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Sensor Circuits: A Complete Guide

Sensor circuits are ubiquitous. Unlike the human eyes, optical sensors have a broader visual spectrum of infrared and ultraviolet. Many applications depend on the use of sensor circuits. Notwithstanding, there are several different kinds of sensors. They range from ultrasonic transducers to gas sensors. Others are commercial sensors that are important in other fields of life. For example, we have radiation detectors, flux, magnetic compass sensors, radio, and microwave. Next, we cover everything you need to know about sensor circuits.  

(A sensor circuit designed to buzz an alarm when it detects rain).

1. What is a Sensor Circuit?

Firstly, a sensor is an electronic device that delivers output from sensing changes in events or quantities. So, sensor devices that convert various inputs into electrical or optical work are sensor circuits. By default, sensors measure changes in their environment. To do this, they generate signals representing these changes. Overall, sensors come in various forms. They include thermometers, barometric pressure gauges, gas detectors, etc.   Most home security systems use Sensor circuits

(Most home security systems use Sensor circuits)


2. The Different Types of Sensors 

  Many different kinds of sensors are used in other fields for varied purposes. The two major groups are
  • Analog and
  • Digital sensors.
Analog sensors, on the one hand, include temperature, humidity, pressure, light sensors, etc. Digital sensors, on the other hand, comprise cameras, microphones, GPS devices, accelerometer sensors, gyroscopes, compasses, and rotary sensors. Furthermore, the most common types of sensor technology available are infrared (IR) sensors, ultrasonic sensors, thermal sensors, pressure sensors, and proximity sensors. These devices are essential parts of consumer electronic products. Therefore, we go on to introduce some of these sensor subtypes.  

Temperature Sensors 

  This type of sensor uses a small transistor circuit to switch loads ON or OFF. Albeit, the loads have to reach specific temperatures first. By standard, the temperature sensor circuit consists of a few electronic components. They include a 9V battery, a temperature sensor (thermistor), two NPN bipolar junction transistor circuits, and an electromagnetic switch.

(A temperature and humidity sensor with an LED to indicate).


IR Sensors

  There are tiny photo chips in some sensor circuits that use photovoltaic cells. These photovoltaic cells detect and emit infrared light. In effect, they group up to form what we term IR sensors.  

(Example of IR sensor circuit used in line followers).

  One of the simplest IR sensor circuits is the TV remote control in everyday use. This type of sensor circuit comprises both an IR emitter and an IR receiver.  

Ultrasonic Sensors

  Also called transceivers, these devices use radar or sonar principles. In other words, they use transducers as a sensor interface. In turn, helps to estimate and interpret a target's attributes.  
Source: (An ultrasonic sensor with a ranging accuracy reaching up to 3mm).   Ultrasonic sensors are of the active type and the passive one. In general, though, an ultrasonic sensor module has a control circuit, a transmitter, and a receiver.  

Touch Sensors

  By default, touch sensors operate like switches. They trigger on or off when they sense touch. However, touch sensors are of different kinds. They include capacitance touch switch, piezo touch switch, and resistance touch switch. You need only a few essential components in a simple touch sensor circuit. These components include a 555 timer in monostable mode operation, touch sensor, battery, and LED.  

(A capacitive touch sensor watch also shows traces of the touchpad).


Proximity Sensors

  The application of proximity sensors is simple. They detect the presence or non-presence of objects and object properties around them. Hence, a proximity sensor detects signals and converts them into user readable forms.  

(The optical proximity sensor in a smartphone).


Proximity sensors types

  There are several types of these sensors in existence. To mention a few, they include:
  • Chemical sensors.
  • Thermal sensors.
  • Photoelectric sensors.
  • Inductive sensors.
  • Capacitive sensors.
Under chemical sensors, however, we have:
  • Fluorescent chloride type.
  • Chemical FET.
  • Electrochemical gas type.
  • Non-dispersive IR.
  • Chemi resistor.
  • Zinc oxide nanorod.
  • pH glass electrode type.
Other types include humidity sensors, acceleration sensors, sound sensors, light sensors, tactile sensors, force sensors, gas sensors, active sensors, and passive sensors.  

3. How Does a Sensor Circuit Work?

  A sensor works by converting external stimuli received into electrical pulses. Such stimuli could be light, sound, heat, or motion. These electrical signals or pulses process it into binary codes via an interface. As a result, the computer can now read and interpret the action or inaction sensed earlier. The output of the sensor is either in binary or analog/digital. Either way, sensing circuits work on the basic principles of motion sensors. For example, output signals from a light sensing circuit indicate the light intensity. In the same way, the sensing circuit uses photocells to measure the minute energy present in a narrow range of frequencies. This range of frequencies covers the infrared, visible, and ultraviolet light spectrum.  

4. How to Design Various Sensor Circuits

  This section explores the designing process of sensor circuits using simple ICs. Here, we design capacitive proximity sensors using IC 555 and IC 741. Also, we explore using a more accurate precision IC PCF8883.  

1. Sensor Circuit with IC 555

  It is a practical capacitive proximity sensor circuit, in general. In this case, however, we consider a darkness detector sensor circuit using IC 555.

(IC 555 pinout image)


How to build a capacitive sensor circuit using IC 555

  You wire IC1 in an astable configuration without actually installing a capacitor. Instead, you introduce a capacitive plate. Preferably, use a larger capacitive scale for better response. After installing the capacitive proximity sensor plate, you introduce the IC 555. Although, set it in standby mode. In some cases, you adjust P1 and P2 for increased capacitive plate sensitivity. Also, introducing a flip-flop circuit helps obtain a latching action for accuracy and responsiveness. (This video tutorial takes you through the building process of a sensor circuit using IC 555)  

2. Sensor Circuit with IC 741

  We also consider another IC that detects darkness. In this case, we use the UA741 Op-Amp with the capacitive sensor plate.  

How to create a capacitive sensor circuit using IC 741

  First, you install Q1 as a high impedance input. We use a regular FET transistor such as 2N3819, for example. Then, you install the IC 741 op-amp to function as a voltage level switch. This IC drives Q2, a PNP bipolar transistor of medium current. To activate the relay, we use Q2 as a current buffer. As a result, it triggers an alarm or buzzer.
(Here is a video that answers a few questions concerning this sensor circuit)  

3. Sensor circuit using IC PCF8883

  The following proximity detector circuit uses IC PCF8883. This IC has a very high sensor accuracy. In other words, it senses the slightest changes in capacitance around the capacitive body.  

The design of the capacitive sensor circuit

  Unlike the others, the circuit senses changes in static capacitance. Hence, it uses an automatic correction system. During installation, you connect it to the accurate pinouts of the IC. These particular capacitive sensor circuit designs often come in little conductive foil packages.  
Source: Wikipedia.

(Circuit diagram for a sensor circuit designed with IC PCF8883)

  Although this is optional, a coaxial cable works for accuracy in remote capacitive proximity sensors.  


  In summary, sensor circuits make up an essential part of our everyday life. These advanced sensors extend the human senses in terms of sight, sound, heat, etc. In practical application, they are useful in security systems and electronic systems, in general. So, you have learned all about sensing circuits and how to build them. But it would be best if you still had some further guidance to accomplish it. Feel free to contact our team of experts.