Project Alarm Foil

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* Objectives

* Justification

* Theoretical Framework

* Circuit diagram

* Procedure assembly

* Alarm Operation

* Calculations for circuit

* Budget

* Conclusion

* Bibliography

* Annexes


Alarms are useful in daily life of people for security. On this occasion, has developed a home alarm, which has performed a full assembly process, calculations of resistors, currents and voltages in different parts of the circuit and elements, etc.

This is not only intended to operate an alarm, but what you want is to understand exactly why and how the alarm. That is carefully studied each of the circuit elements. We have determined the function of each component and the work that plays in the functioning of the alarm.

Likewise, special emphasis was placed on each value calculations, as the circuit works as a system, can not be taken in isolation and manage independent values. Efforts should be all together as a single circuit.

Equally important has been given to the presentation and appearance of alarm, which is installed each item carefully and finally everything is installed in a box with glass on top, so they can see all the components.

The detailed explanation of the operation step by step, so that the reader easily understand why it has used each of the circuit elements and the role they play in the operation of the alarm.


1.1. GOAL

“Do you have an alarm and sensor foil, understanding in detail the role and importance of each element used in the circuit.”


* Learn how to create a circuit from a diagram.

* Implement the knowledge learned in the field Electricity and Magnetism.

* Run alarm foil.

* Understand the function of each element in the circuit.

* Be able to develop and interpret current calculations, voltage and resistance on the components used.


Given that the subject of Electricity and Magnetism can not only be theoretical but must also practice comes to light the need to create a circuit which implement the knowledge learned in the field Electricity and Magnetism.

It is critical to be able to do this alarm from a diagram. After that, understand how all the elements used and finally make calculations on the elements, as learned in class.

For these reasons, it justifies the development of this alarm foil, which are put into practice what they learned in class, and investigate and acquire new skills, abilities and knowledge.



The potential difference between two points (1 and 2) of an electric field is equal to the work done by the unit of positive charge for transportation from point 1 to point 2. Voltage may be defined as the electromotive force that causes free electrons to move.

3.2. POWER

Is the electrical charge passing through a conductor section or unit of time. In the International System of Units is expressed in coulombs per second, called amp unit.

If the intensity is constant in time, the current is said to be continuous, otherwise, called variable. If there is no storage charge distribution at any point of the driver, the current is stationary. According to Ohm’s law, the intensity of the current is equal to the voltage divided by the resistance put bodies:

3.3. OHM LAW

As the electrical resistance in a circuit is very important in determining the intensity of the electron flow, it is clear that it is also very important for the quantitative aspects of electricity. It was discovered long ago that, other things being equal, an increase in the resistance of a circuit is accompanied by a decrease in current. A precise statement of this relationship had to await the develop measurement instruments reasonably safe. In 1820, Georg Simon Ohm, a German schoolteacher, found that the current in a circuit was directly proportional to the potential difference that produces the current, and inversely proportional to the current limiting resistor. Expressed mathematically:

where I is the current, the potential difference V and the resistance R.

This basic relationship is named after the physicist who participated in its formulation more: it is called Ohm’s Law.

If you replace the proportionality sign of Ohm’s Law by an equal sign, we have:


Ohm’s Law to determine electrical current (Amps)

Solving him above equation, there are two equations:

Ohm’s Law to determine values of resistance (ohms)

Ohm’s Law to determine voltage (Volt)

Thus, Ohm’s Law defines the unit of electrical resistance as well as voltage and current, making simple equations presented punt, provided that two values are known and one unknown.

3.4. Potentiometer (variable resistor)

A potentiometer is a resistor that you can vary the resistance value. In this way we can indirectly control the intensity of current to a line if connected in series or potential difference than in parallel.

Normally potentiometers are used in low current circuits, to enhance the current, just as not dissipate power, however on the resistors, which are larger, more current flows and dissipate more power.

Adjustable variable resistor divider through a cursor.

