Fortunately the signal output by the Darlington transistor pair on the receiver is a pretty clean digital signal.  At full power it ranges from (a little above) 0 V to (a little below) 3.3 V.  As the transmitter gets farther away the digital signal remains but the low voltage increases beyond the UART receiver’s ability to read a 0.  For example, at a large distance the UART signal might range from 2.5 V (logical 0) to 3.3 V (logical 1).

There are several options I’ve considered:

The handy old Schmitt trigger is the most straightforward, but it’s touchy to design and it will add a bunch of parts (an op-amp and a bundle of resistors).  I tried using a comparator chip, which is like the Schmitt trigger but simpler, but I found that it had a big limitation.  It worked, but the inputs can’t exceed Vcc – 1.5 V (the “common mode input voltage range” on the datasheet).  It can’t read a digital signal ranging between 1.8 (or more) and 3.3 V, so it would provide some amplification but not a lot.  I’m targeting 90% of Vcc as my threshold, so it should read anything below 3.0 V as low.  I also considered using another PNP transistor to amplify the difference between Vcc and the input voltage, but: that much amplification is risky noisewise, an extra transistor reduces the bandwidth too much, and the PNP transistor will produce a logically inverted signal.

Another option is to amplify the signal using crazy op-amp magic, but aaaaaaah no.

So I tried thinking like I’m supposed to for once, and I’m going to try a software solution.  AVR processors include an analog comparator, which I can (hopefully) use to amplify the digital signal coming from the infrared optotransistor.  The downside to doing it this way is that it bypasses the UART module, which means that the application needs to receive data manually (I can still use UART to transmit the signal).  Receiving data will tie up the processor, but as long as there’s enough time to run the receiver code it should be okay, I don’t foresee much data traffic.  Here’s a potential flow chart for the receive procedure:

Software receiver flow chart.

Some notes:

• The wait process is a euphemism for “disable the timer, store the data, blah blah blah, then go back to the program until the next start bit.”  This process takes place during the stop bit, so if there’s another byte incoming then it should finish in time to receive the next start bit.
• Kevin Rosenberg’s awesome tool AVRCalc calculates that the 8-bit timer running at 8 MHz can be used to generate a 57554 Hz interrupt rate using an OCR value of 0x8A.
• I might need to mess around with the timing so that the AC reads the right bit, but probably not.  There should be plenty of interrupt overhead delay, and the sampling rate is a little slow and will drift toward the end of the bit.
• I’m not sure how long the analog comparator takes to produce a result but I’m guessing it’s pretty fast because it doesn’t have to do much.  The AVR datasheet doesn’t have much information on it.  I also don’t know if the comparator has a large common mode input range.

Failing that, I can look for an external comparator with a large common mode input range, or just suck it up and use an op-amp.

Acoustic transmitter circuit.

Above you can see the transmitter part of my acoustic modem.  From left to right, the components are: the power supply, the acoustic transducer, the amplifier, and the microcontroller.

The problem: I need to send a 40 kHz, 20 V peak-to-peak square wave to the transducer.  The circuit has two outputs, + and -, that are connected to the two transducer pins.  In other words, each output needs to switch from +10 V to -10 V  and back once every 25 microseconds.

As always, there are a few decent solutions, and as always I didn’t implement the best one.

Solution: Transformer

I think the best solution, or at least the awesomest, is to use a step-up transformer.  I don’t need a transformer because the transducer I’m using is pretty low-voltage, so I can use it with a reasonable power supply.  A higher voltage transducer such as the Meas-Spec US40KT-01 [PDF], which operates at up to 300 V peak-to-peak, would definitely need a transformer.  The main downside of using a transformer is that I don’t know very much about using transformers, and also I suspect the circuitry will be more expensive (I do have room in my budget).  The upside is that it will produce a  high voltage signal and should be a simpler circuit once I figure it out.

Solution: Amplifier

I have a little microcontroller generating a 0 V to 5 V square wave at 40 kHz.  Originally I had some kind of vague notion that I would use an amplifer to bump the micro’s output up to 0 V to 20 V.

My stumbling block is that the microcontroller is grounded to the -10 V rail, so according to the op-amp, the micro’s outputs are actually from -10 V to -5 V.  How do you amplify that to a signal that goes from -10 V to +10 V?  I’m sure there’s a slick way of doing it by mucking around with virtual ground, but I gave up pretty quickly.

Solution: Schmitt Trigger

Suddenly it’s clear why I wrote a log entry about the Schmitt trigger!  Another reason: I was too busy to make a long log entry about something more complicated.

Originally I learned about the Schmitt trigger trying to figure out how to turn the receiver’s analog output into a digital signal.  It transforms a signal of arbitrary amplitude into a digital signal flipping back and forth between the op-amp’s two power rails.  I was going to turn the ±10 V output into a 0-5 V output by adding a diode to block the -10 V signal and a voltage divider to turn the +10 V signal into +5 V.

Anyway, the Schmitt trigger produces a ±10 V signal and hey, that’s exactly what I want, so I just made a Schmitt trigger with threshold levels between -10 V and -5 V (that being, again, the microcontroller’s output from the op-amp’s perspective).

It works okay.  The ±10 V waveform is more trapezoidal than square.  Square waves are always are kind of trapezoidal, but this one is really trapezoidal.  But it’s square enough and the acoustic signal it produces is fine.  The raw output of a receiver module receiving my signal is indistinguishable from that of the SRF04 I’m trying to copy, except my signal is marginally more powerful (since I’m sending the transducer a slightly higher voltage).

So that’s what I’m using as a transmitter for now.  The plan is to upgrade the transducer and switch to a transformer amplifier at some point down the line.  This transmitter functions as a proof-of-concept that works well in air (how to get it working underwater is TBD).

Output

Visualization of the output waveform.

The modem encodes data in acoustic pulses, as shown above.  The image is a representation of a single 1 bit, encoded as eight consecutive pulses at 40 kHz.  I will discuss this more later.

Simulation of a Schmitt trigger. The yellow sine wave is the input, the purple square wave is the output.

I was looking for a way to convert an analogue signal into a digital signal, and came across the Schmitt trigger.  This is a great little circuit that you can build with a couple transistors or with an operational amplifier.  I tried to build a transistor-based simulation in Simulink, but it didn’t work right away.  My op-amp simulation did work, so I stuck with that.

The principle is simple: if the trigger input rises above a certain threshold, then the output saturates to the op-amp’s positive power supply voltage.  When the input falls below another threshold, the output saturates to the op-amp’s negative power supply voltage.  If the input lies between the thresholds then the output doesn’t change from whatever it was before.

This is incredibly useful.  To the left you can see a 0.3 V sine wave being converted into a 5 V binary signal (actually a 4.5 V signal, since I added a diode in series with the output to keep it from going down to -5 V).  It also effectively filters out any input jitter that doesn’t cross the threshold needed to change the output state.  Microcontrollers have Schmitt triggers on their digital inputs.  For example, an AVR microcontroller being powered by 5 V typically considers a 0 logic level to be under 1.5 V and a 1 logic level to be over 3 V.  If the input sits between 1.5 V and 3 V then the digital state remains whatever it was before the input entered that region.

Alas, it’s not a perfect circuit.  I used an online calculator to figure out what resistances I needed for my project.  Configurations that worked in the calculator and in simulation didn’t work in reality, either producing no output or something more like a sawtooth wave than the square wave I expected.  It’s probably a limitation in my op-amp—maybe the signal is too high-frequency, I didn’t investigate very deeply.