from breadboard to PCB

Full Stack Electronics

Learn electronics by building a 555 timer from discrete components.

In this course you build a timer from individual transistors and resistors, then take it from breadboard to finished board. You prototype it on a breadboard and measure what it does. Then you design a printed circuit board, have it made, and solder and test it yourself.

The inside of a 555 is mostly analogue. You work with voltages, currents and transistor behaviour, then use those same transistors to build logic gates and memory. Each experiment adds to the timer that you later assemble on your own board.

Four builds of the same blinking-LED astable, numbered 1 to 4 on handwritten cards: an NE555 chip on a breadboard, a build from comparator and logic ICs, a build from individual transistors, and the finished black surface-mount board
The same timer, four ways. You start with an NE555 chip as a reference (1) and rebuild it with comparator and logic ICs (2). Then you build it from individual transistors (3) and turn that circuit into a surface-mount board (4). Each build blinks an LED, so you can compare it with the others.

What you'll learn

Who it's for

If you come from software and want to build hardware, this course takes you from a breadboard to a board of your own. You should already be comfortable with a breadboard and know Ohm's law. The course assumes no calculus, KiCad or surface-mount experience.

The measurement stages need an oscilloscope, a function generator, a power supply with two independently adjustable outputs, a multimeter and a hair dryer or heat gun. Board assembly adds soldering tools and magnification. The Bench lists the minimum capabilities, and The Parts lists the components. Check both before you begin.

How you'll work

Twenty-one experiments cover the four builds. In each experiment you build, measure and explain. Before each measurement, the experiment states the value to expect and the reason for it. When a result does not match, the instructions help you find the cause. Oscilloscope captures from my builds show what to look for on your own screen.

This box explains a new concept in the experiment that first needs it. The experiment then uses the idea on your bench.

This box gives history and context: where a circuit came from, why its designers chose it, and where else it is used.

This box gives the mathematics behind the circuit and connects the equations to the behaviour you measure.

Start the course →

Contents

Before You Start

Meet the course and prepare your bench.

Stage One

The Reference Chip

Build a working reference and measure its timing.

  1. 01The Reference 555Build a repeating timer and measure it as your reference. It flashes about five times per second, and the capacitor voltage cycles between one third and two thirds of the supply voltage.

Stage Two

The 555 from ICs

Turn the chip’s block diagram into a circuit you can probe.

  1. 02The 555, UnpackedRebuild the timer from a comparator IC, a NOR chip, two transistors and a three-resistor divider, and meet the five blocks inside.

Stage Three

The 555 from Transistors

Build and test each block, then join them into a working timer.

  1. 03The Almost-ComparatorOne transistor makes a crude comparator. Measure its limitations.
  2. 04The Differential PairTwo transistors share one current. A second input lets you set the threshold, and the pair drifts much less with temperature than one transistor.
  3. 05The Current MirrorUse a current mirror to keep the differential pair's total current steady, and measure how it improves the comparator.
  4. 06The Active LoadReplace the collector resistors with a PNP current mirror, and narrow the switching band from a quarter of a volt to a few millivolts.
  5. 07The Output StageAdd an output stage so the comparator output can swing close to ground and to the supply voltage. Then move its reference onto the board.
  6. 08The PNP TwinBuild the trigger comparator from PNP transistors as a mirror image of the threshold comparator. Then measure how close to ground the NPN pair keeps working.
  7. 09The Voltage DividerBuild the three-resistor divider that makes both references from the supply. Measure it at two supply voltages, then drive both comparators from one ramp.
  8. 10The NOR GateYou have built a NOT gate three times already. One more transistor makes it a NOR gate, the gate type that flew in the Apollo Guidance Computer.
  9. 11The SR LatchConnect two NOR gates in a loop, and the circuit stores one bit. This latch controls whether the 555 charges or discharges its capacitor.
  10. 12The SwapConnect every block into one loop and add the last two transistors. The all-transistor 555 blinks at the reference rate.
  11. 13The Reset PinThe eighth pin: an input that stops the timer while it is low.

Stage Four

The 555 on Copper

Take your circuit through design, fabrication, assembly and testing.

  1. 14The Master SchematicThe first step from the breadboard to a board. Draw a complete schematic with component labels, named connections and a parts list, and record where the board differs from the breadboard.
  2. 15Schematic CaptureThe second step from the breadboard to a board. Learn KiCad on a small circuit, then enter the master schematic sheet by sheet until the electrical rules check reports zero errors.
  3. 16The Smoke TestThe third step from the breadboard to a board. Run the KiCad schematic in KiCad's simulator with the outside parts of Experiment 12, compare its timing with the breadboard, and test the reset of the board.
  4. 17Going Surface-MountThe fourth step from the breadboard to a board. Choose SOT-23 and 0805 packages that you can solder by hand, give each symbol its footprint, and check them in a bill of materials.
  5. 18The BoardThe fifth step from the breadboard to a board. Draw the outline in the shape of an NE555, place the parts as seven blocks, route the copper and pour the ground until the design rules checker reports nothing.
  6. 19Before You OrderThe sixth step from the breadboard to a board. Check the board by eye, write the Gerber files and check them in a separate viewer, test the size on paper, and order. Milestone: the board design is complete.
  7. 20Assembly DayThe seventh step from the breadboard to a board. Sort the 45 surface-mount parts by their codes, solder them by hand block by block, add the eight pins and inspect each joint.
  8. 21First Power-UpThe last step from the breadboard to a board. Put the board in place of the NE555, switch it on behind a current limit, and measure it against the chip. Milestone: the board blinks the LED.

After the Course

Check the result against the aims of the Introduction.

Reference

Keep these pages open while you work.

Further reading

Start the course →

If you get stuck or something is confusing, write to me at @dnixty.