Depth sounder

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raggy

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With the imminent release of Raspberry PI.
I am looking into ways to use it on the boat.

My plan is to use it as an all in one instrument. I can write the program no problem and can easily get the gps input working.

However I don't know where to start with the echo sounder. It has only one connection which is an aerial style connector. I presume I am going to need to make a control box which creates the pulse and interprets the data to send to the PI.

What I really need is a technical guide for the transducer.
Anyone have any ideas?
 
I am going to have to build a control unit that can create an electrical pulse, listen for the return signal and time the return.

What I am really needing is a description of the pulse so I know where to start
IE: Length, power, Wave type? etc


I could of course cheat and buy either a smart transducer or a raymarine depth pod but the idea is to keep costs down.

Consequently the raymarine depth pod is what I need to build :) with the slight difference that the pod output is nmea mine will just be time in milliseconds
 
I researched echo-sounders a couple of years ago - and found the US Patents Office a rich source of info. It was my intention then to design a 'solid-state' sounder, but gave up when I realised the complexity involved. So - I reverse-engineered an old Seafarer instead, and although the Seafarer is an old design (all transistor, using transformer coupling between stages), it still provides the sort of info you are asking for. So - I'll dig it out, it might take a while - but I know I've got it squirreled away somewhere ...

One of my 'round tuit' ideas was (and still is) to graft a PIC into the circuit and measure infrequent pulses (generated by the PIC) for whenever sailing off-soundings - it would only be when a pre-set depth was encountered that the Seafarer would burst into life with it's spinning prop LED. That way, the Seafarer's battery life could be extended significantly. And, as a bonus, depth info could be relayed in NMEA form to a remote display or computer.

Be aware that a power transistor is needed to drive the transducer - for although the pulses are brief, they are of relatively high power.

Receiver techniques are interesting - for example, one US Patent describes an AGC (Automatic Gain Control) technique which compensates for depth, so that the longer a transmitted pulse takes to be received, the deeper the bottom (or target) must be, and the weaker the signal must also be when being returned from such a depth. So the amplifier gain is duly increased in direct proportion to the time taken for the pulse to be received. All clever stuff (or at least I thought so). It's things like that which made me realise that, although the principles of depth detection appear straightforward, a lot of brainwork has been put into refining the technique over the years.

Another approach for you would be to reverse-engineer a more modern depth-sounder such as the Nasa Target, and to simply copy the business-end of that.
 
I am getting a nasa clipper for £75 which will be fitted to the boat after I have gotten all the info from it.

I have a spare nasa transducer which I can use for testing my project.

At the moment I am looking at using a usb micro controler (off the shelf) coupled to a generator / detector with seperate amplifers for TX and RX.

At the moment I know nothing abouth the pulse I need to produce to send to the transducer. I only know the output of the transducer is 150MHZ

Pulse
P.D: Unknown
Current: Unknown
Duration of pulse: Unknown

I am currently trying to find a good virtual electronics lab to use for designing the bits:)
 
raggy said:
I only know the output of the transducer is 150MHZ
Click to expand...

Are you sure ? - that's VHF.

Transducers for the older systems are 150Khz - which is roughly in the middle of the LF (Low Frequency) radio band. That's why coax is used to connect the transducer, with the coax forming part of the transmitter's tuned circuit. And that's why it should never be cut shorter, or repaired in a willy-nilly fashion, although people do this and seem to get away with it ...

The pulse is itself something around 20-30Khz (i.e. just super-sonic) with a duration of maybe 10 or 12 cycles (i.e. a short 30Khz burst). The transmitted pulse has to remain at or near the audio wavelength as you're transmitting a pressure wave through the water, and not a radio wave.
In practice you want a duration which is short enough to enable reception at a shallow depth (otherwise the trailing end of the pulse would still be being transmitted whilst the leading edge had already bounced back *) - yet you need a long enough pulse for detection of that pulse to be definite (i.e. can be distinguished from any noise present). Unprocessed, such a pulse will be far too short to flash an LED or similar, hence the use of a pulse-stretcher within the circuit.

If you Google for the speed of sound through water, you could figure out a pulse duration from that - say using 1 metre as a minimum depth resolution.

* = I'm assuming you're going to use the same transducer for transmission and reception ?

I am currently trying to find a good virtual electronics lab to use for designing the bits:)
Click to expand...
Try: http://www.sonsivri.to/forum/index.php
 
Raspberry Pi Afloat

Having just ordered a Raspbery Pi, I have also been thinking about Maritime applications, in particular a software-based AIS decoder. There are a couple of these for PC platforms which could probably be ported relatively easily.

Only thing is what to do about the Screen/User interface. Have you had any thoughts about this bit? Waterproof/daylight viewable PC monitors are expensive/large and current hungry. There are plenty ofcheap composite video displays (in-car DVD etc) which could probably be waterproofed with a bit of inginuity, but they are far from daylight-viewable.

Regards, Rob
 
NOt sure on the final screen but there are loads of options.
I might use a mono colour LCD which can be brought from a wholesaler like Farnell as a panel mounted on a pcb with inbuilt controller.
I have a friend who can knock me up and enclosure on a rapid prototyper :P
 
Ok - here's what you wanted - albeit from vintage equipment.
Seems I've got a bit of egg on the face from guessing the audio frequencies - well, that's getting old for you - the ol' memory isn't what it was.


"Seafarer 3 & 4"
"Transducer: Reversible Electric Ceramic. Nominal frequency 150Khz. Standard length of cable 24ft. (7.5m) Capacity of transducer and lead approximately 1000uuF. Since the capacity forms part of the input resonant circuit, the lead length must not be altered without re-tuning the inductor T2."

"Measurements taken with 9 volt supply, sensitivity maximum, set to Fathoms (x6):
Peak to peak drive voltage across transducer socket greater than 250 volts, and between 0.7 and 1.5 mS
Input sensitivity for LED illumination (nominally 150 Khz, 30% modulation) 10 to 20 uV."

The actual transmitted waveform itself is not given, but can be inferred from that used for testing:
"Alignment: Connect a Signal Generator, modulated at 400cps 30% via a capacitor of 1000uF to the centre of the transducer socket ...
Set the frequency of the Signal Generator to that marked on the sensitivity control of the Seafarer, or to the new transducer frequency marked on the transducer plug."



Seafarer 5 & 700

147 Khz, 100% modulation at 125Hz and 600Hz.

(Cable lengths as for Seafarer 3.)

Tx pulse widths (all +/- 25%) >300v pk-to-pk
700 fast 0.15ms
700 med. 0.275ms
700 slow 1.5 ms

5 fast 0.15ms
5 slow 1.5ms



So, the Seafarer 3 uses 400Hz, and the more advanced (in their day ...) 5 and 700 both use 125Hz and 600Hz - the higher frequency/ shorter pulse train width obviously being used at shallower depth to avoid the pulse collision I mentioned earlier.

From the pulse width at the chosen frequency, you can then work out the approximate number of cycles required for each pulse train - but as you can see from +/- 25%, this ain't exactly critical - it's only the leading edge of the pulse train which is important.
 
If you just want to measure depth rather than use as a fish finder, then Airmar do sensors with nmea output. One is available with speed, depth and temp.
 
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