Sensor and Converter

Simple independent electronic circuits can be made to turn on a light repeatedly or play a note.

However, in order for an electronic circuit or system to perform any useful task or function, it must be able to communicate with the "real world" whether by reading an input signal from an "ON/OFF" switch or by activating a form.

In other words, an Electronic System or circuit must have the ability to "do" something, and Sensors and Converters are the perfect components to do so.

The word "converter" is a common term for sensors and actuators that can be used for switching to detect a wide range of different forms of energy, such as motion, electrical signals, radian energy, thermal or magnetic energy, etc.

There are many different types of sensors and converters, both analogue and digital, and input and output to choose from.The type of input or output converter used depends on the type of signal or process that is really "Detected" or "Controlled", but we can define one sensor and converter as devices that convert one physical quantity into another.

Devices that perform an "input" function are often called sensors, since they "detect" a physical change in some properties that vary in response to some stimulation, for example, heat or force, and hide it in an electrical signal. Devices that perform the "output" function are often called actuators and are used to control some external devices, such as motion or sound.

Electrical converters are used to convert one type of energy into another type of energy, so for example, a microphone (input device) converts sound waves into electrical signals for amplifier amplification (a process), and a speaker (output device) returns these electrical signals to sound waves, and an example of this type of simple Input/Output (I/O) system is given below.

Simple Input/Output System Using Audio Converters

There are many different types of sensors and converters on the market, and which one to use really depends on the measured or controlled amount, and the following table provides the more common types:

Sensors and Transducers


Measured Quantity
Input Device
(Sensor)
Output Device
(Actuator)
Light LevelLight-Dependent Resistance (LDR)
Photodiode
Photo-transistor
Solar Cell
Lights and Lamps
LEDs and Displays
Fiber Optics
TemperatureThermocupor
Thermistor
Thermostat
Resistant Temperature Detectors
Heating
Fan
Force/PressureStrain Indicator
Pressure Switch
Load Cells
Elevators and Jacks
Electromagnet
Vibration
LocationPonsiometer
Encoders
Reflector/Grooved Opto-switch
LVDT
Motor
Solenoid
Panel Meter
SpeedTaco-generator
Reflector/Grooved Opto-defragmenter
Doppler Impact Sensors
AC and DC Motors
StepPer
Brake
SoundCarbon Microphone
Piezo-electric Crystal

Ring-Voice
Speaker

Input converters or sensors produce a voltage or signal output response proportional to the change in the amount (stimulus) they measure.The type or amount of the output signal depends on the type of sensor used.But in general, all types of sensors can be classified as two types: Passive Sensors or Active Sensors.

In general, for active sensors to work, an external power supply called an exaspulation signal used by the sensor to generate the output signal is required.Active sensors are self-generating devices, since their properties change in response to an external effect that produces an output current, for example, 1 to 10v DC output voltage or 4 to 20mA DC.Active sensors can also produce signal amplification.

A good example of an active sensor is an LVDT sensor or a strain gauge.Strain gauges are pressure-sensitive resistant bridge networks that are externally polarized (warning signal) to produce an output voltage in proportion to the amount of force and/or voltage applied to the sensor.

Unlike an active sensor, a passive sensor does not need any additional power supply or stimulation voltage.Instead, a passive sensor generates an output signal in response to some external stimuli.For example, it is a thermocouplç that produces its own voltage output when exposed to heat.Passive sensors are direct sensors that change their physical properties, such as resistance, capacitance or inductancy.

However, in addition to analog sensors, Digital Sensorsproduce a separate output that represents a binary number or number, such as the logic level "0" or the logic level "1".

Analogue and Digital Sensors

Analog Sensors

Analog Sensorsproduce a continuous output signal or voltage, which is usually proportional to the measured amount.Physical quantities such as Temperature, Speed, Pressure, Displacement, Strain, etc. are analogue quantities as they tend to be continuous in nature.For example, the temperature of a liquid can be measured using a thermometer or thermocouple, which continuously responds to temperature changes when the liquid is heated or cooled.

Projects with Analog Sensors

LED-Burning Circuit in the Dark (LDR)

Thermocouple used to produce analog signals

Analog sensors tend to produce smooth and ever-changing output signals over time.These signals tend to be of very small value from a few micro volts (uVs) to several miles of volts (mV), so some kind of amplification is required.

