Air Flow Detector Circuit

Air flow detection is often necessary in many applications or systems where it is necessary to detect the presence of air to have a descriptive picture of the proper functioning of the systems. For example, we need air flow detection in engines to get an estimate about the amount of fuel to be added to the engine, we need air flow detection to check the amount of contamination or the transfer of contamination using chemical media like air. For high power density electronic devices, we need air flow detection to ensure the devices from getting over heated.

Principle Behind Air Flow Detector Circuit:

Here, a simple air flow detector circuit is developed which uses a resistance temperature detector as the basic component. This circuit is based on two principles – a) Variation of resistance with temperature, b) Air as an insulator.  As current flows through the resistor, it gets heated up. Now when air is made to flow through the RTD, it being an insulator, allows the resistor to cool down. Thus resistance starts decreasing and the voltage across the RTD decreases. This variation in voltage drop is detected using a timer circuit to give an indication of the air flow.

Air Flow Detector Circuit Diagram:

Air Flow Detector Circuit Diagram – ElectronicsHub.Org

Circuit Components:

• V1 = 12 V
• R1 = 38 Ohms
• D1= 4.7 V Zener diode, 1N4732
• R2 = 100 Ohms
• Rx = HEL-700 platinum RTD
• R3 = 10K
• C2 = 1uF
• C1= 0.01 uF
• LED = 5V, Green LED
• IC = 555 Timer

Air Flow Detection Circuit Design:

This circuit is designed to provide a constant current input to the RTD so that it is heated slightly at the beginning. The RTD selected here is HEL-700 platinum RTD, which works at a maximum operating current of 2 mA. Here we are using a Zener Diode as a voltage regulator to provide a constant current to the RTD.

To design a Zener voltage regulator, we need to first select the Zener diode. Here a Zener diode with Vz = 4.7V is selected. Since input voltage is 12V and required output current is 2mA, we select a load resistance of 100 Ohms, so that maximum current flows through the load and only a small amount of current flows through the RTD. The input resistance selected is given by (Vin-Vz)/(Iz+I_L) and is equal to 38 Ohms. Here a 38 Ohms resistor is used as the input resistor.

The next step requires design of timer monostable multi vibrator. Here the timer is used to provide biasing voltage to the LED, which is about 5V. The LED is required to glow as the voltage across the RTD decreases. Here we select a resistor of 10K and an electrolyte capacitor of 1uF.  A ceramic capacitor of 0.01uF is used to connect the control pin to ground.

Air Flow Detector Circuit Operation:

This circuit is operated using a 12V battery. The battery voltage is regulated using the Zener diode, which produces a constant voltage. Initially as current flows through the RTD, it gets heated up and its temperature increases, thereby increasing its resistance.  Now as current is constant, the voltage across the resistance also tends to increase.  When this voltage is applied to the trigger pin of the timer, it fails to trigger the timer and the LED is in off condition. Now as air flows over the RTD, it starts cooling. This reduces the temperature of the device. As the temperature reduces, the resistance also reduces and so does the voltage across the device. As this voltage reduces below a certain point, the timer gets triggered and the LED starts blinking. As voltage falls further, indicating fall of temperature, the LED starts glowing with full intensity. This indicates the flow the air.

Theory Behind Air Flow Detector Circuit:

The basic theory behind this circuit involves knowledge about three basic parts- Voltage Regulator using Zener Diode, Resistance Temperature detector and a timer circuit.

Voltage Regulator using Zener Diode:

Zener diode is a simple PN junction diode operated in reverse bias condition. It basically works on the principle of breakdown – Avalanche and Zener. Zener breakdown occurs at a reverse bias voltage between 2V to 8V, when highly strong electric field intensity causes the electrons to break free from the atoms and form free electron hole pairs. The avalanche breakdown occurs above 8V, when high speed charge carriers cause disrupt of covalent bond due to collision, leading to formation of free electrons.

As can be seen by the characteristics, for a large variation in current through the diode, the voltage across the diode remains very small or constant. This unique feature is utilized in many applications by using Zener diode as the voltage regulator.

Resistance Temperature Detector:

A resistance temperature detector or RTD is a metal resistor whose resistance changes with temperature. It is based on the fact that in metals, as temperature increases, the lattice vibrations increases. These vibrations cause collision among the electrons. As collisions increase, the energy of the electrons decrease, causing a decrease in the flow of free electrons, leading to low conductivity. Thus, with increase in temperature, the resistance increases. An RTD is constructed basically using platinum. At 0 degree Celsius, resistance of an RTD is about 100 Ohms.

555 Timer Multivibrator:

Multivibrator circuit is used to produce pulsed output signal. It is triggered when a low level signal is applied to the trigger pin of the IC. The 555 timer IC is an 8 pin IC and the timing of the output signal is given by T=1.1 RC. To get detailed information about 555 timer IC, read the post Understanding 555 Timer

Applications of Air Flow Detector Circuit:

This circuit can be used to detect the flow of air in areas like car engine, where it is required to estimate the amount of fuel needed by the engine. Apart from being used as an air flow detector, this circuit can also find its application as a temperature detector circuit. With slight modifications, this circuit can be used to control loads like a fan, based on temperature sensing.

Limitations of Air Flow Detector Circuit:

1. Since Zener diode is being used, the efficiency of the circuit is affected. This is because loss in series resistor causes a decline in efficiency in case of heavy loads.
2. The resistance temperature detector used is expensive and easily affected by shock and vibration.