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This section presents a complete procedure for building the coin-toss circuit on a small preprinted PCB. When you get all your materials together, you should be able to complete this project in about an hour.

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:

Quantity	Description
1	General-purpose, dual-printed circuit board
1	LM555 timer IC
1	1 kΩ, ¼ W resistor (5 per package)
1	10 kΩ, ¼ W resistor (5 per package)
2	470 Ω, ¼ W resistor (5 per package)
1	0.1 μF polyester film capacitor
1	Red LED 5 mm
1	Green LED 5 mm
1	Normally open, momentary-contact push button
1	9 V battery snap connector
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 list is similar to the list I give earlier in this chapter for building the coin-toss circuit on a solderless breadboard. If you have the parts from that project, you can reuse them here.

Figure 6-15 shows the layout of the preprinted circuit board. Before we start building the circuit, study the layout of this board 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, 10 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.

FIGURE 6-15: The layout for the PCB used in the coin-toss circuit.

The strips aren’t numbered on the board, but I’ve numbered them in Figure 6-15. I use the numbers 1 through 10 to number the strips on the left side of the board and the numbers 11 through 20 to number the strips on the right. In the instructions that follow, I use these numbers to indicate which holes to attach components or jumper leads to. Note: To keep things simple, I just specify the terminal strip number and leave it up to you to decide which of the five holes in the strip to use.

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: 1 through 10 is on the right and 11 through 20 is on the left.

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 that 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.

Here are the steps for building the coin-toss circuit on a preprinted PCB.

1 Break the PCB in half.The preprinted circuit board comes with two identical sections. You need just one of those sections for this project, so you can break the board in half and save the other half for another project. (To break the board, just grab an end in each hand and snap it in two.)

2 Insert the 555 timer IC.Remember that the dot or notch on the 555 IC marks pin 1. Install the chip so that pin 1 is in strip 4 and pin 8 is in strip 14. Then solder the chip carefully into place. (See Chapter 7 of this minibook for tips on soldering.)

3 Install the jumper wires.This circuit needs a total of nine jumper wires. Cut the jumper wires from the 22-gauge solid wire and carefully strip the insulation from each end. Use needle-nosed pliers to bend the bare end of each jumper wire down, insert both ends into the appropriate holes, solder the leads to the pads, and then use your wire cutters to snip the excess of the end of each lead.The following table provides the PCB strip locations for each jumper wire. Use your own judgment to determine which hole in the indicated strip to place the jumper wire in. Whenever possible, use the shortest possible path for each jumper wire.Jumper NumberFrom stripTo strip191021920351644105719626715821291419

4 Install the resistors.There are four resistors to be installed. Use the following table to install each resistor into its correct location. Bend the leads down and insert each resistor into the correct holes, solder the resistor in place, and then snip off the excess wire from the ends of the leads.Resistor NumberValueColorsFrom stripTo stripR11 kΩBrown, black, red1520R210 kΩBrown, black, orange1115R3470 ΩYellow, purple, brown1319R4470 ΩYellow, purple, brown310

5 Install the capacitor.Install the capacitor into strips 5 and 10. Push the capacitor all the way in until it’s flush with the board. Then solder the leads to the pad and trim off the excess wire.

6 Install the LEDs.Remember that LEDs are directional and must be installed in the correct direction or they won’t work. One lead is shorter than the other to help you tell which lead is which. This short lead is the cathode, and the longer lead is the anode.The following table shows where to install the LEDs:LED ColorCathode (short lead)Anode (long lead)Red1213Green32When you install the LEDs, do not push the LED in until it is flush with the circuit board. Instead, push just a little bit of the leads into the holes so that the LED stands up about an inch from the top of the board.

7 Install the jumper wires for the metal contacts.Cut two, 2-inch lengths of stranded wire and strip about ⅜ inch of insulation from each end. Solder one end of each wire into holes in strips 1 and 11 and leave the other ends free. When the circuit is installed in its final enclosure, you connect the ends of these wires to the metal posts that the user will touch to activate the coin-toss circuit. Feeding the stranded wire through the holes in the circuit board can be tricky. First, carefully twist the loose strands until there are no stragglers protruding from the end of the wire. Then, carefully push the wire through the hole. If any of the strands get caught and refuse to go through the hole, pull the wire out and try again.

8 Connect the push button.Cut a 2-inch length of stranded wire and strip about ⅜ inch of insulation from each end. Solder one end to either terminal on the push button (it doesn’t matter which). Push the other end through a hole in strip 10 and solder it in place.

9 Connect the battery snap connector.Strip off about ⅜ inch of insulation from the end of both leads. Then, solder the black lead to the free terminal on the push button (the terminal you did not use in Step 8) and solder the red lead to a hole in strip 20 on the PCB.

10 Connect the 9 V battery to the snap connector.The red LED should immediately light up, indicating that the circuit is ready to do its decision-making work.

11 Turn off your soldering iron.You’re done!

Test the circuit by pinching both of the free jumper wires between your fingers. The LEDs should alternately flash until you let go, at which time one or the other will remain lit.

Figure 6-16 shows the completed circuit in operation.

FIGURE 6-16: The completed coin-tosser PCB.