(Parts: 555 timer and maybe a microcontroller, 4x NPN transistors, various resistors, wires, an led and/or piezoelectric buzzer, some sort of metal slug (like a screw or magnet))
Step 1: Gather the Parts
4x NPN transistors (I mostly used 2N2222's but you can cheat and use a 2N4401 or other NPN's)
1x 555 timer chip or microcontroller (depending on how adventurous you want to get)
1x of the following resistors: 100 o, 220 o, 2.2k(opt.)*, 100 k, 1 M,
2x of the following resistors: 1 k, 10 k (opt.)*
Piezoelectric buzzer (can be a siren or buzzer but not a speaker, this circuit only produces a straight voltage signal not a frequency)
wires, including a long stand of spare wire
A metal slug or other metallic object (opt. but desired)
* depending on what you have for resistors and how sensitive you want your alarm you may not need one of each of these, which can be replaced with another single resistor
You may also find it necessary to have a ruler and calculator handy for the formulas later in this Instructable.
Step 2: Building Part 1: the Receiver
First, we start by taking one transistor and bending the collector leg so that when it sits in the breadboard it will have two open holes between it and the base leg. If you aren't familiar with transistors, each has an emitter, base, and collector leg (frankly, explaining these would take a while so you'll have to discover why on your own). For our circuit you only need to know that your emitter, base, collector (on both the 2N2222 and 2N4401) are on the left, center, and right respectively as you look at the transistor with the flat side towards you. Second, take two more transistors and do the same to them placing each so that their collector pins fit in the empty rows between the first transistors collector (C) pin and base (B) pin. The two new transistors must also connect their B pins to the emitter (E) pins of their previous transistors.( If you're lost I suggest the pictures below) Finally, take the last transistor and place it along side the last one, but this time it won't be connecting to the previous transistor in the same way as before. This time you'll need a jumper wire connecting the previous E to the last transistor's B pin.
The receiver is now mostly set up, just take the 1M, 100K, 10K, and 1K resistors and connect each to power and to the C pins of each transistor starting with the 1M resistor on the first transistor's C pin, then continuing till each transistor has a resistor on the collector (the final transistor should have the 1K resistor connected to it's C pin and power). Just so that you know where to plug in something later on, I suggest you locate the first transistor's B pin and connect a jumper wire to it, this will be useful later on.
Step 3: Building Part 2: the Interpreter/Switch
First, start with the timer chip. You'll need to put it so that it's within a moderate range of the last transistor's E pin. Next, you'll need to power your chip. For mine, I'm using a 9v battery with a 100 ohm resistor but you could also use a USB power supply if you wanted (just make sure you test it out with a battery first so you don't blow out a USB port). So with my 100 ohm resistor connecting pin 8 on my 555 timer and a wire connecting pin 1 to ground we can start setting up the Schmitt Trigger. (I heard you like steps, so I put steps into my Instructable steps)
Step 1. Connect a 1K resistor from power to pin 4.
Step 2. Connect a 220 ohm resistor to pin 3.
Step 3. Connect a 2.2K resistor to pin 6 and an empty spot on the board close to the last emitter ( or replace this with a potentiometer connected to pin 6 and the last emitter and skip the next step)
Step 4. Connect a 10K resistor to the emitter and the unused end of the 2.2K resistor.
Step 5. Connect a tactile switch to an empty portion of the breadboard.
Step 6. Hook up wires to the switch so it is normally open (meaning it needs to be pressed to be used).
Step 7. Connect switch wires to pin 4 and pin 6.
Step 8. Connect LED and/or piezoelectric device to the 220 ohm resistor and ground.
Step 9. Connect a jumper wire from pin 2 to the last emitter.
Step 10. Take a breath, stretch, or whatever you need to do to relax before... MATH! (up next)
Step 4: We Interrupt This Instructable...
Basically we just have to make the last component to finish our circuit here which is also the most important part of all, a coil. An inductive coil is essential to this circuit's ability to detect intruders. (I'll explain why in the last step) First, understand this: it takes few measurements to create the best one for your project. Some coils are more naturally sensitive while others are not as sensitive. So what's the rule of thumb? A simple formula: L = (R^2 * T^2)/(9*R + 10*g) where L is inductance in microhenries, R is radius from the outside of the coil to the center in inches(half the diameter of the coil), T is the number of turns of the wire, and g which is the length of the coil itself from one end to the other in inches. This calculates inductance of a single layer coil if it is not filled with a core, like say a screw or magnet, for which you would need an entirely different formula.(which I do not have)
needless to say, follow the pictures below to see how to actually create a coil. You can also use a couple other formulas to help you find a specific inductance by finding the number of turns or by finding the length needed when you know either. ( T= sqrt[(L9R+10g)/R^2] and g= ((R^2 * T^2 / L) - 9R)/10 )
Step 5: Connect and Test
Well, sort of. You see, it's all fine and good that you have your circuit put together but getting it to trigger is another story. What you need to do is test out your coil by turning on the power, resetting the Schmitt Trigger with the switch you installed earlier and then (taking note of how much resistance you put between the 6th pin and emitter) start shifting your body at different speeds, from sudden to slow, to see what speeds will trigger the switch. By decreasing the resistance it will react to more subtle movements easier, but be careful not to turn down the resistance too much or the Schmitt Trigger will never turn off no matter how many times you reset! Turning up the resistance means it will take more sudden movements to trigger. For my circuit the ideal resistance was about 11.28K, which I swapped for a 2.2K and 10K in series but this could change depending on the inductance of your coils (mine were 6 microHenries each with a radius of about a quarter inch and a length of about half an inch with 7 turns of wire and an air core. Though they worked fine with air cores, I opted for magnetite cores which allows me to magnetically fasten them to metal surfaces.)
Step 6: An Explanation
To explain how the circuit works, by my understanding, it goes like this: When the circuit is powered on (after you reset it to be in an off state) the coil acts like an antenna waiting for minute electromagnetic frequencies to come in contact with. While it waits, the transistors are less biased to pass current and are in an off state as well, much like a switch. Since these transistors are off, they don't pass large enough current to trigger the Schmitt Trigger and the lights and sirens remain off. Once there is a large enough disturbance in the EM field (such as a person entering or leaving it), the field that the coil thought was stable becomes unbalanced and more signals are received (because of the human presence receiving the signals and throwing them all over the place) thus more current is produced in the transistors which gain enough oomph to trigger the Schmitt Trigger and sound the alarm. The transistors are recognized in two pairs called Darlington Pairs which use the previous transistors emitted current to attempt to drive itself. With them in a 4x series the current gains through each of the four stages which allows the very, very tiny signals from the coil to be amplified to the point where they can drive the Trigger. In less exciting news, the switch you installed simply counteracts the on cycle by passing current to the reset pin, thus resetting the chip to an off state. You're probably wondering why the EM field created by the circuit itself isn't triggering the alarm. Well, that's a good question, but not unanswerable. The field around these are strong enough to trigger it but since these fields are stable, the coil doesn't really care about them and won't trigger. If you'd like to test this, take the coil and place it near a track that you can roll a metal marble down. Does it trigger? Touch the coil with your hand and touch a radio antenna. Does is trigger?
I hope you had fun building this and I hope you find it useful to you. With a microcontroller and a few modifications you can turn this into a power saving light control (walk in your room and it turns the lights on, stay still for too long and it shuts itself off) or use it to detect those pesky raccoons that always raid your garbage at night. Whatever you do with this, have fun doing it!