Wireless Coupler

Posted on Feb 4, 2013

Here's a way to get a short range bidirectional RF (Radio Frequency) channel on a microcontroller, using the controller's tristatable I/O pins and on-chip comparator with a few passive components. The range with the firmware and simple I came up with for Atmel AVR controllers allows for low error rate communications over 10 to 15 cm using a 5.5 cm diameter loop antenna on each device. This is the answer to my search for the simplest and lowest cost way to connect instruments on my bench top to my computer and to each other without wires. Its an alternative to short-range infrared; except no need to make sure there is a good optical path between the units.

Wireless Coupler
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I presently use this system for communications among a wireless alphanumeric 2 line X 16 character LCD (Liquid Crystal Display), a battery operated frequency meter, a four channel 10 bit voltmeter, and a base unit that connects my computer's RS-232 port to the RF channel. These projects are described on other pages on this site. This RF Link also adheres to the "Minimum Mass" product design concept - not using any thing that is not absolutely essential to obtaining the needed function. The Minimum Mass RF Link makes use on a microcontroller's on-chip analog comparator connected to a loop antenna and a pair of bias resistors as a receiver and uses a tristateable output on one of the comparator pins to drive the same loop antenna as a transmitter. I have written code modules for the AT90S2131 and the ATMega 8. The code is fairly small - only 131 bytes for the ATMega8 version, and for hardware resources, it only uses an 8 bit time and the comparator. Though I have only tried this on AVR controllers, I suspect this technique would work well on other controllers, such as the PIC family, equipped with comparitors. pair of resistors biases the resonant antenna at 1/2 the supply voltage (See Figure 2). This is the sweet spot of the comparator's common mode input range, where the input offset voltage is likely to be minimum, which result in the greatest receiver sensitivity. This is also nice for the transmit mode, when sending...

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