits, producing an oscillation, as shown on the left. In more detail, the capacitor will slowly charge (A) through the external resistors until its voltage hits the 2/3 reference. At that point (B), the threshold (upper) comparator switches the flip flop off turning the output off. This turns on the discharge transistor, slowly discharging the capacitor (C) through the resistor. When the voltage on the capacitor hits the 1/3 reference (D), the trigger (lower) comparator turns on, setting the flip flop and the output on, and the cycle repeats. The values of the resistors and capacitor control the timing, from microseconds to hours.
I especially dig this block-division of the die shot of the IC:
Adafruit publishes a wide range of writing and video content, including interviews and reporting on the maker market and the wider technology world. Our standards page is intended as a guide to best practices that Adafruit uses, as well as an outline of the ethical standards Adafruit aspires to. While Adafruit is not an independent journalistic institution, Adafruit strives to be a fair, informative, and positive voice within the community – check it out here: adafruit.com/editorialstandards
Stop breadboarding and soldering – start making immediately! Adafruit’s Circuit Playground is jam-packed with LEDs, sensors, buttons, alligator clip pads and more. Build projects with Circuit Playground in a few minutes with the drag-and-drop MakeCode programming site, learn computer science using the CS Discoveries class on code.org, jump into CircuitPython to learn Python and hardware together, TinyGO, or even use the Arduino IDE. Circuit Playground Express is the newest and best Circuit Playground board, with support for CircuitPython, MakeCode, and Arduino. It has a powerful processor, 10 NeoPixels, mini speaker, InfraRed receive and transmit, two buttons, a switch, 14 alligator clip pads, and lots of sensors: capacitive touch, IR proximity, temperature, light, motion and sound. A whole wide world of electronics and coding is waiting for you, and it fits in the palm of your hand.
Have an amazing project to share? The Electronics Show and Tell is every Wednesday at 7pm ET! To join, head over to YouTube and check out the show’s live chat – we’ll post the link there.