Is a resistance track formed by a thin carbon exit end of which two terminals to said track runs along a slide which is linked to a third terminal.

Applying a voltage between terminals 1 and 2, the cursor will have a voltage proportional to the position on the track.

Potentiometer or variable resistor


The transistor is a semiconductor element has the property that power will govern the intensity of current between two of its three terminals (emitter and collector), by circulating a small current applied to the third terminal (emitter).

This effect is known as current amplification and allows the transmitter to apply a very small current with any form of variation over time, and obtain the same current, with the same variation in time but larger amplitude .

They are mainly used in circuits that perform amplification, control, data processing, etc..

Transistor and represent it in the form of a circuit


It is called electrical resistance, R, of a substance is the opposition that the electrical current to tour. Its value is measured in ohms and is designated by the Greek letter capital omega (). The matter has 4 states in relation to the flow of electrons. These are Conductors, Semi-conductors, resistors and dielectrics. They are defined by the degree of opposition to electric current (flow of electrons).

This definition is valid for the direct current and alternating current for the case of pure resistive elements, ie without inductive or capacitive component. Of these reactive components exist, the opposition presented to current flow is called the impedance.

Depending on the magnitude of this opposition, the substances are classified as conductors, insulators and semiconductors. There are also certain materials that, under certain conditions of temperature, a phenomenon called superconductivity occurs, in which the resistance value is almost zero. The electrical resistance is measured with Ohm meter is a device for measuring the electrical resistance in ohms. Because the resistance is the potential difference that exists in a conductor divided by the intensity of the current passing through it, an ohmmeter must measure two parameters, and it must have its own generator to produce electrical power.

Group of resistors of different values3.7. Electric condenser

In electricity and electronics, a capacitor, sometimes referred to incorrectly Anglicism capacitor is a device consisting of two conductors or armor, generally in the form of plates or foils separated by a dielectric material, which, subjected to a potential difference (ddp ) acquire a certain electrical charge.

This storage property is called load capacity or capacitance. In the international system of units is measured in farads (F), with 1 farad capacitor capacity in which an armor subjected ddp 1 volt, they acquire an electric charge of 1 coulomb.

1 farad capacity is much larger than that of most capacitors, so that in practice the capacity is usually indicated in micro-uF = 6.10, F = 9.10 nano-or pico-F = 10-12-farads. The capacitors obtained from supercapacitors (EDLC) are the exception. They are made of activated carbon to achieve a relatively large area and have a molecular separation between the “plates”. Thus capabilities are achieved by hundreds or thousands of farads. One of these capacitors is incorporated Seiko Kinetic clock with a capacity of 1/3 of Farad, making unnecessary the stack. Is also being used in prototype electric cars.

The value of the capacitance is defined by the following formula:


C: Capacity

Q: Electric Charge

V: potential difference

As regards the constructive aspect, both the shape of plates or armor as the nature of the dielectric material are highly variable. There capacitors formed by plates, usually of aluminum, separated by air, ceramic, mica, polyester, paper or an aluminum oxide layer obtained by means of electrolysis.

Capacitors or condensers

3.8. SCR or thyristor

The SCR is a device composed of four semiconductor layers PNPN semiconductor material or NPNP structure. Its acronym is SCR (Silicon Controlled Rectifier). The name comes from the union of thyratron (thyratron) and Transistor.

An SCR has three connections: anode, cathode and gate. The gateway is responsible for controlling the passage of current between the anode and cathode. Basically functions as a controlled rectifier diode, allowing circulating current in one direction only. While not apply any voltage at the gate of the SCR does not start driving and the instant when the voltage is applied, the thyristor starts conducting. Once started, can override the gate voltage and the thyristor will continue to conduct until the load current drops below the holding current. Working in the SCR AC drops out in each rotation or alternation.

Thyristor symbol

When a sudden variation in voltage between anode and cathode of a thyristor, and it can be fired into conduction yet gate current. Therefore property is given as the maximum rate of voltage rise which keeps the SCR locked. This effect occurs due to the parasitic capacitor between the gate and anode.

