LED Dice Printed Circuit Board

LED Dice

Abstract

The objective of this project was to design and assemble a printed circuit board for an LED dice. The project uses seven light emitting diodes (LED’s) that light up as random dice numbers. A 14017 Decade Counter IC randomizes the number while a 555 IC oscillator provides a slowdown feature. A push switch is used to reset the IC’s and “roll” a new number when pushed. The PCB was designed with DipTrace software and manufactured elsewhere. The components (all through-hole) were assembled and soldered in our lab. The design was successful with the exception of incorrect on/off switch placement.

Project Description

This project was based on a kit provided by Carl’s Electronics. I purchased the kit which contained a schematic (Figure 1) and all of the necessary components.

LED Dice diagram

Figure 1: LED Dice Circuit: My replication of the purchased schematic with Diptrace software.

Before pressing the “roll” push-switch, a BC547 n-p-n transistor (Q2) is turned off (its base is pulled by the 3.3 megaΩ transistor) and the 555 IC (U1) is not oscillating. Pressing the push-switch charges the 470 nF capacitor (C1) and turns the Q2 and U1 on.  C1 steadily discharges through the 10 megaΩ and 3.3 megaΩ resistors and Q2 turns off. When Q2 turns off, the frequency of oscillation becomes dependent on the voltage.

The 14017 decade counter IC (U2) randomizes the sequence of powered LED’s. Seven output pins were used for the six numbers and an additional output was reset to pin 15. The LED sequence and corresponding pins are shown in Table I. The resistors are used as voltage dividers for the LED combinations and to limit the current to the transistors’ bases.

Table 1: 14017 Decade Counter Corresponding Pin and Number
U2 Pin Number Dice Number LED's On
1 1 4
2 3 2, 4, 6
3 4 1, 2, 6, 7
4 6 1, 2, 3, 5, 6, 7
7 5 1, 2, 4, 6, 7
10 2 2, 6
Reset 5 to 15

A printed circuit board (PCB) was designed with DipTrace software (Figure 2). All of the components were placed on one side to simplify assembly and the board was routed on two sides. I was able to design a slightly smaller PCB than the PCB given with the kit. After design, the board was sent to an outside fabrication company to be laminated and patterned.

Board graphic

Figure 2: PCB layout created in Diptrace. Light-colored routing is on the backside, black routing is on the front, and green is the silkscreen layer.

Once I received my board, I began assembling. My components were soldered onto the board with PbSn solder. While soldering, I found several mistakes that required extra assembly steps to correct. U1 and U2 were given the wrong widths. I had measured the width of the entire IC, and placed the pin pads around the perimeter. In actuality, the pins of the IC were set slightly inside of the perimeter. I was able to bend the leads to fit through the corresponding vias in the PCB. I also realized that I had indicated two capacitors’, C1 and C2, polarities on my schematic that didn’t require directional placement. My diode (D10) should have been labeled D8.

After it was assembled, I attached the 9V battery to power the device (Figure 3). The device worked with the ON/OFF switch ON, but was malfunctioned with the ON/OFF switch OFF. Apparently, my switch was misplaced and provided power to U1 but not U2.

LED Dice

Figure 3: Fabricated LED Dice showing the number three.

Conclusion

The completed device functions as an LED dice with “roll” feature. It was a simple, inexpensive kit that was complicated enough to learn from. I have many suggestions for improvement:

  • The IC via widths should be modified (reduced).
  • D10 should be renamed D8.
  • The switch should be placed so that it cuts off all power to the circuit.
  • The board should be designed to a size that can easily be placed in a case.
  • The mounting holes should be big enough to place screws into.

I also recommend buying additional parts from an online parts supplier to replace any faulty, lost, or broken components. The parts are all inexpensive and can be purchased for much less when purchased separately. My kit was missing a transistor, so I had to use a backup for assembly. Testing this kit on a breadboard was difficult and required bending of the IC leads. My IC’s were not damaged but I did purchase backups in fear that they would not work correctly.

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