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Prototype an Electronic Coin-toss Step 3: Connect the Finger-touch Circuit
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Prep to Build an Electronic Coin-Toss: Parts List and PCB Layout

Once you have assembled and tested the prototype of your electronic coin-toss, you can begin to build the actual electronic coin-toss on a printed circuit board (PCB). The parts you will need and the layout of the PCB are outlined here.

All the parts required to build this prototype circuit can be purchased from your local RadioShack. Or, you can order them online from any electronic parts supplier. For your convenience, here is a complete list of the parts you'll need to build this prototype circuit, along with the RadioShack catalog part numbers:

Part Number Quantity Description
276-159 1 General-purpose, dual-printed circuit board
276-1723 1 LM555 timer IC
271-1321 1 1 kÙ, 1/4 W resistor (5 per package)
271-1335 1 10 kÙ, 1/4 W resistor (5 per package)
271-1317 2 470 Ù, 1/4 W resistor (5 per package)
272-1053 1 0.1 ìF polyester film capacitor
276-041 1 Red LED 5 mm
276-022 1 Green LED 5 mm
275-1547 1 Normally open, momentary-contact pushbutton
270-325 1 9 V battery snap connector
n/a 1 9 V battery

You will also need about a foot each of 22-gauge solid insulated wire and 22-gauge stranded insulated wire. The color doesn't matter.

Note: This parts list is similar to the list for prototyping the coin-toss circuit on a solderless breadboard. If you have the parts from that project, you can reuse them here.

Before you start building the circuit, study the layout of the PCB for a moment to familiarize yourself with it. As you can see, this board doesn't contain bus strips like those found on a breadboard. However, the overall layout of the board is similar to the layout of the terminal strips on a breadboard.

The center portion of the board contains a total of 20 terminal strips, ten on each side of the ditch. Each strip has three holes but is also connected to a second strip of two holes along the edge of the board. Thus, each strip effectively has five holes.

The layout for the PCB used in the coin-toss circuit.
The layout for the PCB used in the coin-toss circuit.

The strips aren't numbered on the actual printed circuit board, but the layout can serve as a reference template. The numbers 1 through 10 number the strips on the left side of the board and the numbers 11 through 20 number the strips on the right.

The numbers used in the PCB layout are relative to the bottom of the board — that is, the surface of the board with the copper traces and pads. Thus, the numbers 1 through 10 are on the left and the numbers 11 through 20 are on the right. When you flip the board over to insert components from the top of the board, you'll have to mentally reverse the numbers.

When installing components onto the PCB, use an alligator clip as a temporary clamp to hold the components flush against the board. This will enable you to turn the board upside down so you can solder the leads to the pads.

If you don't clamp the component to the board, the component will fall out when you turn the board upside down, or you'll be tempted to hold the component in place with one finger while you solder the leads. Bad idea: Resistors get really hot when you solder them.

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