The SCR applications are used in power electronics and control. We could say that an SCR functions as an electronic switch. The image of this element is shown below:

SCR or thyristor


For the development of this project, is part of a seemingly simple, but that is the foundation of everything, and the circuit diagram is developed. To understand all the symbols and values of each element, we proceed to buy them, and begin the process of assembly and test operation, so that eventually they can also present calculations done on the components, which were useful when going arming the circuit and when to study and explain what each component.

The diagram is as follows:


The first thing that should be done is to prepare the cobreada plate, where the circuit will. This is presented below step by step process that has been followed to the plate and be ready to start assembling the circuit design:

* Cut cobreada giving plaque measures 15.5 x 9.5 cm.


* Lining with masking tape at the top of the plate cobreada.

* Draw masking tape on the circuit diagram.

* Cut with a knife on the masking tape, the drawn diagram and then exposing the copper where the parts will be assembled.

* Apply nail polish on the exposed portion is no longer masking tape on top, letting it dry.

* Remove the masking tape, so that it is drawn the diagram with glaze.

* Heat 1 cup water and dissolve in it 2 ounces of perchloride.

* Place the plate in the vessel containing the water with perchloride and shake until the copper is apparent from the unglazed cobreada plate.

* The goal of applying enamel on the diagram on the board is made that the perchloride not remove the copper from which it has become the diagram (for which that party conducting electricity), but the copper is removed from all parts around the diagram, so that there exists no electric conduction. The principle of this is that the enamel with perchloride are like water and oil, they never meet. To verify the above is sufficient to apply the glaze onto perchloride is dissolved in water, and observe how the perchloride remains as a layer on the water surface with perchloride.

* Place the plate in water with baking soda to clean any residue completely perchloride that are left. Following that, find the copper using a polish remover, and the plate will be ready to start assembling the circuit, thus:

* Weld each of the elements on the plate using the iron and tin, according to the circuit diagram.

12. For better aesthetic enclosure installed on the circuit, protecting and giving a better appearance.


This alarm is activated when the circuit is opened, ie when tearing the foil.

It essentially works with 5 elements: a potentiometer, a transistor, a resistor, a capacitor (capacitor) and an SCR or thyristor.

We are working with a 5V output transformer, but also works with 6V, which it can be obtained using a power adjustable.

The potentiometer or variable resistor acts as a voltage regulator, allowing varying input voltage. In this circuit, the pot causes the alarm to sound stronger or weaker, depending on the amount of ohms, which regulates the potentiometer. The potentiometer then functions as a regulator of alarm volume, and the volume is intensified or weaker depending on the step voltage to the circuit there.

The transistor is operating as a pulse amplifier. This element has three pins. The first step was to identify the base, the collector and emitter of the transistor. Using the voltmeter, it emerged that the center pin is the base and the other two pins are collector and emitter. Therefore, it was learned that this transistor is NPN. The driving is always given from collector to emitter. When breaking the foil, the transistor sends a pulse to the SCR, yielding the excitation and activating the SCR gate, so that conduction is generated to the part where the alarm, and this starts ringing. When reattached cables where was the foil, the alarm goes off.

The capacitor stores charge for its part and is useful when the alarm is sounding. Subsequently, when switching off the circuit, the capacitor reaches discharged. This element, being a filter, causes the current reduction that may have more pulses, causing the alarm to sound perfectly.

We are also using a 1K resistor, which is protecting both the transistor as well as the excess current SCR.

Which activates the alarm is then cutting the foil, that is when the circuit is opened. This is because the cutting, there is a pulse, a minimum of shock energy that flows through the circuit, which allows power received at the base of the transistor and which is then amplified by the collector and emitter, reaching the gate of the SCR, which is excited. As this happens, the cathode and anode are activated, and energy reaches the alarm, which starts ringing. As shown, the SCR acts as an electronic switch, because the excitement of the gate, is like closing the circuit (anode and cathode), or open. With a mechanical switch, it is people that open or close the switch, but in this case, the excitation of the gate which performs this task, so the SCR can be called a power switch.

