Description.

Many fire alarm circuits are presented here,but this time a new circuit using a thermistor and a timer to do the trick. The circuit is as simple and straight forward so that , it can be easily implemented.The thermistor offers a low resistance at high temperature and high resistance at low temperature. This phenomenon is employed here for sensing the fire.

The IC1 (NE555) is configured as a free running oscillator at audio frequency. The transistors T1 and T2 drive IC1. The output(pin 3) of IC1 is couples to base of transistor T3(SL100), which drives the speaker to generate alarm sound. The frequency of NE555 depends on the values of resistances R5 and R6 and capacitance C2.When thermistor becomes hot, it gives a low-resistance path for the positive voltage to the base of transistor T1 through diode D1 and resistance R2. Capacitor C1 charges up to the positive supply voltage and increases the the time for which the alarm is ON. The larger the value of C1, the larger the positive bias applied to the base of transistor T1 (BC548). As the collector of T1 is coupled to the base of transistor T2, the transistor T2 provides a positive voltage to pin 4 (reset) of IC1 (NE555). Resistor R4 is selected s0 that NE555 keeps inactive in the absence of the positive voltage. Diode D1 stops discharging of capacitor C1 when the thermistor is in connection with the positive supply voltage cools out and provides a high resistance path. It also inhibits the forward biasing of transistor T1.

Circuit diagram with Parts list.

Notes.

• The circuit can be powered from a 6V battery or a 6V power supply.
• Click Here ! for the circuit diagram of a power supply circuit for this project.
• 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.

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Description.

The circuit diagram present here is that of a street light that automatically switches ON when the night falls and turns OFF when the sun rises.In fact you can this circuit for implementing any type of automatic night light.

The circuit uses an LDR to sense the light .When there is light the resistance of LDR will be low.So the voltage drop across POT R2 will be high.This keeps the transistor Q1 ON.The collector of Q1(BC107) is coupled to base of Q2(SL100).So Q2 will be OFF and so do the relay.The bulb will remain OFF.

When night falls the resistance of LDR increases to make the voltage across the POT R2 to decrease below 0.6V.This makes transistor Q1 OFF which in turn makes Q2 ON. The relay will be energized and the bulb will glow.

Circuit diagram with Parts list.

Notes.

• POT R2 can be used to adjust the sensitivity of the circuit.
• You can use bulb of any wattage ,provided that relay should have the sufficient rating.
• The circuit can be powered from a regulated 9V DC power supply.
• Click Here! to get the power supply circuit for this project.
• The relay K1 can be a 9V SPDT relay.

Description.

No big deal here!You can make a decent power amplifier using just three transistors.All components used here can be obtained from your electronic junk box.Also you just need a 9V transistor radio battery to power this circuit. The circuit is ideal for a pocket radio or other hand held audio equipments.

The first part of the circuit here is a preamplifier consisting of transistor Q1(2N2222).The collector of the Q3 is coupled to the base of Q2 (2N2905A),which forms a complementary symmetry pair with Q3 (2N3053).The amplified signal is available at the junction of emitter of two transistors.

Circuit diagram with Parts list.

Notes.

• For Q1 you can try with any general purpose NPN transistors like BC107,BC148,etc.
• For Q2 and Q3 you can try any complimentary power transistors. The result may vary slightly.No problem.
• The can be also powered from a well regulated and filtered 9V DC power supply.
• Click Here!.To get the power supply circuit for this amplifier.

Description.

Here is the simplest scheme for converting a CD ROM drive of your computer to a Audio CD player.The minimum requirement for the player is that, it should have a audio output and skip button. Luckily most of the Cd ROM’s are equipped with both of these.

The CD ROM drive needs two voltages ,12V & 5V for its operation.So the main objective is to build a suitable power supply for the CD ROM drive.The IC1 (7812) together with associated components produce a regulated 12V DC.The IC2 (7805) together with associated components produce a regulated 5V DC.These voltages as well as ground can be connected to the corresponding voltage pins of the CD ROM drive using a male type CD ROM drive power connector.The 12V can be connected to the yellow wire of connector, 5V to red wire and GND to black wire as shown in figure1.Now the power supply is ready.

Testing.

Make the circuit as shown in the circuit diagram.Power up the circuit after connecting the power connector to the CD ROM drive. Now the power LED of the drive will glow.Insert the audio CD.Now the music will be available at the audio output socket of the drive.It can be heared using a headphone.The skip button of the drive can be used to play next song.

By connecting the audio output to a power amplifier you can enjoy the music in a greater wattage.Click Here! to get the amplifier circuits suitable for this project.

Circuit Diagram with Parts list.

Notes.

• For car stereo applications you don’t need the transformer,rectifier and the 7812 regulator.12V will be available from battery.You just need to produce a 5V from it using a 7805 based regulator. Connect the corresponding voltages to the connector as shown in figure 1 and connect the connector to drive.Done.

• The amplifier for the car audio CD player must be one operating from 12V.

• Do not connect the voltages to CD ROM drive in wrong polarity.Double check the voltages using a multimeter. Wrong polarity could easily damage the drive.

Description.

Here is a simple fire alarm circuit based on a LDR and lamp pair for sensing the fire.The alarm works by sensing the smoke produced during fire.The circuit produces an audible alarm when the fire breaks out with smoke.

When there is no smoke the light from the bulb will be directly falling on the LDR.The LDR resistance will be low  and so the voltage across it (below .6V).The transistor will be OFF and nothing happens.When there is sufficient smoke to mask the light from falling on LDR, the LDR resistance increases and so do the voltage across it.Now the transistor will switch to ON.This gives power to the IC1 and it outputs 5V.This powers the tone generator IC UM66 (IC2)  to play a music.This music will be amplified by IC3 (TDA 2002) to drive the speaker.

The diode D1 and D2 in combination drops 1.4 V to give the rated voltage (3.5V ) to UM66 .UM 66 cannot withstand more than 4V.
Circuit diagram with Parts list.

Notes. 

• The speaker can be a 8Ω tweeter.
•  POT R4 can be used to adjust the sensitivity of the alarm.
• POT R3 can be used for varying the volume of the alarm.
• Any general purpose NPN transistor(like BC548,BC148,2N222) can be used for Q1.
• The circuit can be powered from a 9V battery or a 9V DC power supply.
• Instead of bulb you can use a bright LED with a 1K resistor series to it.