Simple Proximity Sensor Circuit and Working with Applications

In our day-to-day life, we got used to implement different different types of sensor circuits using various types of sensors such as IR sensors, temperature sensor, pressure sensor, PIR sensor, and so on. Frequently, we observe PIR sensor circuit based automatic door opening system, LDR sensor circuit based automatic street light system, piezoelectric sensor circuit based power generation system, IR sensor circuit based traffic signal system, ultrasonic sensor circuit based obstacle detection system, and so on.

Here, in this article, let us discuss about the simple proximity sensor circuit and working. But, before discussing in detail about the proximity sensors, primarily, we must know what actually the proximity sensor means?

Proximity Sensor

A sensor that can be used for detecting the presence of objects surrounding it without having any physical contact is termed as a proximity sensor. This can be done using the electromagnetic field or electromagnetic radiation beam in which the field or return signal changes in the event of the presence of any object in its surrounding. This object sensed by the proximity sensor is termed as a target.

Thus, if we discuss about different types of targets such as plastic target, metal target, and so on requires different types of proximity sensors such as capacitive proximity sensor or photoelectric proximity sensor, inductive proximity sensor, magnetic proximity sensor and so on. The range in which the proximity sensor is able to detect an object is termed as nominal range. Unlike the other sensors, proximity sensors can last for long life and have very high reliability as there are no mechanical parts as well as no physical contact exists between the sensor and sensed object.

Proximity Sensor Circuit Diagram

Let us discuss about the inductive proximity sensor circuit which is most frequently used in many applications. The proximity sensor circuit diagram is shown in the above figure which consists of different blocks such as oscillator block, electrical induction coil, power supply, voltage regulator, etc.

Proximity Sensor Working Principle

The inductive proximity sensor circuit is used for detecting the metal objects and the circuit doesn’t detect any objects other than metals. The above proximity sensor circuit diagram represents the field produced by the coil, which is generated by providing a power supply. Whenever, this field is disturbed by detecting any metal object (as a metal object enters this field), then an eddy current will be generated that circulates within the target.

Due to this, load will be caused on the sensor that decreases the electromagnetic field amplitude. If the metal object (called as target, as we discussed earlier in this article) is moved towards the proximity sensor, then the eddy current will increase accordingly. Thus, the load on the oscillator will increase, which decreases the field amplitude.

The trigger block in the proximity sensor circuit is used to monitor the amplitude of the oscillator and at particular levels (predetermined levels) the trigger circuit switches on or off the sensor (which is in its normal condition). If the metal object or target is moved away from the proximity sensor, then the amplitude of the oscillator will increase.

The waveform of for the inductive proximity sensor oscillator in the presence of the target and in the absence of the target can be represented as shown in the above figure.

Proximity Sensor Circuit Operating Voltages

Nowadays, inductive proximity sensors are available with different operating voltages. These inductive proximity sensors are available in AC, DC, and AC/DC modes (universal modes). The operating range of the proximity sensor circuits is from 10V to 320V DC and 20V to 265V AC.

Proximity Sensor Circuit Wiring

The proximity sensor circuit wiring is done as shown in the below figure. Depending on the transistor condition based on the absence of target, proximity sensor outputs are considered as NC (normally closed) or NO (normally open).

If PNP output is low or off while the target is absent, then we can consider the device as normally opened. Similarly, if the PNP output is high or on while the target is absent, then we can consider the device as normally closed.

Proximity Sensor Circuit-Target Size

A flat and smooth surface with a thickness of 1mm and made of mild steel  can be considered as standard target. There are various grades in which steel is available and mild steel is made of composition of carbon and iron (higher content). The standard target with shielded sensors will be having sides that are equal to the diameter of sensing face. The sides of the target with unshielded sensors is equal to a greater one among the two, i.e., the diameter of the sensing face or thrice the rated operating range.

Even though, the size of the target is greater than the standard target, there will be no change in the sensing range. But, if the size of the target becomes less than the standard target or irregular, then the sensing distance will decrease. Thus, we can say that, as small as the size of the target, then the target must be moved closer to the sensing face to get detected.

Proximity Sensor Circuit Applications

The proximity sensor circuit can be used for different applications, a few proximity sensor circuit applications are described below:

A simple metal detector can be designed using proximity sensor, buzzer, and LC circuit (inductor connected in parallel with capacitor), which are connected as shown in the above circuit diagram. This circuit will make the LED to glow and buzzer to sound whenever it detects metal objects or targets.

These proximity sensor circuit is frequently used in mobile phones (smart phones or touch screen phones) which we use in our day-to-day life. If this sensor is made to move near to the ear or made to fall shadow or touched, then the display light of mobile gets turned off, such that it avoids the mobile screen touch (avoids screen contact with the face or fingers) during calls (based on requirement). The touch sensitive switches can be implemented using proximity sensor circuits and proximity sensor circuit can be used to design metal detector robotic projects.

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