Fire Sensor Circuit

This fire sensor circuit utilizes the temperature-sensing capability of a standard signal diode, specifically the IN 34, to detect heat from a fire. Upon detecting heat, it triggers a loud alarm that mimics the sound of a fire brigade. The circuit is highly sensitive and can detect a temperature rise of 10 degrees Celsius or more in its vicinity. Signal diodes such as the IN 34 and OA 71 demonstrate this property, wherein their internal resistance decreases as the temperature increases. This effect is more pronounced when the diode is in reverse-biased mode. Typically, the diode can generate approximately 600 millivolts at 5 degrees Celsius. For every degree rise in temperature, the diode produces an output voltage of 2 mV; thus, at 5 degrees Celsius, it outputs 10 mV, and at 50 degrees Celsius, it generates 100 millivolts. This voltage is utilized to trigger the remainder of the circuit. Transistor T1 acts as a temperature-controlled switch, with its base voltage reliant on the voltage from the diode, as well as from variable resistor VR and resistor R1. Under normal conditions, T1 conducts (due to the voltage set by VR), causing an LED to light up, indicating a normal temperature. When T1 conducts, the base of T2 is grounded, keeping it off to prevent the alarm generator from activating. The circuit employs IC UM 3561 to produce a fire force siren. This ROM IC features an internal oscillator capable of generating various tones based on its pin connections. In this configuration, pin 6 is connected to the Vcc pin 5 to achieve the fire force siren sound. When the temperature near the diode exceeds 50 degrees Celsius, it conducts and grounds the base of T1, turning T1 off and T2 on. Consequently, the alarm generator receives current from the emitter of T2, which is regulated by ZD to 3.1 volts and buffered by capacitor C1. Resistor R4 (220K) determines the oscillation frequency, and the specified value of 220K is essential for producing the correct tone. To calibrate the fire sensor circuit, place a lit candle near the diode and wait for one minute. Gradually adjust VR until the alarm sounds, then remove the heat. After one minute, the alarm will turn off. VR can be further adjusted to set specific temperature thresholds.

The fire sensor circuit operates on the principle of temperature detection using a diode's characteristics, making it both efficient and cost-effective. The choice of the IN 34 or OA 71 diodes is crucial, as their response to temperature changes allows for precise monitoring of heat levels. The diode's output voltage is directly proportional to the temperature increase, providing a simple yet effective means to trigger the alarm system.

The transistor configuration plays a vital role in the circuit's operation. T1 is normally in a conducting state, allowing the LED to indicate a safe temperature environment. The grounding of T2’s base when T1 conducts ensures that the alarm remains inactive during normal conditions. However, once the temperature exceeds the set threshold, the circuit transitions to an alarm state, effectively alerting individuals in the vicinity.

The UM 3561 IC is integral to the alarm's sound generation. Its versatility in tone production is leveraged to create a loud and distinct alarm, ensuring that it can be easily heard in an emergency. The resistor R4's value is critical, as it directly influences the sound frequency, which is important for creating an effective alerting tone.

Calibration of the circuit is straightforward, allowing for easy adjustments based on environmental conditions. This feature enhances the usability of the fire sensor circuit, making it adaptable to various settings and ensuring reliable operation when needed. The combination of these components results in a robust fire detection system that can provide early warnings, potentially saving lives and property.This fire sensor circuit exploits the temperature sensing property of an ordinary signal diode IN 34 to detect heat from fire. At the moment it senses heat, a loud alarm simulating that of Fire brigade will be produced. The circuit is too sensitive and can detect a rise in temperature of 10 degree or more in its vicinity.

Ordinary signal diodes li ke IN 34 and OA 71 exhibits this property and the internal resistance of these devices will decrease when temperature rises. The fire sensor circuit is too sensitive and can detect a rise in temperature of 10 degree or more in its vicinity.

Ordinary signal diodes like IN 34 and OA 71 exhibits this property and the internal resistance of these devices will decrease when temperature rises. In the reverse biased mode, this effect will be more significant. Typically the diode can generate around 600 milli volts at 5 degree centigrade. For each degree rise in temperature; the diode generates 2 mV output voltage. That is at 5 degree it is 10 mV and when the temperature rises to 50 degree, the diode will give 100 milli volts.

This voltage is used to trigger the remaining circuit. Transistor T1 is a temperature controlled switch and its base voltage depends on the voltage from the diode and from VR and R1. Normally T1 conducts (due to the voltage set by VR) and LED glows. This indicates normal temperature. When T1 conducts, base pf T2 will be grounded and it remains off to inhibit the Alarm generator. IC UM 3561 is used in the circuit to give a Fire force siren. This ROM IC has an internal oscillator and can generate different tones based on its pin connections.

Here pin 6 is shorted with the Vcc pin 5 to get a fire force siren. When the temperature near the diode increases above 50 degree, it conducts and ground the base of T1. This makes T1 off and T2 on. Alarm generator then gets current from the emitter of T2 which is regulated by ZD to 3. 1 volt and buffered by C1. Resistor R4 ( 220K) determines the frequency of oscillation and the value 220K is a must for correct tone.

To set the fire sensor circuit, keep a lighted candle near the diode and wait for 1 minute. Slowly adjust VR till the alarm sounds. Remove the heat. After one minute, alarm will turns off. VR can be used for further adjustments for particular temperature levels. 🔗 External reference