- 1 Simple Fire Alarm Circuit using Thermistor
- 2 Fire Alarm Circuit Using LM341
- 3 Fire Alarm Circuit Using Basic Components
- 4 Fire Alarm Circuit Using Germanium Diode
- 5 Related Articles
Fire alarms are major necessity in modern buildings, theatres, restaurants etc. There are many ways to design a simple fire alarm circuit. Fire can be detected in many ways such as by the emission of smoke, by the raise in temperature etc.
In the previous articles we have seen Smoke detector circuit, in this article let us see fire alarm circuits. Different methods were used to design the fire alarm .These circuits produce alarm when any fire is detected.
Simple Fire Alarm Circuit using Thermistor
Components of Fire Alarm Circuit
- Variable resistor(POT)
- BC547 Transistor
- The circuit consists of a 10k ohm thermistor. This is an NTC thermistor, which decreases its resistance with increase in the temperature.
- At room temperature it had a resistance of 10kohms.
- Another resistance is connected to the thermistor to form voltage divider circuit and this is connected to the transistor through a diode.
- Buzzer switches on only when the transistor is grounded. As the temperature increase the buzzer sound also increases
Fire Alarm Circuit Using LM341
Here is another small project on fire alarm. When a fire accident is happened in home or office, it will detect the fire and give the alarm.
Block Diagram of Fire Alarm Circuit Using LM341
The thermistor is the main component which detects the fire by sudden change in the room temperature because of the heat generated by the fire. The thermistor will detect the heat and give the information to the LM741 OP-AMP. The op-amp will make the NE555 to generate pulse which has been given to a buzzer to buzz.
LM741: LM741 is an operational amplifier which will work according to the difference in the input voltages. LM741 has following features like high current driving, voltage gain, noise amplification and also provide low output impedance. LM741 can also used as a short circuit protection.
Circuit Diagram of Fire Alarm Using LM741
- Circuit principle is similar to the first circuit i.e. Thermistor is used to sense the raise in temperature. But it rises only after a fixed temperature.
- Here op amp acts as non-inverting comparator i.e. Vout is positive only if Vin (voltage at pin 2)< VRef(voltage at pin3).
- When there is no any fire, voltage at pin 2 of the comparator is greater than the voltage at pin3.
- When there is no fire resistance of thermistor is 10k. So 10K and 4.7k forms voltage divider circuit.
- Voltage at pin2 is calculate using formula. V= (100*12) / (100+4.7) =11.4
- Voltage at pin 3 =50*12/100=6v (Variable pin of the pot is at 50% of total resistance.)
- When there is any fire thermistor temperature raises and its resistance decreases. So voltage at pin2 starts decreasing. Thus Vout is goes to positive i.e it is equal to Vcc.
- Here reference voltage selected is 6v.Fire alarm starts only if the input voltage is less than 6v.To increase the reference voltage decrease the resistance of pot.
Fire Alarm Circuit Using Basic Components
We are aware that there are many integrated circuits which can be used to generate the siren effect but we preferred to use basic electronics components like resistors, capacitors and transistors to generate it so that you will clearly understand the internal working of it and it will be much useful for you as you will gain more knowledge by analysing it instead of simply going or pre designed integrated circuits.
Fire Alarm Circuit Diagram
- This circuit uses a thermistor to sense the temperature. When it senses that the temperature of the environment is increasing above a given threshold, then it gives a signal.
- The temperature at which the circuit detects fire can be adjusted by using the potentiometer arrangement at RV2.
Get an idea about Thermistor Temperature Sensing Alarm if you are interested.
- When the temperature increases above the set value, the potentiometer arrangement produces a high voltage.
- This voltage is then given to BC547 transistor in common emitter mode. When the base is given a high input, it gets turned on.
- When the transistor is turned on, its collector voltage is reduced to low as the collector to emitter voltage decreases. The collector output voltage of the first transistor is given to the base as an input to the second BC 547 NPN transistor.
- This transistor too is in common emitter mode and as the input is low when the temperature threshold is reached, the output at the collector will rise high.
- In this state, it will turn on the next transistor, i.e BC107. This transistor will now act as a switch for the siren circuit. This transistor can bear power quite larger than the BC547 and it is also equipped with a heat sink for that purpose.
- When the BC107 transistor turns on, it allows current to pass from power supply to ground through collector thereby acting as an electronically controlled switch.
- When the current is passing, the siren circuit which is assembled as the load to the circuit is turned ON. Then you can hear the siren sound through the buzzer.
The capacitors used in the circuit are the main components in producing the siren effect.
- The principle involved in generating the siren effect is to make an oscillator with an envelope which periodically increases and decreases so as to generate that effect.
Fire Alarm Circuit Using Germanium Diode
This is a simple fire alarm circuit using Germanium Diode and 555 timer. In this circuit Germanium Diode play very important role in detecting the fire. This circuit is very easy to construct, cost effective and implementable.
Block Diagram of Fire Alarm Circuit Using Germanium Diode
Here is the simple fire alarm circuit which costs less than 100 rupees. The key component in the circuit is DR25 (germanium diode) whose resistance will decrease with increase in temperature. The conduction of germanium diode will start at 70 degrees. So we may use germanium diode as a heat sensor. When the temperature is more than 70 degree, the germanium diode will conduct and trigger the NE555 timer through a transistor. The NE555 is configured in astable multivibrator and make the buzzer to alarm when germanium diode conducts. So that we can get alert and act according to the alarm.
Circuit Diagram of Fire Alarm Using Germanium Diode
- The DR25 germanium diode is heat sensor which will conduct when temperature is increased at certain point. The DR25 is made reverse biased in the circuit. It will conduct only when it is more than 70degree of room temperature.
- The DR25 is connected to the transistor in reverse bias, which has high reverse resistance (more than 10K ohm) and does not make the transistor to turn off which is connected to the reset pin of 555 timer. The reset pin of 555timer will be in ground level when the transistor is turned off. Here, the 555 timer is configured as astable multivibrator.
- When more than 70degrees in room temperature occurred, the resistance of DR25 diode drops to 1k ohm which will make the transistor to turn off and make the reset pin to go high. This will generate the output at pin3 and make the sound through the alarm.
- We can use 3 or more diodes in reverse bias connected in parallel and placed in different room. If there is fire accident, it will sense and make the alarm.
- If DR25 germanium diode is available, you can still use AC128, AC188 or 2N360 germanium transistors. Use base and emitter junctions in place of cathode and anode.
- Diode must be connected to the circuit in reverse bias.