Assembling the five-pointed star LED kit

This board has parts on both faces, so work out which is which before you start.

The side printed MH-T12, with the chip outline in the middle, is the component side. The resistors, IC socket, power jack, switch and header go in from here.

The other face, with fifty circles printed in a star, is the star side. Only the LEDs go in from there.

Every part is soldered on the face opposite the one it sits on.

Lay everything out and check it against the parts table above.

The kit includes two spare resistors and five spare LEDs, so you’ll have a few left over at the end. Sort out anything that’s missing before you pick up the iron.

All twelve resistors are 1 kΩ: ten to fit and two spares. Because they’re all the same value, any resistor can go in any position.

Colour code: brown-black-black-brown-brown (5-band) or brown-black-red-gold (4-band).

The resistors sit in five pairs: top left, top right, bottom left, and one pair on each side of the chip outline.

Push each one through from the MH-T12 side, then bend its legs outwards on the star side so it can’t drop out. Press the body flat against the board. Resistors aren’t polarised, so either way round is fine.

Turn the board over and solder all twenty joints. Each one should be full and rounded, not a dull blob sitting on the pad.

Snip every leg flush with its joint.

The socket goes into the long outline in the middle of the MH-T12 side.

The socket and the printed outline both have a half-moon notch at one end, and they must line up. On this board the notch points towards the star’s top point.

Fit the socket now and leave the chip out. The chip pushes in later, well away from the heat.

Hold the socket against the board (a blob of Blu Tack helps with the board upside down) and solder two diagonally opposite corner pins.

Check the socket is sitting flat. If one end has lifted, reheat that corner and press it down before you go on.

Work down each row. The pins are only 2.54 mm apart, which is where solder bridges happen.

When you’re done, check that no two neighbouring joints have run together. If they have, drag the iron between them to separate them.

The DC jack goes in the lower outline on the right-hand edge of the MH-T12 side.

Its round opening faces out over the edge of the board, where the plug can reach it. Push it right down until the body sits flat.

The switch goes in the rectangular outline just above the jack. It lies on its side, and the button on its long stem reaches out past the edge so you can press it.

It’s a latching switch: press once for on, and again for off.

Turn the board over. These pins are much chunkier than a resistor leg, so hold the iron on each one for a few seconds longer and feed in enough solder to fill the hole.

The switch has two larger mounting tabs as well. Solder those too, because they’re what keep it steady when you press the button.

The header is only for loading a new program into the chip (see the last step), so you can leave it out.

If you fit it, it goes in the small outline below the chip on the MH-T12 side, with the short end of the pins into the board and the long end pointing up.

Four joints on the star side. Tack one pin, check the header is standing straight, then solder the other three.

Match the chip’s notch to the socket’s notch, towards the star’s top point.

If the two rows of pins splay outwards, roll them gently against the table until they’re parallel. Line every pin up with its hole, check none has folded under, then press down evenly with your thumb.

The kit’s paper sheet fits the chip last. We fit it now so you can test the first LED in a few steps’ time. None of the joints still to come are on the chip’s own pins, and they’re all out round the star.

Each LED has a tiny chip inside that cycles its own colour. That chip only works one way round, so the LEDs are polarised.

The long leg is positive (+). The short leg is negative, and the rim of the LED usually has a flat edge on that side too.

The LEDs go in from the star side, into the fifty printed circles.

Start with just one, at the star’s top point.

Look closely at its printed circle. LED outlines usually mark polarity with a small + or a flat edge. The long leg goes to +, and the LED’s flat rim lines up with the flat edge.

Push it down until the rim sits on the board.

Turn the board over, solder the LED’s two legs and trim them.

Plug the USB lead into the jack and into a USB port or charger, then press the button. Give the pattern a few seconds to come round, and the LED should flash through its colours.

If it stays dark, unplug it, desolder the LED, turn it round and test again. Once it works, you know which way the other forty-nine go.

Press the button to switch off and unplug before you carry on soldering.

Work one star point at a time, ten LEDs to a point.

Fit the other nine the same way round as the one you tested, and push each one down flat. Bend the legs slightly outwards underneath so they stay put when you turn the board over.

For a neat star, solder just one leg of every LED in the point first.

Then look along the row from the side. If an LED is leaning or sitting high, reheat its one joint and push it into place.

When the whole point looks right, solder all the second legs and trim.

Repeat for the other four points: fit ten, tack one leg each, straighten, solder the second legs, trim.

Doing a point at a time stops the underside turning into a forest of legs.

Look across the star from the side. Every LED should be sitting on the board and standing square.

That’s a hundred joints. The LEDs are packed tightly, so their pads are close together.

Look for any joint that has run into its neighbour, and any that look dull or haven’t filled the hole. Fix them now, before the board goes on its feet.

Put a brass standoff under each corner hole on the MH-T12 side, and screw an M3 screw into it from the star side.

Snug is enough, so don’t crank them down. The standoffs are the feet that let the finished star stand face-up.

Plug the USB lead into the jack and a USB port or charger, and press the button. The whole star should come to life.

If some LEDs stay dark, the table in If it doesn’t work above narrows it down quickly.

The chip steps through its patterns, switching LEDs on and off in pairs while each LED picks its own colour.

Press the button again to switch off. That’s the build finished. Nice work.

Each of the chip’s 25 output pins is wired to the negative legs of two LEDs. When the program pulls a pin low, both light, each through its own 1 kΩ resistor from +5 V.

The ten resistors are shared across all fifty LEDs. R1, for example, feeds D2, D4, D6, D8 and D10.

That’s why a dead resistor shows up as five scattered dark LEDs, while a bent chip pin darkens a single pair such as D9 and D10.

The STC15F204EA is a standard 8051-family microcontroller, and the 4-pin header (J1) is its serial port: P3.0 (RXD), P3.1 (TXD), VCC and GND. With a 5 V USB-serial adapter and STC’s ISP tool or the open-source stcgal, you can write your own patterns in C (for example with SDCC) and flash them. The chip checks for new code each time it powers up. Connect TX to RX and RX to TX, and check the pin labels printed beside J1 before you connect anything.

Be aware: STC chips can’t be read back, so once you flash your own program the factory light show is gone for good.

Pin map (from the kit’s schematic). Pull a pin low to light its pair: P2.6 → D9/D10, P2.7 → D7/D8, P1.0 → D5/D6, P1.1 → D3/D4, P1.2 → D1/D2, P1.3–P1.7 → D19/D20 down to D11/D12, P0.1 → D29/D30, P3.0–P3.3 → D27/D28 down to D21/D22, P3.4–P3.7 → D39/D40 down to D33/D34, P2.0 → D31/D32, P2.5–P2.1 → D49/D50 down to D41/D42.