I came across an advertisement from a sporting-goods store for sub-$150 fish locators, and was curious to find out what made these things tick. The FishFinder 535 from Humminbird, Figure 1, is an old guard of the industry.
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Its design sports a sizable 5.3-inch-diagonal gray-scale LCD with many of the basics of pricier alternatives.
Like all devices of its type, the 535 relies on the ultrasonic "ping-and-response" of sonar to image activity in the water. Here, a single 200-kHz frequency burst is sent from a waterproof transducer that mounts underneath the boat. This circular piezoelectric "puck" is excited by metallized electrodes on the top and bottom faces, and driven by output energies of several hundred watts (rms). While instantaneous power is high, the sound pulse is brief, typically in the 20- to 400-microseconds range.
Imaging is based on collecting return ping data, and here the transducer works in reverse, with return signals bouncing off the target (fish, trees, lake bottoms) and creating a voltage in the same piezoelectric device.
Because ultrasound travels at a fixed rate, the delay of the return signal describes the object depth. Correspondingly, the strength of the return signal (at a given depth/delay) correlates to the size of the object being imaged in the roughly 20 degrees cone of sound produced by the transducer. By gathering "time slices" of imaging data and painting them across the width of the screen as the boat motors along, a two-dimensional image is created.
A Samsung S3C44B0X microprocessor forms the control system, along with NAND and SDRAM memory, Figure 2.
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The MPU drives the display, almost certainly doing some pretty sophisticated image processing to translate raw return signals into usable LCD imagery. The CPU manages the analog elements of the 535's design. A semi-sinusoidal pulsed signal is boosted by a push-pull amplifier, sizable storage capacitor and step-up transformer; the intense burst is sent out and a timer in the processor starts a counter for later determination of the return pulse delay.
In generating a usable return signal, the NXP SA604A FM intermediate-frequency subsystem chip is used, but only for its received-signal strength indicator (RSSI) circuitry. The return transducer ping creates a logarithmic RSSI voltage, which is digitized using Samsung's built-in A/D converter. Its timer is also stopped when the return ping is detected/digitized, and now both the delay and signal-strength data needed to build the picture are available.
Fancier echo sounders use multitone transducers to leverage deeper-penetrating lower frequencies and finer-resolution higher frequencies. Given the $140 price tag, the model studied here must be done on a budget. But the design is elegant in its simplicity.
My thanks to Dave Betts of Humminbird for helping me understand the basics of the 535 echo-sounding design. Now let's go fishing!
About the Author
David Carey is president of Portelligent (www.teardown.com). The Austin, Texas, company produces teardown reports and related industry research on wireless, mobile and personal electronics.
