Fire alarm circuit

Fire alarm circuit

Fire alarm circuit

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.

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Metal Detector Circuit

Metal Detector Circuit

 

Description

This is the circuit diagram of a low cost metal detector using a single transistor circuit and an old pocket radio..This is nothing but a Colpitts oscillator working in the medium band frequency and a radio tuned to the same frequency.First the radio and the circuit are placed close.Then the radio is tuned so that there is no sound from radio.In this condition the radio and the circuit will be in same frequency and same frequencies beat off to produce no sound.This is the set up.When the metal detector circuit is placed near to a metal object the inductance of its coil changes , and so do the frequency of oscillations.Now the two frequency will be different , there will be no canceling and radio produces a hissing sound.The metal is detected.

Note

To make L1 make 60 turns of 36SWG enameled Copper wire on a 1 cm PVC tube.
Powering the circuit using a adapter rather than a battery induces noise. It is always good to power radio projects from battery.

Circuit Diagram & Parts List .

 

Description

This is the circuit diagram of a low cost metal detector using a single transistor circuit and an old pocket radio..This is nothing but a Colpitts oscillator working in the medium band frequency and a radio tuned to the same frequency.First the radio and the circuit are placed close.Then the radio is tuned so that there is no sound from radio.In this condition the radio and the circuit will be in same frequency and same frequencies beat off to produce no sound.This is the set up.When the metal detector circuit is placed near to a metal object the inductance of its coil changes , and so do the frequency of oscillations.Now the two frequency will be different , there will be no canceling and radio produces a hissing sound.The metal is detected.

Note

To make L1 make 60 turns of 36SWG enameled Copper wire on a 1 cm PVC tube.
Powering the circuit using a adapter rather than a battery induces noise. It is always good to power radio projects from battery.

Circuit Diagram & Parts List .

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Mobile incoming call indicator

Mobile incoming call indicator

Mobile incoming call indicator

By admin

Description.

This circuit can be used to escape from the nuissance of obile phone rings when you are at home.This circuit will give a visual indication if placed near a mobile phone even if the ringer is deactivated.

When a call is coming to the mobile phone, the transmitter inside it becomes activated.The  frequency of the transmitter is around 900MHz.The  coil L1 picks up these oscillations by induction and feds it to the base of Q1.This makes the transistor Q1 activated.Since the Collector of Q1 is connected to the pin 2 of IC1 (NE555) , the IC1 is triggered to make the LED connected at  its output pin (pin 3) to blink.The blinking of the LED is the indication of incoming call.

 

Circuit diagram with Parts list. 

 

 

Notes. 

• The coil L1 can be made by making 150 turns of 36 SWG enameled copper wire on a 5mm dia plastic former.Or you can purchase a 10 uH coil from shop if available.
• The circuit can be powered from a 6V battery.
• Assemble the circuit on a good quality PCB.
• C1 & C3 are to be polyester  capacitors.
• The electrolytic capacitor C2 must be rated 10V.

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Digital temperature sensor

Digital temperature sensor

Description.

Here is a straight forward ,yet effective digital temperature controller that can be used with temperature control systems.

The circuit uses IC LM 35 (IC3) to sense the temperature.The output of LM 35 is given to the IC ADC 0804 (IC4) which converts the analog output of the LM 35 to digital output.The ADC 0804 is a 8 bit ADC .Eight LED’s are connected to the output of ADC to show the output logic.If needed the LED’s can be avoided and the output can be connected to any suitable external circuit like temperature controller etc.A 5V fixed voltage regulator based on 7805 (IC1) powers the circuit.Another variable voltage regulator based on LM317(IC2) is used to provide the reference voltage to the ADC 0804.

Circuit diagram with Parts list.

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digital-temperature-sensor-circuit 

Notes.

•POT R10 can be used to adjust the scale factor of the ADC.The reference voltage at pin 7 of ADC0804 should be 2.5 V for full scale.
•Assemble the circuit on a good quality PCB or common board.
•If 1A bridge is not available make one with four 1N 4007 diodes.