Super Sensitive Intruder Alarm

In the project first I will have do research for three or more circuit which have at least three active components. I will do the research by using the internet, books, magazines etc. After the research I will choose the circuit which I think is simple and easy for me to do my project on. I will do further research on the chosen circuit to understands how it works and how the components work. So when I start to making the project I will be able to deal with the faults if I find any in the circuit.

Circuit 1 – Fire alarm

This circuit is used is used when a fire occurs. The circuit relies on the smoke produced in the event of fire. The light which falls on LRD decreases when the smoke passes between bulb and LDR. This increases the resistance of the LDR and the voltage increases at the base of transistor. Then the supply to COB is complete to set off the alarm. The sensitivity of the circuit is the distance between bulb & LDR and it can also be preset setting of VR1.


Circuit 2 – Fire alarm

The 555 timer (IC1) is an oscillator at audio frequency which is configured to run freely. The T1 and T2 (transistors) build IC1 value. The pin 3 of IC1 is couples to the base of transistor, which makes the speaker to make the alarm go off. The thermistor is the key to the alarm. Also when the resistance is low and the temperature is high the alarm will sound. The frequency of NE555 depends on the values of resistances R5 and R6 and capacitance C2. The circuit can be powered from a 6V battery or a 6V power supply. The thermistor can be mounted on a heat resistant material like mica to prevent it from damage due to excessive heat. The LED acts as an indication when the power supply is switched ON.


Circuit 3 – super sensitive intruder alarm

In this circuit the alarm will set off when the shadow of an intruder passing few meters nearby the circuit is enough to trigger the alarm. IC2 uA 741 is wired as sensitive comparator; its set point is by R6 and R7. The voltage divide by LDR and R9 is given at non inverting pin of IC2. When there is an intruder near by or close to the LDR the shadow will make the resistance to increase. In that case the voltages at the inputs of comparator will be different and the out put of IC2 will be low. There fore the Q1 will turn on. This makes a negative going pulse to trigger the IC1 which is wired as a mono-stable multi-vibrator. The out put of IC1 will be amplified by Q2 (SL 100) to produce alarm. The LDR can be housed in a dark tube to increase sensitivity. The sensitivity is very important here. If you cannot adjust the required sensitivity properly, use one LOW resistance (~1K) POT in series with R9 for fine adjustment.


Choosing circuit:

The circuits have done research on are very simple and interesting. The circuit I am going to do for my project is the intruder alarm circuit. The components in the circuit are simple and I have used them in the past. I understand how the components work and how the components behave so it will be easy for me to make the circuit. I can also make the other two circuits but they are not as simple as the intruder alarm circuit and they have components which haven’t used before.

Components list




R1, R2, R3 & R5

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1k ohm resistors



1M resistor


R6 & R7

10 k ohm resistors






47k potential meter






Operational amplifier – uA741



555 timer



PNP transistor – BC 157



NPN transistor – SL100



Capacitor – 0.01 uF



Capacitor – 10 uF 25v





Three subsystems and active component s

Subsystem 1 – Operational amplifier uA741

An operational amplifier is usually said as an op-amp. It’s a DC coupled high-gain electronic voltage amplifier, with deferential inputs but it’s usually a single output controlled by negative feedback which determines magnitudes of its output voltage gain, and has high input impedance, low output impedance. Used with split supply, usually +/- 15V.

The most common and most famous op-amp is the mA741C or just 741, which is packaged in an 8-pin mini-DIP. The integrated circuit contains 20 transistors and 11 resistors

Pin layout


Large input voltage range

No latch-up

High gain

Short-circuit protection no frequency compensation

Subsystem 2 – PNP transistor

Transistor PNP – it consists of – two P-N junctions transistor back to back like this:

The arrows are there to represent the conventional flowing of current from positive to negative. Some semiconductors have an excess of electrons, n-type material that is due to doping of the semiconductors, while others have a deficiency of electrons called holes, they are found in P-type materials.

When base emitter voltage is about 0.7 v, that’s when the transistors conducts. Current are controlled by transistors devices. The bigger the base-emitter current, the bigger the collector-emitter current. The transistors are used as: an amplifier or a solid state switch.

Pin layout

Subsystem 3 – NE555 timer

555 timers is an extremely stable regulator which is capable of producing precise time delays. In the time delay process the time is controlled by one external resistor and capacitor. The frequency and the cycle are controlled by two external resistors and one capacitor for a stable operation as an oscillator. The output can drop down to 200mA.


Turn-off time less than 2 ms

Max. operating frequency greater than 500 kHz

Timing from microseconds to hours

Operates in both a-stable and mono-stable modes

High output current

Adjustable duty cycle

TTL compatible

Temperature stability of 0.005% per °C

Pin configuration


Input voltage – 0-9V

Output source – sound and light

Num of functions – 1

Product design – Intruder alarm

Reliability – good quality

Technical functionality – Sense shadows of intruder

Process – The circuit is able to sense intruders shadow and make the alarm go off.

Scale of operation – Normal

Size – length 20cm x 8 cm, height 2cm

Capability – It only takes 1 action

Cost – £5

Quality standards – has to meet the health and safety act of 1974

Legislation – Health and safety at work act 1974

Timescales – 18 weeks

Physical and human resource implications – the circuit that can be built manually. It will be built Using breadboards and components or with the use of circuit wizard.

Other components

LED – When LED is forward bias then it going to emit lights and converts electrical energy to light.


The capacitor is a component to stores up electronic charge and releases when it’s needed. Capacitor comes in massive range of sizes and types for to be used in adaptable power, conditioning, smoothing and isolating signals. They are made from various materials and all the electronic system uses them.