He said the sound intensity or the number of decibels of alarm sound is controlled by the potentiometer. The explanation is simple, considering that the potentiometer is regulating the voltage input to the circuit, then the higher the voltage, the greater the intensity of the sound, and at a lower voltage, lower sound intensity. When the potentiometer is 1000 ohms, there is a high input voltage and thus the maximum sound that can be produced by the alarm circuit. When the potentiometer is at 0 ohms, there is a high input voltage and therefore the slightest sound that can be produced by the alarm, it would just total silence, not having a voltage input. This is proved by Ohm’s Law: V = I R. According to the above equation, if the resistance has a value of 0 omnios, consequently the voltage automatically will be zero, which corresponds to silence in the case of this alarm. As he is giving added strength to the alarm, the voltage increases, and when it reaches the maximum value of the potentiometer, which is 1000 ohms, the alarm sounds at its maximum value, because it has allowed the maximum possible input voltage from the source or transformer. If the potentiometer is at zero ohms, means not working transistor, whereby the capacitor can not become charged, or if it was loaded, unloaded.

The capacitor remains charged whenever it’s voltage. The knob is only controlling the voltage input to the circuit, but what determines the alarm sounds pulse is generated directly by breaking the foil. Do not think that what the alarm is the potentiometer, as though he were a 1K potentiometer, but if you have not broken the foil, not command the amplification transistor to the gate of the SCR, and the latter does not reach excited, so the alarm would not sound. This alarm then works through pulses, which are amplified by the transistor and the gate and the SCR is excited and ignited, setting the cathode-anode junction.

All elements of this circuit are important and necessary for the operation of this circuit, but there are two notable elements, which are the transistor and SCR. These sets are in playing the central role in the activation and deactivation of the alarm, but would be useless if not for the other elements, of course. The transistor is also called driver controller means, as is the controller amplification of the pulses that are injected.

This alarm can be used to protect areas advantages or glass, etc.. It is known that the foil is a switch, and you can also say it is the sensor that triggers the alarm. Whenever the circuit is opened, the alarm will sound and will remain so indefinitely until the circuit is closed again or until you turn the power voltage. It is a very useful tool to protect doors, windows, etc..


Below are some calculations on the elements of the circuit was developed.

First calculations performed to determine the transistor currents. However it should be noted that not only the transistor there is a current, which should be determined by three streams: the base, the collector and the emitter.

Base power Base

where IB is the supply voltage, VCC is the supply voltage or transformer, VBE is the base-emitter voltage, which is 0.7V to measure, although we know by formula that will always have this value when in its “on” state, RB is the resistance of the base, which is the 1K potentiometer or 1000.

Emitter current and collector current

We start from the following equation:

IE = IB + IC

where IE is the emitter current, IB is the base current and IC is the collector current. However, it will be seen that the base current is much smaller than the emitter current and the collector, so that if the currents of the emitter and collector were given in milliamps of base current would be given in microamperes, and if the current of the emitter and collector are given in amperes, the base current in milliamps, whereby the formula can be reduced to:


From the above equation, it appears that sufficient find one of the two currents, and will know the value of both.

IB IC = ss

CI = (200) (0.0043A)

IC = 0.86A

Where Ic is the collector current ss is a constant that has been searched in the ECG Manual transistor used according to, IB is the base current, which is calculated above.

Therefore since the emitter current is equal to the collector current is obtained as follows:

IE = IC = 0.86A

Current potentiometer

Is obtained using Ohm’s law:

I = V / R

I = 5V / 1000

I = 0.005A

where I is the current through the potentiometer, V is the supply voltage reaching the potentiometer, and R is the resistance that has the same potentiometer.

