The simple ultrasonic fire alarm circuit explained below detects a fire hazard situation by picking up the variations in the surrounding air waves, or the air turbulence. The high sensitivity of the circuit ensures that even the slightest of air turbulence created by a temperature difference or fire is quickly detected and an attached alarm device is sounded.
Conventional fire sensors utilize diverse systems to identify fire, and they come with all sorts of complexities.
An ordinary fire alarm system uses a temperature sensor to sense the unusually high-temperature variance caused by a fire.
It is not fundamental that only an electronic part like a thermistor or a semiconductor temperature device is used, but simple material like a low-temperature fusible link or bimetal temperature switch.
Although the simplicity of such alarm types is preferred, their reliability is questionable because detection occurs only when a fire has already matured.
More complex fire alarm systems exist, for example, smoke detectors that are equipped with a distinct semiconductor part that senses the existence of smoke particles, combustible gas and vapor.
Other than that, there are optoelectronic fire alarm systems which get triggered when the smoke of any form blocks their light beams. Such type of fire detection system was published on Hobby Electronics.
Heat Detection Using Doppler Shift
A novel method of fire detection using ultrasonic sound is described in this article. Bearing the same operating principles as the famous Doppler Shift ultrasonic intruder alarms, this fire detection system is tremendously sensitive to turbulence in the air, in addition to solid object’s motion.
The heat from an electrical fire produces immense turbulence and triggers the alarm. Often, false alarms are set off because of the turbulence. As a result, this type of fire alarm is perfect for a home even though people living in it would often not appreciate it.
How Sound Discrimination Happens
One drawback of using a Doppler Shift burglar alarm as a fire alarm is the massive detection area this unit delivers. Somehow, here this turns out to be a boon because quick detection becomes possible even though a fire starts in a small corner of the detection area.
The standard principle of conventional fire alarms is to detect fires while ignoring people who are scrambling around the room. This is crucial as the alarm system is set to run until activated.
A typical ultrasonic Doppler Shift alarm fails to differentiate between people and turbulence. Therefore, it makes more sense for a fire alarm system to use a circuit which governs a small area of operation.
The alarm unit can be placed in a location in the room where human motion is minimal, but still, be able to swiftly identify the turbulence resulting from a fire.
A basic ultrasonic alarm is equipped with two independent circuits that are connected via the same power supply.
The simpler electronic circuit acts a transmitter which emits uniform sound frequencies to the receiver, which is the more complicated circuit.
A block diagram of the fire alarm is shown in Figure 1.
As described, the transmitter circuit works to produce ultrasonic sound using an oscillator and feeds the signal through a loudspeaker.
The electrical signal is converted into sound waves by the speaker, but humans cannot hear them because they are pitched above the hearing range.
Common sound amplifiers do not work well at ultrasonic frequencies because of the Piezoelectric type of transmitting transducer.
Usually, an output level moderator is included so that the sensitivity of the circuit can be attuned to the right level.
A microphone at the receiver detects the soundwaves from the transmitter and converts them to back to electrical signals.
Once more, a specialized Piezoelectric transducer is utilized on the receiving microphone because the normal ones are unsuitable to operate at high, especially ultrasonic frequencies.
The extremely maneuvering state of ultrasonic sound causes detection troubles between the microphone and the loudspeaker in case both the devices are installed almost next to each other.
In practical situations, the captured signals are reflections from walls or furniture in the room.
Moreover, the output from the microphone is relatively low and typically around 1 mV RMS. So, an amplifier is incorporated to enhance the signal to a working level.
Normally, two high-gain stages of amplification are used at minimum in an ultrasonic burglar alarm. However, since the discussed fire alarm system requires lesser sensitivity, so a single stage of amplification is more suitable.
The next section of the circuit is an amplitude modulation detector. In a practical situation, the detected signal is a direct 40kHz output wave from the transmitter.
This signal is collected using various paths and arbitrarily phased. But, both amplitudes of the signal and its phase relationships are preserved without any alteration. Thus, no output is generated from the amplitude generator under ready situations.
Whenever there is motion in front of the detector or the air is turbulent, the whole scenario changes.
The famed Doppler Shift takes charge and produces a frequency swing on the signals that are reflected from the object in motion or disorder in the air.
A portion of the communicated signal is collected either directly or using motionless items through the air which is resistant to the turbulence.
After that, two or more frequencies are channelled into the amplitude demodulator. At this stage, the phase relationship is beyond regulation because the signals have varying frequencies.
When looking at the waveform diagram in Figure 2 below, envision that the upper waveform is the standard 40 kHz signal and the lower waveform is the frequency-altered signal. In the beginning, the signals are in-phase or they increase and decrease homogeneously in scale while maintaining the same polarity.
The in-phase signals are summed up inside the demodulator to generate a huge output signal. Afterwards, during the waveform sequence, they enter the anti-phase zone.
This means the signals still increase and decrease their amplitude uniformly but now have opposite polarities.
As a result, the demodulator produces a weak output signal as the two other signals cancel each other. But in the end, the signals jump back to be in-phase and release a sturdy output from the demodulator.
The moment the circuit is activated, a changing output level from the demodulator is measured.
The output signal’s frequency is the same as the variance between the double input signals.