Capacitor Symbol = C, Unit = Farads

Charge Symbol = Q, Unit = Coulombs

Potential difference = V, Units = Volts

The capacitor can charge and discharge quickly. It’s made of two plates separated by an insulator or air. When a plate is charged the other plate is charged oppositely. The charge can build up and remain after the current is gone.

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Materials in capacitor

6 volt battery

Two large electrolytic capacitors, 1000 µF minimum (Radio Shack catalogue # 272-1019, 272-1032, or equivalent)

Two 1 kΩ resistors

One toggle switch, SPST (“Single-Pole, Single-Throw”)


A resistor is a passive electrical component which controls the flow of current. There are two types of resistor one is called fixed resistor and the other is called variable resistor. The resistance of a resistor is measured in ohms and it can be measured by colour coding band and measurement.

Potentiometer (pot)

This type of variable resistor with 3 contacts (a potentiometer) is usually used to control voltage. It regulates the voltage in the circuit.

Light dependent resistor (LDR)

It’s an electrical component which converts brightness (light) to resistance. The LDR depends on the intensity of light for its value.


It’s an electrical device that produces and buzzing sounds when a signal has been received.

Testing the circuit

I first built the circuit on circuit wizard. I put all the components out the page first then I connected all the components and added ground to the circuit. Then I played the circuit which shows the circuit is working perfectly because the Led lit up and the buzzer went off when I turned on the circuit. This shows that all the components are working correctly and there are no faults in the circuit.

Safe working procedures

Every activity must be carried out in accordance with a developed Safe Working Practice.

This will be validated by means of a Risk Assessment.

The control measures identified in the risk assessment shall form the basis for the written Safe Working Practice.

Consider what must be done before the task starts, How the task is done, what training is needed

What competence is required or certification?

The action to be taken when the task is complete.

Five steps for risks assessment and hazard

Identify the hazards

Wire cutter – sharp edges

Long nose pliers – sharp edge

Components – sharp edge from components

Decide who might be harmed and how

The person who is using equipment will get hurt by cutting them themselves and

The people who are around the work could get hurt by leaning on to a sharp component.

Evaluate the risks and decide on precaution

The risks are very similar, there are all involved with sharp edges. The user of the equipment could get hurt by losing concentration and accidently cut their finger. The precautions would be to always have full concentration when cutting wires and the user should make sure they are careful when cutting the wires and keeping their fingers away from the sharp edges.

Record your findings and implement them

The results of my risk assessment are that most people are cutting their fingers as they cut the wires. The reason behind that is people are not concentrating when using the wire cutters and pliers. To avoid these risks there should not be more then two people in the workshop. When there are lots of people in the workshop the users could lose their concentration easily.

Review your assessment and update if necessary

To review the risk assessment I have decided to update the wire cutter with a wire cutting machine so when people use it they won’t cut their fingers.


PPE refers to protective clothing and other devices that are designed to protect an individual while in potentially dangerous areas or performing potentially hazardous operations. Examples of PPE include gloves, hard hat, steel toed boots, and safety glasses.

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Making the subsystems

This is the first subsystem where the operational amplifier uA741 is. It also had LDR, pot and two resistors. I connected the circuit with black wiring to the negative and red wiring to the positive.

This is my second subsystem where the BC157 NPN transistor is at. It also had components such as resistors and LED. As I connected the first subsystems wiring with black to negative and red to positive I continued to follow the same procedures.

This is my third subsystem where the 555 timer is at. It also has components such resistors, transistors, capacitors and a buzzer. I connected the circuit with black wiring to the negative and red wiring to the positive.

Testing the circuits

The method I used to test my circuit is voltmeter, logic probe and oscilloscope. I used the voltmeter to check the circuit has correct reading on each outputs and inputs. I used the logic probe to test the circuit by checking if the circuit has the correct readings. And I used the oscilloscope to make a sine wave when the alarm goes off.

Firstly I tested my subsystem which had some faults in it. The faults which did not give any reading on the voltmeter, was some wires were misplaced. So after I correct my mistake the circuit was working the out of the circuit was an LED which lit up and I got reading on the voltmeter.

In the first image the LED is lit up and the voltmeter reading is 1.88v but when the LDR I covered on the image on the right the LED switches off and the voltmeter reading decreases to 1.31v. The readings on the voltmeter, and LED switching on and off shows that the circuit is working correctly.

My second subsystem had faults with it which I could not find a solution to it. I tried testing it with voltmeter and logic probe but the result was negative. So I decided to put my second subsystem with my third subsystem. I connected both circuits together and put the power on to find out that the buzzer is working but the Led wasn’t lighting up. I tried replacing the LED quit few times to if the Led had a fault with it. I also had some minor problems with the wring but that was simple enough to fix.

Putting all subsystem together

I started to put the subsystems together by firstly connecting all three breadboards together, and then I connected to the power supply to one of the breadboard. I had to connect all three circuits so I cut out some wires and connected positive to positive and negative to negative so all the circuits are connected.

So now to test the circuit I put the power supply on and then connected a voltmeter to check all the components in the circuit are working correctly. Then I connected the buzzer which was buzzing but did not stop until I disconnected the circuit and the LED did not light up either. To find the faults I went over the circuits to check for any faults. I did find some faults but they were just simple wring problems which I fixed with ease but the circuit still not work. After replacing some components the circuit was still not working. After doing all the tests I could do I decided to take out the second subsystem because it did not work when I was testing it but it worked with the third subsystem. As I removed the second subsystem the circuit started to work. I then check it with voltmeter to make sure the circuit was working correctly. The Led lit up too and went off when I covered the LDR.

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