Make: Electronics by Charles Platt (classic books to read TXT) 📗

blocks positive current. In this circuit, that’s the lower end of the diode.

Try it to make sure that it works. Short the sensor wires together and then apply power. The alarm should remain silent. You can use your meter to check that no voltage exists between the side rails. Now separate the sensor wires, and the relay should click, supplying power to the side rails, which activates the noisemaker. Even if you bring the sensor wires back together, the relay should remain locked on. The only way to unlock it is to disconnect the power supply.

When the circuit is active, the transistor followed by the diode drops the voltage slightly, but the 12-volt relay should still work.

In my test circuit, trying three different relays, they drew between 27 and 40 milliamps at 9.6 volts. Some current still leaked through the transistor when it was in its “off” mode, but only a couple of milliamps at 0.5 volts. This low voltage was far below the threshold required to trip the relay.

Ready for Perfboarding

If the circuit works, the next step is to immortalize it on perforated board. Use the type of board that has a breadboard contact pattern etched on it in copper, as shown in Figure 3-72 on page 116. Check the following section,

“Essentials: Perfboard procedure,” for guidance on the best way to make this particular kind of solder joint—and the subsequent section for the most common problems.

Essentials

Perfboard soldering procedure

Carefully note the position of a component on your breadboard, and then move it to the same relative position on the perfboard, poking its wires through the little holes.

Turn the perfboard upside down, make sure that it’s stable, and examine the hole where the wire is poking through, as shown in Figure 3-96. A copper trace surrounds this hole and links it with others. Your task is to melt solder so that it sticks to the copper and also to the wire, forming a solid, reliable connection between the two of them.

Take your pencil-style soldering iron in one hand and some solder in your other hand. Hold the tip of the iron against the wire and the copper, and feed some thin solder to their intersection. After two to four seconds, the solder should start flowing.

Figure 3-96.

Figure 3-97.

Figure 3-98. To establish a connection between a section of wire and a copper trace on perforated board, the wire is pushed through the hole, and solder (shown in pure white for illustrative purposes) completes the connection. The wire can then be snipped short.

Allow enough solder to form a rounded bump sealing the wire and the copper, as shown in Figure 3-97. Wait for the solder to harden thoroughly, and then grab the wire with pointed-nosed pliers and wiggle it to make sure you have a strong connection. If all is well, snip the protruding wire with your cutters. See Figure 3-98.

Because solder joints are difficult to photograph, I’m using drawings to show the wire before and after making a reasonably good joint, which is shown in pure white, outlined with a black line.

Actual soldered perfboard is shown in the photographs in Figures 3-99 and 3-100.

Figure 3-99. This photograph was taken during the process of transferring components from breadboard to perforated board. Two or three components at a time are inserted from the other side of the board, and their leads are bent over to prevent them from falling out.

Figure 3-100. After soldering, the leads are snipped short and the joints are inspected under a magnifying glass. Another two or three components can now be inserted, and the process can be repeated.

Tools

Four most common perfboarding errors

1. Too much solder

Before you know it, solder creeps across the board, touches the next copper trace, and sticks to it, as depicted in Figure 3-101. When this happens, you have to wait for it to cool, and then cut it with a utility knife. You can also try to remove it with a rubber bulb and solder wick, but some of it will tend to remain.

Even a microscopic trace of solder is enough to create a short circuit. Check the wiring under a magnifying glass while turning the perfboard so that the light strikes it from different angles (or use your solder wick to suck it away).

Figure 3-101. If too much solder is used, it makes a mess and can create an unwanted connection with another conductor.

2. Not enough solder

If the joint is thin, the wire can break free from the solder as it cools. Even a microscopic crack is sufficient to stop the circuit from working. In extreme cases, the solder sticks to the wire, and sticks to the copper trace around the wire, yet doesn’t make a solid bridge connecting the two, leaving the wire encircled by solder yet untouched by it, as shown in Figure 3-102. You may find this undetectable unless you observe it with magnification.

You can add more solder to any joint that may have insufficient solder, but be sure to reheat the joint thoroughly.

Figure 3-102. Too little solder (or insufficient heat) can allow a soldered wire to remain separate from the soldered copper on the perforated board. Even a hair-thin gap is sufficient to prevent an electrical connection.

3. Components incorrectly placed

It’s very easy to put a component one hole away from the position where it should be. It’s also easy to forget to make a connection.

I suggest that you print a copy of the schematic, and each time you make a connection on the perforated board, you eliminate that wire on your hardcopy, using a highlighter.

4. Debris

When you’re trimming wires, the little fragments that you cut don’t disappear. They start to clutter your work area, and one of them can easily get trapped under your perforated board, creating an electrical connection where you don’t want it.

This is another reason for working with something soft, such as polyurethane foam, under your project. It tends to absorb or