# Unseen burglar alarm.

## Step 6: An Explanation

Ok, you got it built and working... by why does it work and how? Well, I'll tell you. At least to my understanding. You see this circuit works almost exactly like a traffic light intersection or fast food drive-thru. When a car reaches a certain point the body of the car passes over an inductive coil (a larger version of what you made) set in the pavement. This triggers something in the circuitry of the traffic light that counts cars and after a while will switch the lights from green to yellow to red and so forth. At a drive-thru the same happens only instead of lights, an alert is sent to the order takers in the restaurant when you get near the speaker (after all you can't expect them to stare at a screen all day looking for cars right?). Our circuit works the same way but a little differently. Since you probably aren't building this to sense cars but rather people, you need to have a more sensitive circuit to detect the disturbances in magnetic flux caused by humans. What's magnetic flux and why can humans trigger disturbances? Simply put, magnetic flux is the magnetic field around a magnet (you've seen it done with iron fillings on paper) which also flows around a wire that has an electric current passing through. Humans are similar to the car frames because of their constant bombardment by WiFi, Radio, and cell phone signals. The disturbances caused by humans is smaller than that of a car but our circuit is tuned to be extra sensitive to these disturbances.
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!
Happy Building!
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Rohanlopes says: Sep 19, 2011. 10:50 PM
nice idea!
i used 3 * BC547 transistor.it's working but sensitivity was bad it's working from 15 cm distance . can u plz tel me how to increase it's range to pull trigger from 6 ft (distance from coil.)

lopesphilipson@gmail.com