MAX7219 Vintage Tube LED Clock Kit

What it does

Eight seven-segment digits, driven by one chip, over three wires. The MAX7219 holds the eight bytes in its own memory and refreshes the display from its own oscillator at about 800 scans a second — so a number you send once stays lit while your sketch reads a sensor, joins Wi-Fi, or blocks for a second.

That is the whole reason to use a driver chip rather than wiring the digits directly. Eight digits is sixty-four LEDs; multiplexing them in software is sixteen pins and a refresh loop that can never stop.

What is in the box

Three of the six displays arrive finished in their tubes: one red with decimal points, one blue with decimal points, one red with colons. The rest of the box is three driver boards, six loose panels and three empty tubes — which is exactly enough to make three more.

Two panels make one display

The driver board has a twelve-pad footprint either side of the chip, and each takes an ordinary four-digit 0.28-inch panel. The left one becomes digits 0 to 3 and the right one digits 4 to 7. Nothing in the wiring or the software knows which panel is which, so red beside blue works, and a colon panel beside a decimal one gives you a clock and a readout on one display.

Which panel

The colon is not a separate LED with its own pin. Each clock panel wires it to the decimal-point line of its second digit, so on the assembled eight the colons are digits 1 and 5 — and on the other six digits the point bit lights nothing at all. Eight segment lines, and the eighth is either the points or the colon. Never both.

Wiring, in five lines

1. GND to your board's GND.
2. VCC to 5 V — the 5V pin on an Uno, VIN on an ESP32, VBUS on a Pico. Not the 3.3 V pin.
3. DIN to any digital pin — D12 on an Uno, GPIO 23 on an ESP32.
4. CLK to any other — D11, or GPIO 18.
5. CS to a third — D10, or GPIO 5.

VCC is the one that is not a free choice. The chip is specified from 4.0 V, and the 3.3 V pin on an ESP32 or a Pico is a regulator output that cannot supply the few hundred milliamps a lit display wants in any case.

Soldering

The three loose driver boards need their two panels and their five-pin header fitted. That is twenty-nine through-hole joints per display on 2.54 mm pitch, which is about as forgiving as soldering gets — but it is not optional.

A wire pushed through an unsoldered hole makes a connection whose resistance changes every time anything moves, and what you get is missing digits, segments lighting at random, or a display that works until you nudge the desk. All three read like a wiring mistake or a library problem, and people spend the evening in the sketch.

Where to start

The handbook below runs from why eight digits need a driver chip through to a clock that sets itself over Wi-Fi. If you only read two of them, read Five volts, and the threshold before you wire anything and The first number to get it lit.

Example

Install LedControl by Eberhard Fahle from the Arduino Library Manager — the header is LedControl.h.

// Eight digits on a MAX7219: GND->GND, VCC->5V, DIN->D12, CLK->D11, CS->D10
#include <LedControl.h>

LedControl lc = LedControl(12, 11, 10, 1);   // DIN, CLK, CS, one chip

void setup() {
  lc.shutdown(0, false);     // it powers up blanked
  lc.setScanLimit(0, 7);     // all eight digits
  lc.setIntensity(0, 8);     // 0 dimmest, 15 brightest
  lc.clearDisplay(0);

  // 08:30 17:45. On colon panels the point of digit 1 is the left colon
  // and the point of digit 5 is the right one.
  lc.setDigit(0, 0, 0, false);
  lc.setDigit(0, 1, 8, true);
  lc.setDigit(0, 2, 3, false);
  lc.setDigit(0, 3, 0, false);
  lc.setDigit(0, 4, 1, false);
  lc.setDigit(0, 5, 7, true);
  lc.setDigit(0, 6, 4, false);
  lc.setDigit(0, 7, 5, false);
}

void loop() {
  // Nothing. The chip holds the bytes and refreshes the display itself.
}