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Fish Finders – Description of a fish finder

If you would like to know where the fish are, you can use a fishfinder. This is the most commonly used term but in Australia, they call it “siren”.

To locate the fish, the device uses reflected sound detection as in sonar. The latest models display the reflected sound on a graphical representation. This gives the operator a lot of information about locating schools of fish, underwater debris, etc … The devices are used in both recreational and professional fishing. The fishfinders are also provided to work with the marine radar, compass, and GPS navigation systems.

The whole concept of a fishfinder is based on the earlier “fathometers”. These were active sonar instruments that could determine navigation and safety in the water. “Fathometer” is a word that gets its name from the word “fathom” and is used to determine the water depth like a sonar. A real-time display can be very important here. The fish finders also work in the same way and so they can also be used to detect fish. The techniques of both devices were thus combined.

Fish Finders – How does a fish finder work?

When the device is active, an electrical pulse from a transmitter is converted into a sound wave by an underwater transducer. This transducer is a device that converts energy (that electrical pulse) into another form. Those other forms are position, movement, energy, power, light, torque,… That is why they are often used at borders to analyze the automation systems and the measurement and control systems. The process of converting energy into another form is called transduction.

Fish Finders – Types of fish finders

There are two types of transducers: the mechanical and the electrical transducers. The mechanical devices convert the physical quantities into mechanical quantities and the electrical devices convert the physical quantities into electrical quantities. Examples include a thermocouple that converts temperature differences to a small voltage, or a “linear variable differential transformer” (LVDT) that is used to measure displacement.

Fish Finders – Sensors and actuators

Transducers can be categorized by the direction in which information passes through them:

A sensor is a transducer that receives and responds to a signal or stimulus from a physical system. It produces a signal that represents information about the system, which is used by some type of telemetry, information, or control system.

An actuator is a device responsible for moving or controlling a mechanism or system. It is controlled by a signal from a control system or manual control. It is powered by an energy source, which can be a mechanical force, electrical power, hydraulic fluid pressure, or pneumatic pressure, and converts that energy into motion. An actuator is a mechanism by which a control system interacts with an environment. The control system can be simple (a fixed mechanical or electronic system), software-based (e.g. a printer driver, robot operating system), a human, or some other input.

Bidirectional transducers convert physical phenomena into electrical signals and also convert electrical signals into physical phenomena. An example of an inherently bidirectional transducer is an antenna that can convert radio waves (electromagnetic waves) into electrical signals that can be processed by a radio receiver, or translate an electrical signal from a transmitter into radio waves. Another example is voice coils, which are used in speakers to convert an electrical audio signal into sound and in dynamic microphones to convert sound waves into an audio signal.

Fish Finders – Active versus passive sensors

Passive sensors require an external power source to operate, this is called an excitation signal. The signal is modulated by the sensor to produce an output signal. For example, a thermistor does not generate an electrical signal, but by passing an electrical current through it, resistance can be measured by detecting variations in the current or voltage across the thermistor.

Active sensors, on the other hand, generate an electrical current in response to an external stimulus that serves as an output signal without the need for an additional energy source. Such examples are a photodiode and a piezoelectric sensor, thermocouple.

Fish Finders – Characteristics

Some specifications used to rate transducers are:

  • Dynamic range: This is the ratio between the signal with the greatest amplitude and the signal with the smallest amplitude that the transducer can effectively translate. Transducers with a wider dynamic range are “more sensitive” and more accurate.
  • Repeatability: This is the ability of the transducer to produce an identical output when stimulated by the same input.
  • Noise: All transducers add some random noise to their output. In electrical transducers, this can be electrical noise due to the thermal movement of charges in circuits. Noise corrupts small signals more than large ones.
  • Hysteresis: This is a property where the output of the transducer depends not only on the current input but also on the input from the past. For example, an actuator using a gear train may have some backlash, which means that if the actuator’s direction of motion reverses, there will be a dead zone before the actuator output reverses, caused by backlash between the gear teeth.
  • Electromagnetic:
  • Antennas – converts propagating electromagnetic waves to and from conducted electrical signals
  • magnetic cartridges – converts relative physical movement to and from electrical signals
  • Tape head, disc read, and write heads – converts magnetic fields on a magnetic medium to and from electrical signals
  • Hall effect sensors – convert a magnetic field level into an electrical signal
  • Electrochemical:
  • pH probes
  • Electro-galvanic oxygen sensors
  • Hydrogen sensors
  • Electromechanical (electromechanical output devices are generally referred to as actuators):
  • Accelerometers
  • Airflow sensors
  • Electroactive polymers
  • Rotary motors, linear motors
  • Galvanometers
  • Linear variable differential transformers or rotary variable differential transformers
  • Load cells – Convert power to mV / V electrical signal using strain gauges
  • Micro-electromechanical systems
  • Potentiometers (if used for measuring position)
  • Pressure sensors
  • String potentiometers
  • Tactile sensors
  • Vibration-driven generators
  • Vibrating structure gyroscopes
  • Electro-acoustic:
  • Speakers, earphones – converts electrical signals into sound (amplified signal → magnetic field → movement → air pressure)
  • Microphones – converts sound into an electrical signal (air pressure → movement of conductor/coil → magnetic field → electrical signal)
  • Pickup (music technology) – converts the movement of metal strings into an electrical signal (magnetism → electrical signal)
  • Tactile transducers – convert an electrical signal into vibration (electrical signal → vibration)
  • Piezoelectric crystals – converts solid crystal distortions (vibrations) to and from electrical signals
  • Geophones – converts a ground movement (displacement) into voltage (vibrations → movement of conductor/coil → magnetic field → signal)
  • Gramophone elements – (air pressure → movement → magnetic field → electrical signal)
  • Hydrophones – converts changes in water pressure into an electrical signal
  • Sonar transponders (water pressure → movement of conductor/coil → magnetic field → electrical signal)
  • Ultrasonic transceivers, which transmit ultrasound (transduced by electricity) and receive it after sound reflection from target objects, using for imaging those objects


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