Dot with attached Particle and AAA battery
These were developed for a proof of concept research project. Slap one of these onto every printer, fax, PDA and gadget. Then slip a Relate dongle into your laptop. As you walk around your building your software will automatically detect and locate each of the devices. The fancy schmancy interface allows you to locate the devices and share services - drag and drop documents for instance. The Relate project hyperbole for the Dots states something like 'Small ultrasonic transducers that allow the automatic recognition and location of distributed services.' I was tasked to design and implement the hardware and no-one is more surprised than me that it all actually worked! The initial prototypes were two layer, the final item slimmed down to a four layer board. The RF communication was done via Techo Particle boards - these had been selected for all RF communication for the Relate project before my arrival. I made the outline of the four layer Dots fit the footprint of a Particle. See below for a .jpg of the Dot schematic. The schematics and layouts were all done using the Eagle software package, available for free from cadsoftusa.com. The schematic and board layout files for the four layer version can be downloaded here Dot_4layer_7.sch and Dot_4layer_7.brd.
Dots, top and bottom view
Magic. That's how.
The ultrasonic transducers need a 40kHz driving signal. As the transducers resonate at this frequency they can be driven with any old shape of wave. I used square wave. The bigger the wave, the louder they shout. A lot of circuits on t'internet use a microprocessor output pin directly connected to the transducer, but this will only allow it to operate with the voltage that is supplying the processor. So you need voltage, lots and lots of voltage to make them ring louder and annoy their neighbours. How to get a big voltage swing? There are lots of potential ways, using DC-DC converters or transformers for instance. But I used an RS232 interface chip. This converts lowly microprocessor signal voltages (circa 3V) to a mighty 15V swing. Woo hoo. Feel the power. Fast and throbbing. The LTC1348 by Linear Technology does the job nicely. Three channels in the same IC - which is exactly what I needed. You can poggle free samples too. Jolly nice people. I have used quite a few of their chips now. They do a good range of filter chips too. But I digress.
Once I had the idea of using the transceiver I had a quick search on the internet. I was relieved to find that this idea had been shown to work on http://www.robot-electronics.co.uk/htm/srf1.shtml.
Disabling the LTC1348 sets the transmitter lines from the LTC1348 to the transducers to a high impedance state. This is excellent, as it allows the transducers to be immediately used as receivers. Otherwise the transducers would need to be isolated from the from the driver chip by transistors, adding extra components and routing complexity to the board. The signal from the transducers passes through a standard two stage amplifier. Why two stages and not just one stage with loads of gain? Bandwidth. With a 40kHz signal, each stage gives about a gain of 25. This requires an op-amp with a bandwidth of about 1MHz. Cheap as chips.