Electronics_Projects_For_Dummies
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In this part . . .
The chapters in this part all deal with things that vibrate in some fashion, from sound waves in the air
to a furniture cushion. Intrigued? Here’s what’s covered:
Radio waves form the basis of the remote control unit that you use to manage a character we call Sensitive Sam (Chapter 13). Oscillating signals sent by a metal detector (Chapter 12) produce a corresponding oscillating signal in metal, helping you identify when you’ve hit gold (or more likely, iron). Finally, our Couch Pet-ato (Chapter 14) responds to the vibration that occurs when your cute kitty jumps up on your couch in your absence, producing a sound to scare off the little dear.
Chapter 12
A Handy-Dandy Metal Detector
In This Chapter
Exploring the detector circuit
Checking off the parts list
Assembling the handle
Mounting the circuit to the handle
Finding metal!
Finding little bits of metal is a time-honored task, right up there with finding free music to download off the Internet. With a metal detector, you
can scout out coins in your couch, nails in your walls, keys in somebody’s pocket — face it, the applications are endless.
In this project, we show you how to build a small, handheld metal detector that can detect certain kinds of metal — especially ferrous (iron-containing) metals — even if that metal is under a half-inch or so of drywall, sand, or soil (but never water!).
The Big Picture: Project Overview
In this project, you use an IC that generates an AC signal that goes through a coil. Because metal objects conduct electricity, you can induce a current in those objects. When the coil in the metal detector comes near a metal object, the electromagnetic field in the coil induces currents in the metal object. The electromagnetic field generated by the metal changes the current in the coil. When the signal changes, IC turns on an LED, alerting you to the presence of metal.
You can see the finished metal detector in Figure 12-1.
282 Part IV: Good Vibrations
Figure 12-1:
Our handheld metal detector.
Here are the types of activities you’ll be doing to build the metal detector.
You will
1.Build a pretty simple electronic circuit containing a coil, a proximity detector IC, a transistor, a couple of resistors, and an LED.
2.Install the circuit in a box with batteries and an on/off switch.
3.Mount the box on a handle made from plastic pipe.
Scoping Out the Schematic
This project is really easy. You have only one breadboard to put together for this project. You can see the schematic for the board in Figure 12-2.
Chapter 12: A Handy-Dandy Metal Detector 283
Figure 12-2:
The schematic of the metal detector.
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1 |
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3 |
2 |
R3 |
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L1 |
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R1 |
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C1 |
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+ |
7 |
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6V |
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R2 |
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- |
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LED1 |
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C2 |
4 |
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5 |
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S1 |
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6 |
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IC1
R4
Q1
R5
Here’s a list of the schematic elements:
L1 is a coil (also called an inductor) wired in parallel to capacitor C1; the combination of this dynamic duo is a parallel LC (inductor/capacitor) circuit. When a signal that oscillates at several kHz passes through this circuit, the signal creates an electric field around the coil. When you bring the coil near a metallic object, that electric field induces an oscillating signal in the object. Turnabout is fair play, so when the oscillating signal has been induced in the metallic object, the signal in the object creates an electric field that induces current in the coil. This current changes the oscillating signal running through the LC parallel circuit.
IC1 is a TDA0161 proximity detector. This IC is designed to supply the oscillating signal that’s sent through the LC parallel circuit. The IC also responds to any changes in the signal: the IC has an output of 1 milliamp (mA) or less if the coil is far from a metallic object and an output of 10 mA or higher if the coil is near a metallic object.
R1 is a resistor, and R2 is a variable resistor. These resistors are used to calibrate the circuit in IC1 to the LC circuit. You calibrate the circuit by
284 Part IV: Good Vibrations
adjusting the value of the variable resistor when the coil is not near any metal objects.
R5 is a resistor connected between the output of IC1 and ground. When the output of IC1 is on, current flows through this resistor and provides a positive voltage to the base of Q1.
Q1 is a 2N3904 transistor that you connect to the output of IC1. When the output of IC1 is high, Q1 turns on and allows current to flow through LED1.
LED1 is the indicator light used to indicate that the device has detected metal in the vicinity.
