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Competition #1: Miniature Christmas Tree

Discussion in 'Entries' started by ElectroMaster, Jul 3, 2008.

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  1. ElectroMaster

    ElectroMaster Administrator

    Joined:
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    Location:
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    ***** To vote for this project please answer the poll above. *****

    Project: Miniature Christmas Tree

    This miniature Christmas tree has one steady white LED at the peak and 6 red and 6 green blinking LED’s. It uses a standard 9V (PC1604) battery for power. In addition to supplying power for the miniature Christmas tree the 9V battery acts as a stand.

    Theory of Operation

    Referring to the schematic, an astable multivibrator is used to blink the red and green LED’s. The red and green LED’s are grouped in four series strings of three LED’s and a 220W current limiting resistors. The single white LED shows two 470W in series. One resistor of 1k would have been adequate, but during the construction of the circuit I needed to jump some traces on the PCB. With a fully charged battery the individual strings of the red and green LED’s draw approximately 8mA from the battery and the white LED draws approximately 6mA. The colored LED’s continue to flash when the battery is discharged below 7V. The oscillation frequency with the base resistors of 560kW and 0.47uF capacitors between the base and collectors is approximately 5Hz. The 560kW base resistors were calculated to provide 12uA of base current for the darlington transistors, and the 0.47uF capacitors were found experimentally.

    [​IMG]

    Construction

    The miniature Christmas tree was constructed on a 3” X 6” piece of fiberglass single sided copper clad board that was positive resist sensitized. The layout of the resist pattern was done looking thru the top of the board. The image was printed on a transparency sheet with my EPSON R200 inkjet printer. When the image was placed on the PCB for exposing it was reversed so the image was showing the bottom of the board. The PCB image has 2 extra triangles of resist on either side of the sloping sides, and two extra rectangles on either side of the stand . I did this to save a little on etching solution, since these parts of the board would be trimmed off and discarded. The stand portion of the pattern also shows three rectangles. The two smaller ones are used to solder the battery clip to the circuit board. The larger one was used to save a little on the etching solution and is covered by the battery when it is in place. Once the board was exposed, developed, etched, drilled and then tin plated I painted the top of the board with some spray paint.

    When I was laying out the PCB, I decided to have the resistors on the top of the board with the LED’s, as the resistors are colorful and could appear as other decorations on the tree.

    I installed all the parts on the board except for the 0.47uF capacitors. By not installing the capacitors and applying power, all the LED’s should light up. When I applied power there was one string of 3 LED’s that didn’t light and the white LED didn’t light. The string of LED’s had both ends connected to the collector of its associate transistor and had two current limiting resistors in the string. This problem was solved by removing the extra resistor and installing a jumper on the bottom of the board. The white LED has both ends of its circuit connected to the 9V positive trace. The negative side of the white LED circuit was connected to ground thru a 470W resistor to jump some traces. The 1kW resistor in the positive lead was changed to 470W

    [​IMG]

    The PCB layout shown has these revisions done. The completed miniature Christmas tree shown in the pictures is the original etched board and has one jumper and one resistor on the bottom of the circuit board

    The battery clip (Kestone #079) was soldered to the PCB on the trace side by melting extra solder on the two small rectangles and then tinning the two flat edges of the clip. The clip was centered on the board over the rectangles, first on edge was heated and then the other. The clip was now soldered to the board.

    The ON/OFF switch was also soldered the bottom of the board, but a small piece of insulating tape was fastened to the metal housing as when in position the metal housing was very close to a trace on the board.

    [​IMG]

    [​IMG]

    Photograph Notes

    To take the picture of the front of the board I left the .047uF capacitors off the back of the board so all the LED’s would be lit. The photograph labeled blinking shows two strings on and two strings off. My digital camera must have caught the half cycle when Q1 was saturated and Q2 was cutoff. The photo of the back of the board shows the trace side of the board and the jumper and extra resistor previously mentioned.


    ***** To vote for this project please answer the poll above. *****
     
    Last edited: Jul 3, 2008

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