Proximity Detectors

Air Flow Detector Circuit

Air flow detector is frequently required in various applications or systems where the presence of air must be detected in order to provide a descriptive image of the systems’ proper operation. For example, we require air flow detection in engines to estimate the amount of fuel to add to the engine, and we need air flow detection in chemical media like air to assess the amount of contamination or contamination transmission. We require air flow sensing for high power density electronic devices to prevent them from overheating.


  • Principle Behind Air Flow Detector Circuit:
  • Air Flow Detector Circuit Diagram:
    • Air Flow Detection Circuit Design:
    • Air Flow Detector Circuit Operation:
    • Theory Behind Air Flow Detector Circuit:
    • Applications of Air Flow Detector Circuit:
    • Limitations of Air Flow Detector Circuit:

Principle Behind Air Flow Detector Circuit:

Air Flow Detector Circuit Diagram:

Air Flow Detector

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 intended to deliver a steady current input to the RTD, causing it to be slightly heated at first. The RTD chosen for this application is the HEL-700 platinum RTD, which has a maximum operating current of 2 mA. To supply a constant current to the RTD, we’re utilising a Zener Diode as a voltage regulator.

We must first choose a Zener diode before designing a Zener voltage regulator. A Zener diode with Vz = 4.7V is chosen here. We choose a load resistance of 100 Ohms because the input voltage is 12V and the desired output current is 2mA. This ensures that the maximum current travels through the load and just a tiny portion through the RTD.The input resistance selected is given by (Vin-Vz)/(Iz+IL) and is equal to 38 Ohms. Here a 38 Ohms resistor is used as the input resistor.

The creation of a timer monostable multi vibrator is the next phase. The timer is employed to supply a biassing voltage of roughly 5V to the LED. As the voltage across the RTD falls, the LED must illuminate. We’ll use a 10K resistor and a 1uF electrolytic capacitor in this example. The control pin is connected to ground via a 0.01uF ceramic capacitor.

Air Flow Detector Circuit Operation:

A 12V battery is used to power this circuit. The Zener diode, which creates a constant voltage, is used to manage the battery voltage. As current passes through the RTD, it heats up, increasing its temperature and hence increasing its resistance. As the current remains constant, the voltage across the resistance rises. When this voltage is connected to the timer’s trigger pin, the timer does not trigger, and the LED is turned off. As air passes over the RTD, it begins to cool. 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:

A zener diode is a basic PN junction diode that operates in reverse bias. It is based on the breakdown principle – Avalanche and Zener. At a reverse bias voltage of 2V to 8V, zener breakdown occurs when the electrons break free from the atoms and create free electron hole pairs. Above 8V, avalanche breakdown happens when high-speed charge carriers collide, disrupting the covalent bond and resulting in the production 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, often known as an RTD, is a metal resistor whose resistance changes in response to temperature. It is based on the fact that the lattice vibrations in metals increase as the temperature rises. The electrons collide as a result of these oscillations. As the number of collisions rises, the energy of the electrons decreases, reducing the flow of free electrons and resulting in low conductivity. As a result, as the temperature rises, the resistance rises as well. Platinum is primarily used in the construction of RTDs. An RTD’s resistance is roughly 100 Ohms at 0 degrees Celsius.

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 air movement in regions such as automotive engines, where the amount of gasoline required by the engine must be estimated. This circuit can also be used as a temperature detector in addition to being utilised as an air flow detector. This circuit can be used to control loads such as a fan using temperature sensing with minor adjustments.

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.


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