Wireless Coupler Terminal Interface

The purpose of the terminal interface is to allow my computer to communicate with Minimum Mass Wireless Coupler equipped devices using a terminal emulator program. Consequently, the Base Unit is basically an RS-232 interface and Minimum Mass Wireless Coupler. The Base Unit is the larger enclosure in Photo 1. The RS-232 interface to the computer is 9600 baud, 1 stop bit, no parity, plenty fast to keep up with the 1200 baud maximum data rate from the Coupler. The antenna is made of 12 turns of #30 enameled wire threaded through metal guide loops arranged around the perimeter of the board (See the photograph.) This antenna's area is about 160% that of the prototype 5.5 cm loop so it has a little more range for both transmitting and receiving.
Wireless Coupler Terminal Interface - schematic

To further increase the transmitter's range, I used 220 ohm bias resistors (See the schematic, Figure 4). This sets the maximum peak current from the microcontroller’s output pin at about 23 milliamps, safely below the maximum current specified on the microcontroller’s data sheet. Since the antenna is resonant, the peak antenna current is greater than the microcontroller's drive current, about 70 milliamps peak-to-peak in this cae. The Base Unit uses a trick to increase the receiver's sensitivity. I added an adjustment to compensate for some of the comparator's offset. The smaller the offset voltage, the smaller the signal needed from the antenna to trip the comparator. Adjusting R1 causes the voltage drop across R3 of up to ± 60 millivolts, and this effectively compensates for offset voltages across the comparator input up to the maximum shown on the controller’s data sheet. Increasing the sensitivity can be a good thing, but too much can be a problem. When the offset is adjusted to near zero, noise from the microcontroller itself causes the receiver to repeatedly detect false signals. I've also noted that when I put the base unit too close to a backlight power supply in my notebook computer it picks up interference. Not only does the RS-232 output send out garbage, but the noise interferes with the desired signals from being properly decoded. The solution is to back off on the sensitivity adjustment. to the...

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