LED Starlight

This is an ornamental, if somewhat seasonal item that is in the shape of a star.

However, I wanted something different to the usual two-dimensional construction.

As a result, I created a three-dimensional version using three PCB’s.

One for the base and two shaped boards that when locked together form the 3D star.

These boards are pre-shaped as part of the manufacture although a rectangular needle file is required to adjust the slot width for an optimal fit.

Connections to the interlocked boards that form the star are made by pads that align with the control board and the other two boards making up the star.

These are connected by solder joints bridging the 2 pads forming a right angle.

I rejected a socket of other similar arrangement for simplicity as the connection was to be permanent.

The two-star boards have LED’s on both sides and are therefore visible from multiple angles.

There are 3 LED’s (Red, Green and Amber), on each side of the 4 arms for a total of 24 LED’s

In order not to detract from the overall form of the finished item and enabling visibility of the screen printing of a cubic form the LED’s are surface mounted versions.

The cubic form design was created directly on the PCB during the design phase and not imported from another application.

The LED pattern can be changed by adjustment of the hex switch.

Additionally, flashing speed can be changed adjustment of the potentiometer that changes the oscillator frequency.

The star is powered by a 3V CR2032 which sits under the bottom of the control board, this is situated in the centre of the board and being flat allows the star to be free standing on a flat surface.

Power can also be supplied externally from a larger battery (I.e. 9V PP3), via the screw terminals or modified USB cable.

This is accomplished by the suitable positioning of a link on a header that selects the power source.

Holes are at the top of each arm to allow the star to be hung if required.

The double-sided PCB has been designed using EagleCAD and manufactured at OSH Park.

Qty DEVICE

1 BATTERY CLIP-20MM

3 0.1uF C-EUC1812K

1 1uF C-EUC1812K

1 10uF C-EUC1812K

6 1N4148 SMA-DO214AC

1 1N4004 DO41-10

3 CD4013D SO14

1 CD4070D SO14

2 CD4069D SO14

1 NA555D S08

12 LED POINTLED (3 x Red, 3 x Green, 3 x Amber)

12 220R R-EU_R1206

14 10K R-EU_R1206

2 2K2R R-EU_R1206

1 0R R-EU_R1206

1 500KR-TRIM 3314G

1 SWS001 SPST Momentary

1 BCD SWITCH

2 MPT2 2.54mm screw terminal

4 MPT3 2.54mm screw terminal

The majority of the components are SMD, the exceptions being the pattern selection switch, timer frequency control resistor, the external power connector, the supply selection jumper and the supply polarity protection diode.

The circuit comprises an oscillator made from a 555 Timer (8 pin SOIC), whose frequency can be varied from a few hertz to a few hundred hertz. ~1.25Hz to 220Hz although the actual values will vary subject to component tolerances but are not critical.

The output of the timer is used to clock 3 dual D type Flip Flops (CD4013, 14pin SOIC), these are configured as a Linear Feedback Shift Register (LFSR), using an EXOR (CD4070), to provide feedback.

CD4070 truth table.(See image).

LH = Low to High transition, HL = High to Low transition, X = Don’t care, NC = No change.

The Q outputs of each register are fed to the D inputs of each successive stage.

The first 4 registers have the R inputs connected to the HEX switch enabling them to be preloaded with a pattern to pre-initialise the start sequence.

The S inputs of all the registers are connected together to enable the registers to be reset, using the reset button.

The remaining registers allow further randomness using links to connect the Q or /Q outputs to the next stage. The default links connect the fifth register Q output to the sixth registers D input and the sixth register /Q output to one of the EXOR inputs, completing the feedback loop.

Both outputs of the registers are each connected to an inverter (CD4069, 14 pin SOIC), with 2 LED’s connected to each of the 12 outputs.

Power consumption is dependent upon the supply voltage and the specific pattern.

However, guidelines of current consumption for the following voltages are listed.

3V = 3mA, CR2032 capacity can be between 210-240mAH meaning the battery will last ~70-80hrs.

5V = 11mA

9V = 38mA

Each board is assembled separately.

Starting with the control board mounting all the SMD components on the front than the battery clip at the back.

Following this the through hole components are mounted.

The boards that form the star just contain LED's and resistors, checking the orientation of polarised components is recommended to prevent rework or damage.

The star boards have solder pads for connection to the control board at both ends meaning that they can be mounted either way up as long as they are orientated correctly to the centre slot that extends half way into the board. Allowing the two boards to be slotted together prior to fixing to the control board.

Problems can occur and if they do how can they be tackled.

The first thing to do is look for the obvious.

IC in the wrong location, wrong orientation or pin(s) not soldered or poorly soldered, poor socket insertion or bent pin.

Component in wrong position, wrong value, wrong orientation or poor soldering.

Solder bridging, Supply voltage on the wrong terminals, supply leads swapped, incorrect voltage.

Even the PCB could have an open or shorted track(s).

Don't tell yourself it cannot possibly be a particular issue without verifying it