Circuits that then measure analog signals usually react slowly and/or have low accuracy.In addition, analog signals can be easily converted to digital signals for use in microcontroller systems using analogue-to-digital converters or ADCs.

Digital Sensors

As the name suggests, Digital Sensorsproduce a separate digital output signal or voltage, which is a digital representation of the measured amount.Digital sensors produce a binary output signal in the form of "1" or "0" ("ON" or "OFF").This means that a digital signal produces discrete (non-continuous) values that can only be output as a single "bit" (serial transmission) or by combining bits to produce a single "byte" output (parallel transmission).

Digital Sensor Projects

PIR Motion Sensor

Light Sensor Used to Generate Digital Signals

In our simple example above, the speed of the rotating shaft is measured using a digital LED/Opto-detector sensor.Fixed to a rotating shaft (e.g. from an engine or robot wheels), the disc has a series of transparent slots in its design.As the disk rotates with the speed of the shaft, each slot passes through the sensor, in turn, producing an output pulse that represents the level of logic "1" or logic "0".

These pulses are sent to a meter record and finally to an output screen to show the speed or revolutions of the shaft.By increasing the number of slots or "windows" within the disk, more output pulses can be produced for each rotation of the spindle.The advantage of this is that a larger resolution and accuracy is achieved, since the fractions of an era can be detected.This type of sensor arrangement can then also be used for position control with one of the disk slots representing a reference position.

Compared to analog signals, digital signals or quantities have very high accuracy and can be both measured and "sampled" at a very high clock speed.The accuracy of the digital signal is proportional to the number of bits used to represent the measured amount.For example, using an 8-bit processor will produce 0.390% accuracy (at 1 piece 256).When using a 16-bit processor, it provides 0.0015% accuracy, 1 part or 260 times more accuracy at 65,536.This accuracy can be maintained millions of times faster than analog signals, as digital quantities are manipulated and processed very quickly.

In most cases, sensors and, more specifically, analog sensors often need an external power source and some kind of additional amplification or filtering of the signal to produce a suitable electrical signal that can be measured or used.A very good way to achieve both amplification and filtration in a single circuit is to use Operational Amplifiers, as seen before.

Signal Conditioning of Sensors

As we have seen in the Operational Amplifier training, op-amps can be used to amplify signals when connected in configurations that do not invert or invert.

Very small analog signal voltages produced by a sensor such as several milli-volts or even pico-volts can be repeatedly amplified with a simple op-amp circuit to produce a much larger voltage signal, such as 5v or 5mA, for example. It can be used as an input signal to a microprocessor or analogue to digital-based system.

Therefore, to provide any useful signal, a sensor output signal must be upgraded with an amplifier with up to 10,000 voltage gains and up to 1,000,000 current gain, together with linear amplification of the signal and complete reproduction of the output signal.

Then amplification is part of signal conditioning.Therefore, when using analog sensors, a type of amplification (Gain), impedance matching, isolation between input and output, or perhaps filtering (frequency selection) may be required before the signal is used, and this is carried out appropriately by Operational Amplifiers.

Also, when measuring very small physical changes, the output signal of a sensor can be "contaminated" with unwanted signals or voltages that prevent the correct measurement of the required actual signal.These unwanted signals are called "Noise".This noise or interference can be greatly reduced or even eliminated using signal conditioning or filtration techniques, as we discussed in active filter training.

Using a Low Pass or High Pass or even Band Switch filter, the "bandwidth" of noise can be reduced only to leave the required output signal.For example, many input types from switches, keyboards or manual controls are not capable of quickly changing the state, and therefore a low-pass filter can be used.

Op-amp Filter Examples

If some random noises still remain after filtering, it may be necessary to take several samples and then average them to give the final value to increase the signal-to-noise ratio.In both cases, both amplification and filtration play an important role in the interface of both sensors and transducers to microprocessor and electronically based systems in "real world" conditions.

In the next tutorial about sensors, we will look at Positional Sensors that measure the position and/or displacement of physical objects, which means movement from one location to another for a specific distance or angle.