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

these you will see that when you need to drill a hole for a 5 mm LED, a 3/16-inch drill bit is about right. (In fact, it results in a better, tighter fit than if you drill an actual 5 mm hole.)

Figure 3-83. Because units of measurement are not standardized in electronics, conversion is often necessary. The chart on the right is a 5x magnification of the bottom section of the chart on the left.

Figure 3-84. This chart allows conversion between hundredths of an inch, conventional U. S. fractions of an inch, and fractions expressed in thousandths of an inch.

Experiment 15: Intrusion Alarm Revisited

Time now to add some of the enhancements to the intrusion alarm that I discussed at the end of Experiment 11. I’m going to show you how the alarm can be triggered if you install various detectors on windows and doors in your home. I’ll also show how the alarm can be wired so that it locks itself on and continues to make noise even after a door or window is reclosed.

This experiment will demonstrate the procedure for transferring a project from a breadboard to a piece of perforated board that has copper connections laid out identically to the ones inside the breadboard, as shown earlier in Figure 3-72. And you’ll mount the finished circuit in a project box with switches and connectors on the front.

When all is said and done, you’ll be ready for wholesale circuit building. The explanations in the rest of this book will get gradually briefer, and the pace will increase.

You will need:

15-watt pencil-type soldering iron

Thin solder (0.022 inches or similar)

Wire strippers and cutters

Perforated board etched with copper in a breadboard layout

Small vise or clamp to hold your perforated board

The same components that you used in Experiment 11, plus:

2N2222 NPN transistor. Quantity: 1.

DPDT relay. Quantity: 1.

SPDT toggle switch. Quantity: 1.

1N4001 diode. Quantity: 1.

Red and green 5mm LEDs. Quantity: 1 each.

Project box, 6 × 3 × 2 inches.

Power jack, type N, and matching power socket, type N.

Binding posts.

Stranded 22-gauge wire, three different colors.

Magnetic sensor switches, sufficient for your home.

Alarm network wiring, sufficient for your home.

Magnetic Sensor Switches

A typical alarm sensor switch consists of two modules: the magnetic module and the switch module, as shown in Figures 3-85 and 3-86. The magnetic module contains a permanent magnet, and nothing else. The switch module contains a “reed switch,” which makes or breaks a connection (like a contact inside a relay) under the influence of the magnet. When you bring the magnetic module close to the switch module, you may faintly hear the reed switch click as it flips from one state to the other.

Figure 3-85. In this simple alarm sensor switch, the lower module contains a magnet, which opens and closes a reed switch sealed into the upper module.

Like all switches, reed switches can be normally open or normally closed. For this project, you want the kind of switch that is normally open, and closes when the magnetic module is close to it.

Attach the magnetic module to the moving part of a door or window, and attach the switch module to the window frame or door frame. When the window or door is closed, the magnetic module is almost touching the switch module. The magnet keeps the switch closed until the door or window is opened, at which point the switch opens.

The only question is: how do we use this component to trigger our alarm? As long as a small current flows through all our magnetic sensor switches, the alarm should be off, but if the flow of current stops, the alarm should switch on.

We could use a relay that is “always on” while the alarm is armed. When the circuit is interrupted, the relay relaxes and its other pair of contacts closes, which could power up the alarm noisemaker.

But I don’t like this idea. Relays take significant power, and they can get hot. Most of them are not designed to be kept “always-on.” I’d prefer to handle the task using a transistor.

Figure 3-86. This cutaway diagram shows a reed switch (bottom) and the magnet that activates it (top), inside an alarm sensor. The switch contains two flexible magnetized strips, the upper one with its south pole adjacent to an electrical contact, the lower one with its north pole adjacent to an electrical contact. When the south pole of the magnet approaches the switch, the magnetic force (shown as dashed lines) repels the south contact and attracts the north contact, causing them to snap together. Two screws on the outside of the casing are connected with the strips inside.

A Break-to-Make Transistor Circuit

First, recall how an NPN transistor works. When the base is not sufficiently positive, the transistor blocks current between its collector and emitter, but when the base is relatively positive, the transistor passes current.

Take a look at the schematic in Figure 3-87, which is built around our old friend the 2N2222 NPN transistor. When the switch is closed, it connects the base of the transistor to the negative side of the power supply through a 1K resistor. At the same time, the base is connected with the positive side of the power supply through a 10K resistor. Because of the difference in resistances and the relatively high turn-on voltage for the LED, the base is forced below its turn-on threshold, and as a result, the transistor will not pass much current. The LED will glow dimly at best.

Figure 3-87. In this demonstration circuit, when the switch is opened, it interrupts negative voltage to the base of the transistor, causing the transistor to lower its resistance, allowing current to reach the LED. Thus, when the switch is turned off, it turns on the LED.

Now what happens when the switch is opened? The base of the transistor loses its negative power supply and has only its positive power supply. It becomes much more positive, above the turn-on threshold for the transistor, which tells the transistor to