Hi Everyone!
Huge shout-out for all your work and massive amount of knowledge you're sharing. Bit TXH!
I'm here to ask you for help. You know many things, I don't. So I'm asking and hoping someone will answer
I want to create a device. Requirements it has to follow:
The Thing will be working as two separate devices. One needs to send signal - other needs to receive it.
Communication suppose to work in both ways. A > B, B > A
Device, when receives signal, should react by ligtning up (propably) LED's. It's the same for A, or B.
Part A transmits the frequency, part B is showing this by ligtning up. And vice versa.
Both devices should be working in maximum range around 30 meters (mostly much closer), in higher density environment - salt water.
Everything should as small as possible, and as simple as possible.
If someone can help with any of this subject (for example, by just explaining the way transmitters and receivers are lightning up other device) or in any other way - I'll be grateful.
That's what I know (let's say) for now:
Device A transmits frequency (I don't know how), and in B device there's receiver (propably some small electronical mainboard?).
Within the signal, there's coded information, which works as a instruction what B device has to do. In 'my case' it must be propably connected also to LED's, to have a possibility to light them up.
Thanks in advance - for your time and answers. All the best for Everyone.
Only sensible approach that would work in salt water would be acoustic. Maybe based on the type of ultrasonic transducers used echo sounders which are widely used in boats and ships from dinghies and yachts to supertankers.
Would need custom electronics to drive them in a "communications" mode but given that echosounders can be very cheap it can't be that hard.
Realistically you are going to have to use wire communication. That is what all submersibles do. Unless you have a budget close to the US Navy you will not be able to get a better system for any sophisticated communication underwater.
Why? I'm not saying use an echosounder but use an echosounder transducer. Sound travels at ~1500 m/s in water, so the echo in say 10m of water, the kind of depth when you start needing to know how deep the water is underneath your keel, comes back in about 20/1500 = 133ms. So you would need a pulse no longer than say 13ms to resolve the depth, preferably less, lets say 10ms. Should allow at least 50 baud transmission rate, higher with a cleverer modulation system. Actually from my sailing days I recall sounders working fine at 3m depth so probably 10x these rates should be possible.
Why do I need a plan? I'm just pointing out that transducers are available that at face value could do the job the OP wants. However if it was my project then to keep it simple and if the range doesn't have to be too long I would start by trying on/off keying, possibly just using a UART. So a MARK turns on the sound and SPACE turns it off. If longer range and / or higher data rate was needed more complex modulation could be looked at. All standard modem stuff.
I just noticed that the max range is 30m or closer. Since echosounders work like radar except with sound, the received signal varies as the inverse 4th power of distance. So using the same power and transducers as an echosounder, but for communications, the range looks like it should be much greater than the echo range.
I just found a Hackaday page on making an echosounder:
If one can make a sounder a comms variant should be possible.
Reading a bit of that linked project it transmits a 512kHz carrier which is pulsed on and off. Carrier generation by a Teensy. Should be easy to do quite sophisticated modulation of that.