Voltage and current in the resistance of 1000

Using the voltmeter was learned that the voltage across this resistor is 1.2V. Note that this occurs only when the alarm is activated, otherwise the voltage is 0V. To determine the current in this resistor using Ohm’s law and we have:

I = V / R

I = 1.2V / 1000

I = 0.0012A

Charge on the capacitor when the alarm sounds

To determine this loading formula was used capacitance. We also measured the voltage on the capacitor there when the alarm is sounding, which were 4V. The following value was obtained for loading the capacitor:

Q = C V

Q = (0.0000047F) (4V)

Q = 0.0000188 Coulomb

Where Q is the charge of the capacitor, which is measured in Coulombs, C is the capacitance value of the capacitor which is used and is measured in Faraday, and V is the voltage in the filter or capacitor.

Current SCR gate

For this measurement, the following values are obtained:

Where IG is the current in the gate of the SCR; VDC is the voltage that is in the SCR, after being measured with the voltmeter, R is the resistance of 1000.


Finally we present the budget for this alarm, detailing the prices of each of the elements used:

Item Price

1K Potentiometer ………………………………………………………………… $ 1.00

Transistor HEP 53 …………………………………………………………………… $ 0.51

Resistor 1K and 0.5W ………………………………………………………………. $ 0.31

Capacitor 47 microF x 16V ……………………………………………………… $ 0.25

HEP R110 SCR ……………………………………………………………………… $ 1.53

Cobreada Plate ……………………………………………………………………… $ 5.00

2 ounces of perchloride ………………………………………………………………. $ 4.00

6 tones Siren S1-136 ……………………………………………………………… $ 5.85

Wooden box with glass and switch ……………………………………………. $ 6.00

TOTAL …………………………………………………………………………….. $ 24.45


Having done this project has been a very important experience because it has put in practice the knowledge acquired in school, yet have acquired new skills in the process of research and development of this project.

He has learned to create a circuit from a given diagram, recognizing the parts and components, to join them later.

This project applied formulas and knowledge studied in classes of Electricity and Magnetism, which was one of the key objectives of this.

Not only the components are assembled, but also have made the alarm function satisfactorily, it has been thoroughly understand the role that each element performs within the circuit, so it is possible to give a detailed explanation of what each item does, This has been made herein shall also be made the official presentation of the project will be in class.

Finally calculations were developed current, voltage and resistance on the components used, this being useful in the assembly process as well as to complete the project and have made the alarm function, as these calculations allow us to study in depth the current formulas and relationships , voltage and resistance, confirming that electrical principles studied in class in relation to these values obtained in practice are real and match perfectly with what has been studied theoretically. So, they have met all the objectives, and has successfully completed this project.


1. Mystical Sciences. Introduction to Electronics. Retrieved November 2, 2006 in

2. Wikipedia. Thyristor. Retrieved on November 5, 2006 in

3. The University Of Arizona. The Department of Electrical and Computer Engineering. Photo transistor. Retrieved on November 5, 2006 in ~ ece220/Course_Notes/transistor.JPG

4. Wikipedia. Potentiometer (variable resistor). Retrieved on November 5, 2006 in C3% B3metro_ 28resistencia_variable% 29%

5. BR Market. Photo potentiometer. Retrieved on November 5, 2006 in

6. Wikipedia. Electrical resistance. Retrieved on November 5, 2006 in C3% A9ctrica

7. Wikipedia. Capacitor. Retrieved on November 5, 2006 in C3% A9ctrico

8. Handtools and electrical components. Photo capacitor. Retrieved on November 5, 2006 in

9. Wikipedia. Thyristor. Retrieved on November 5, 2006 in

10. Photo of SCR. Retrieved on November 5, 2006 from


Are some pictures of the finished project as annexes:


Esmeralda Jennifer Carranza Chacon

Glenda Maritza Spain Canalez

Jaime Montoya Oswaldo Guzman

Santa Ana, November 8, 2006

El Salvador