This is normally seen on a low-audio frequency or a subsonic frequency. Without a doubt, the signal from the output is effortlessly captured after the high-gain amplifier enhances it.
Once the signal is amplified, it is used to control a standard latch circuit that once activated, the alarm continues to blare until the system is reset. The latching operation is governed by a switching transistor that links control voltage to the alarm detection circuit.
The alarm generator is built using a Voltage Controlled Oscillator (VCO) moderated by a low-frequency oscillator.
A ramp waveform is produced by the low-frequency oscillator and an output from the VCO will gradually increase in frequency until its peak pitch.
Then, the signal will revert to the minimum pitch and progressively increase in frequency again. This cyclic process continues and provides an efficient alarm signal.
How the Circuit Works
The complete circuit drawing of the ultrasonic fire detection system or the receiver, is depicted in the figure below.
The transmitter is built using a 7555 timer device, IC1. This CMOS component is the low power type of the 555 timer.
For this type of alarm generator, a 7555 is ideal compared to a 555 because the total power consumption of the circuit is maintained to only around 1mA or less, which contributes to efficient usage of battery power.
Moreover, the 7555 IC is used in a typical oscillating method whereby the timing parts R13, RV1 and C7 are selected specially to generate a frequency of 40 kHz.
The preset is regulated to generate the output frequency that delivers ideal efficiency from the receiving and transmitting circuits. The preset is identified as RV2 in the circuit schematic.
X1 is the signal-capturing sensor in the receiver circuit, and its output is connected to the input of a common emitter amplifier which is designed around Q1.
At this juncture, a low collector current of around 0.1 A is maintained to ensure the power consumption of the whole part is low.
Typically, one would think this causes less gain from an amplifier of this sort, but overall, it is more than enough for the existing operation.
Capacitor C2 combines the enhanced output from Q1 to a usual AM demodulator by employing D1, D2, R3 and C3.
Later, the consequential low-frequency signal is ramped using a second common emitter amplifier located at Q2.
Another IC1 timer is utilized as the latch. Contrary to normal practice, the timer IC1 is used in the monostable approach that provides a positive output pulse if pin 2 is reduced by 33% from the supply voltage.
Usually, the output pulse width would be regulated by a pair of timing resistor and capacitor, but this circuit is without those components.
Instead, pins 6 and 7 of IC1 are linked to the minus supply rail. When activated, the output of IC1 gets switched on and continue being in that state, allowing the latching action.
From the collector of transistor Q2, pin 2 of IC1 is connected and regulated to equal half of the supply voltage.
Thus, under standby condition, IC1 is not activated. The moment the unit is started, the collector voltage at Q2 oscillates.
Moreover, during the negative half-cycles, it becomes lower than the trigger threshold voltage. Using operating switch SW1 and the reset input of IC1 to 0V supply voltage, the complete circuit can be reset.
The component that is utilized to channel power to the alarm circuit when the IC1 is activated is transistor Q3. For safety reasons, R8 acts as a current limiting resistor.
IC2 is the last chip, which is a CMOS 4046BE phase-locked loop. However, in this design, only the VCO part is crucial. A phase comparator is aptly utilised but only as an inverter to the alarm circuit.
The inversion of the output of the VCO results in a two-phase output which allows ceramic resonator LS1 to receive a peak-to-peak voltage is two times the supply voltage.
As a result, a shrieking alarm signal is produced. If needed, the output from pin 4 of IC2 can be enhanced and utilised to energise a standard loudspeaker. Capacitor C6 and resistor R12 function as timing parts for the VCO. The electronic components provide a stable output frequency around 2kHz which is the zone where the ceramic resonator reaches peak efficiency.
The modulation signal is produced by a typical unijunction relaxation oscillator from transistor Q4. This delivers a divergent ramp waveform at 4 kHz.
How to Set Up
Begin with RV1 in the halfway point and RV2 determined for maximum output which is fully turned in the counterclockwise direction.
Using a multimeter (if available), set RV2 to its minimum DC voltage and join it across R3 as the negative probe is attached to the negative supply line.
Turn on the power of the unit and place the transducers facing a wall or any smooth surface with around 10 or 20 cm away.
When RV1 is actuated, there will be reading or movement on the multimeter, and then RV1 is attuned to reach the maximum reading possible.
It is highly recommended to fix a conductor across SW1 when the regulation is done because the alarm generator is silenced, and its output cannot affect the measurements.
In the event a multimeter is unavailable, RV1 can be tuned by employing the trial and error approach to discover a value that works for the whole part.
Although RV2 is well protected, the alarm unit is still sensitive. The mounting location must be well planned for the unit. A good spot would be slightly above the operator’s workbench where the highest risk of fire is present because of the electrical tools and soldering materials.
Another advantage of placing the unit higher is because hot air will rise and makes it easier to trigger the alarm without the risks of false signals created by people running about the room.
With a few trials, a suitable position without the consequence from human factors and stable sensitivity can be achieved for the fire alarm generator.
To test the effectiveness of the unit’s position, a working soldering iron is placed under and in front of the component.
When adequate turbulent air is produced, it should activate the alarm. At switch on, the circuit ill be energised but this can be immediately negated by placing the SW1 on reset.
The ultrasonic fire alarm circuit is not designed with on-delay switch but your presence behind the unit must be ensured when operating SW1. There is no risk if you remove your hand after engaging the switch.
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