Proximity Detectors

Air Flow Detector Circuit

Air flow detector finds frequent application in various systems and scenarios where it’s essential to ascertain the presence of air for a comprehensive understanding of the system’s correct functioning. For instance, in engines, airflow detection is essential to gauge the necessary fuel input for optimal performance. Similarly, in chemical environments such as air, airflow detection is crucial for assessing contamination levels or the transfer of contaminants. Additionally, airflow sensing is indispensable in high-power-density electronic devices to prevent 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:

The purpose of this circuit is to provide a constant current input to the RTD (Resistance Temperature Detector), initially causing it to undergo a slight heating process. In this application, we have chosen the HEL-700 platinum RTD, which operates with a maximum current of 2 mA. To maintain a stable current supply for the RTD, we have opted to employ a Zener Diode as a voltage regulator.

Before designing a Zener voltage regulator, it is crucial to select an appropriate Zener diode. In this case, we have selected a Zener diode with a Vz (Zener voltage) of 4.7V. To accommodate an input voltage of 12V and achieve a desired output current of 2mA, we have chosen a load resistance of 100 Ohms. This configuration ensures that the majority of the current flows through the load, with only a small portion passing through the RTD. The selected input resistance is determined by the formula (Vin – Vz) / (Iz + IL), and it equals 38 Ohms. Consequently, we have employed a 38 Ohm resistor as the input resistor.

The next step involves creating a timer monostable multivibrator. The timer’s purpose is to provide a biasing voltage of approximately 5V to the LED. As the voltage across the RTD decreases, the LED is expected to illuminate. For this example, we have utilized a 10K resistor and a 1uF electrolytic capacitor. The control pin is connected to ground through a 0.01uF ceramic capacitor.

Air Flow Detector Circuit Operation:

The circuit is powered by a 12V battery, and it employs a Zener diode to establish a stable voltage level from the battery’s supply. As current flows through the RTD, it undergoes heating, causing an increase in temperature and consequently a rise in its resistance. While the current remains constant, this resistance elevation results in a higher voltage drop across the RTD. When this voltage is applied to the trigger pin of the timer, the timer does not activate, and the LED remains off.

When air flows over the RTD, it initiates a cooling process, leading to a reduction in its temperature. As the device’s temperature decreases, its resistance decreases as well, leading to a decline in the voltage drop across it. This voltage falls below a certain threshold, the timer is triggered, and the LED commences blinking. As the voltage continues to decrease, indicating a further drop in temperature, the LED brightens to its maximum intensity. This illumination signifies the presence of airflow.

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 fundamental PN junction diode designed to operate in reverse bias. Its operation is based on the breakdown principle, specifically Avalanche and Zener breakdown. Zener breakdown occurs when the diode is subjected to a reverse bias voltage ranging from 2V to 8V, causing electrons to break free from their atomic bonds and generate free electron-hole pairs. On the other hand, above 8V, avalanche breakdown takes place when high-speed charge carriers collide, disrupting covalent bonds and leading to the creation of additional free electrons.

The diode’s characteristics reveal an intriguing property: even with significant variations in current passing through the diode, the voltage across it remains quite small and consistent. This distinctive characteristic is harnessed in numerous applications, where Zener diodes are employed as voltage regulators.

Resistance Temperature Detector:

A resistance temperature detector, commonly referred to as an RTD, is a metallic resistor that exhibits variations in resistance as a reaction to temperature changes. This phenomenon is rooted in the observation that lattice vibrations within metals intensify with increasing temperatures. These vibrations lead to electron collisions, causing a reduction in the energy of electrons. Consequently, the flow of free electrons diminishes, resulting in decreased conductivity. Thus, as temperature increases, the resistance of the RTD also increases. Platinum is the primary material used in constructing RTDs, and at 0 degrees Celsius, the resistance of an RTD is approximately 100 Ohms.

555 Timer Multivibrator:

The multivibrator circuit serves the purpose of generating a pulsed output signal, which is initiated when a low-level signal is introduced to the trigger pin of the IC. The 555 timer IC, consisting of 8 pins, determines the timing of the output signal, and this timing can be expressed as T=1.1 RC. For a more comprehensive understanding of the 555 timer IC, you can refer to the article titled “Understanding 555 Timer.”

Applications of Air Flow Detector Circuit:

This circuit finds applications in various scenarios, including automotive engines where it’s employed to estimate the required fuel quantity. Additionally, apart from serving as an air flow detector, it can function as a temperature sensor. With minor modifications, this circuit can also be adapted for temperature-based load control, such as operating a fan.

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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