R3 is a resistor that limits the amount of current flowing through LED1, which prevents it from burning out.
S1 is the on/off switch.
Building Alert: Construction Issues
You use PVC cement (a glue) to connect the PVC fittings that form the microphone handle. You can get this glue at any building supply store.
This glue creates a very strong joint. However, be sure to wear some form of work gloves when using this glue because it melts plastic — and you defi-
nitely don’t want it on your hands. Also read the label for safety tips, such as using the glue in a well-ventilated area and what to do if some comes in contact with your skin.
Perusing the Parts List
Even though this project doesn’t require many parts, you still have to go out and buy or assemble them. Several of the parts are shown in Figure 12-3. Here’s what you need:
Two 1 kohm resistors (R1, R4)
10 kohm potentiometer (R2)
330 ohm resistor (R3)
Chapter 12: A Handy-Dandy Metal Detector 285
120 ohm resistor (R5)
Two 0.0047 microfarad ceramic capacitors (C1, C3)
One 2N3904 transistor (Q1)
One T-1 3⁄4 LED (LED1)
LED panel-mount socket, T-1 3⁄4
TDA0161 proximity detector (IC1)
Battery pack for 4 AA batteries
680 picofarad bobbin-type inductor (L1)
We used C&D Technologies part #1468420C that we picked up at Mouser (www.mouser.com).
SPST (single-pole, single-throw) toggle switch, used as the on/off switch
400-pin breadboard
Four 2-pin terminal blocks
Knob (for the potentiometer)
Two phono jacks
Two right-angle phono plugs
We used right-angle plugs to avoid having a loop of wire coming out of the box. You can also use banana plugs and jacks.
Enclosure
We used a plastic box, RadioShack part #270-1806.
An assortment of different lengths of prestripped, short 22 AWG wire
Two PVC 45° joints with 1" slip fitting on both ends (one male and one female)
PVC 1" end cap with 1" female slip fitting
1" diameter PVC pipe (referred to as schedule 40 pipe), 1' in length
1" clamp to attach the enclosure to the pipe
Two 1⁄2" 8-32 panhead screws
Two 8-32 nuts
286 Part IV: Good Vibrations
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Right-angle |
IC |
Inductor |
phono pug LED |
Figure 12-3:
Key components of the metal detector.
PVC |
SPST |
Potentiometer |
Phono |
Two-piece |
45° joint |
switch |
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jack |
LED socket |
Taking Things Step by Step
Although the circuit is simple, there are a few steps you need to do to assemble it and then put the whole shooting match together, including building the handle and attaching the circuit to it. As usual, we start with the circuit.
Building a metal detector circuit
The circuit in this project controls sending a detector signal and processing the signal that comes back to light up the LED. Here are the steps involved:
1.Place TDA0161 (IC1), 2N3904 (Q1), and four terminal blocks (TB) on the breadboard, as shown in Figure 12-4.
The transistor pinout is shown in Figure 12-5.
Chapter 12: A Handy-Dandy Metal Detector 287
Coil TB IC1 Potentiometer TB Battery TB
Figure 12-4:
Place the IC, transistor, and terminal blocks on the
breadboard.
Q1 |
LED TB |
Figure 12-5:
The 2N3904
pinout.
Emitter pin |
Collector pin |
Base pin
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2.Insert wires to connect the battery terminal block and the transistor emitter pin to the ground bus. Then insert a wire between the two ground buses to connect them, as shown in Figure 12-6.
Two shorter wires connect components to ground bus; the long wire on the right connects the two ground buses.
Figure 12-6:
Connect components to ground bus; then connect the two ground buses.
3.Insert wires to connect IC1 and the battery terminal block to the +V bus. Then insert a wire between the two +V buses to connect them, as shown in Figure 12-7.
Chapter 12: A Handy-Dandy Metal Detector 289
Pin 1 of IC1 to +V Battery terminal block to +V
Figure 12-7:
Connect components to the +V bus.
4.Insert wires to connect the IC and discrete components, terminal block for the coil (L1), terminal block for the potentiometer (R2), and terminal block for the LED, as shown in Figure 12-8.