Category: detector

Fire alarm circuit

Fire alarm circuit


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



  • 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.

Infrared motion detector circuit


Here is the circuit diagram of an infrared motion detector that can be used to sense intrusions.Infra red rays reflected from a static object will be in one phase, and the rays reflected from a moving object will  be in another phase.The circuit uses this principle to sense the motion.

The IC1 (NE 555)  is wired as an astable multivibrator .The IR diode connected at the output of this IC produces infrared beams of frequency 5Khz.These beams are picked by the photo transistor Q1 .At normal condition ie; when there is no intrusion the output pin (7) of IC2 will be low.When there is an intrusion the phase of the reflected waveforms has a difference in phase and this phase difference will be picked by the IC2.Now the pin 7 of the IC 2 goes high to indicate the intrusion.An LED or a buzzer can be connected at the output of the IC to indicate the intrusion.

Circuit diagram with Parts list.


  • Comparators IC2a and IC2b are belonging to the same IC2  (LM1458).So the power supply is shown connected only once.No problem.
  • When there is disturbance in the air or vehicles passing nearby,the circuit may get false triggered.
  • POT R5 can be used for sensitivity adjustment.



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.


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 .