AE5K's Fox Hunt Keyer

INTRODUCTION

One of the joys of ham radio is looking for a "fox". Fox Hunting in Amateur Radio is really trying to locate a hidden transmitter. It's also known as bunny hunting, DFing, and by other names.

In our area of the Northern Arkansas Ozarks, we hold a fox hunt every month. Our usual pattern is to have the fox (hidden transmitter) transmit on a 2-meter FM simplex frequency for a one minute period, followed by a four minute period of silence. This has usually been done by carefully watching a watch with seconds readout and keying the microphone at the proper time and then spending a minute wondering what to say besides identification and 15-second interval countdown before end of transmission.

After being the fox for a couple times, this watching the watch and wondering what to say became old and a little boring. Why not let a microprocessor or microcontroller to the job?

I use a Kenwood TM-261A mobile transceiver and a quick look in the manual gave the RJ-45 microphone connector (8-circuit) pinout. Aha! It is capable of supplying 8 vdc at low current. Hence, if we design a circuit which draws low current, it might work by just plugging into the mike jack of the transceiver.

SPECFICATIONS

The main thing I wanted this fox hunt keyer (FHK) to do was to duplicate the timing and to provide ID. So to start with, I decided on the following 5-minute sequence (to be repeated over and over until switched off at the end of the hunt):

Time/Action
0:00 - 0:05 -- Transmitter on, steady tone
0:05 - 0:20 -- Morse code ID (time approximate)
0:20 - 0:55 -- One second ticks or beeps
0:55 - 1:00 -- Steady tone
1:00 - 5:00 -- Transmitter off

The Morse code ID was to be: "FOXHUNT DE AE5K" and I wanted it at some reasonable speed.

THE DESIGN

In choosing a microcontroller, I wanted one which could be reprogrammed easily and was not too expensive. The Atmel AVR series, introduced several years ago, was very attractive, and in particular the AT90S2313 chip in a 20-pin narrow DIP (0.3 inch spacing) package had everything I needed, and then some. It has flash ROM for program storage, EEPROM for changeable data storage (like call signs and timing?), and sufficient RAM memory. It also has a couple timer/counters, interrupts, and a UART. Best of all, it can easily be reprogrammed after it is in the final circuit with a simple 10-pin header and dongle connection to a PC printer port. The assembler, simulator, and programmer software are all free, and a $50 "starter kit" (which I already had) to check out a program before even building up a final circuit.

I'll not go through all the design steps and decisions, but just mention that I opted for 4 MHz. crystal control and used the 16-bit timer to generate 1 millisecond interrupts. The interrupts are then used to decrement software counters for all other timing and also to toggle the tone-out pin (eventually going into the microphone input of the radio) whenever the tone was "on". This produces a 500 Hz. tone with lots of harmonics since it is square wave.

At the present time the Morse ID string is in the code ROM space, but with later revisions will be moved to EEPROM. The string is ASCII. It is moved to RAM and translated to a dot-dash format before being sent. There is nothing new in this translation method as it has been used for at least 25 years. Basically, a dot is represented by a 0 (zero), while a dash is represented by a 1 (one). Fill the rest of the byte with zeros and a single 1. An "A" by this coding would be 00000110. Read from right to left. When being sent, the byte is shifted to the right with the rightmost bit going into the "carry" bit. If the remaining byte (after the shift) is zero, we are done with this letter or number. Examination of the carry bit determines if a dot or dash should be sent. Once sent, the shift right is done again, etc.

One pin of the 2313 is used to control the PTT line of the radio. A 2N2222 transistor was used for isolation and could possibly be eliminated if the output sense was reversed in the software.

The 10-pin (2 x 5) header is shown but could be eliminated if the 2313 were programmed out of circuit. It's pinout is that suggested by Atmel and used by the starter kit.

CIRCUIT DIAGRAM

Until I can figure out how to go from EAGLE schematic software output to gif or some other web compatible format, I have had to print and then scan the schematic. I'll send anyone a printed copy of the schematic on receipt of a SASE. There's really not much to this keyer. It should be able to be adapted to other radios.

You may view it here.

FIRMWARE

If anyone is interested in viewing or using the firmware source, it may be found here (in two parts): Main Firmware Listing and Code Lookup Table Listing.

FUTURE

I have this circuit all laid out as a PCB pattern and may opt to have it etched one of these days. The original prototype, used for the first time in a foxhunt on Feb. 18, 2001, was just thrown together on an old salvaged Radio Shack proto board, but worked the first time power was applied and made the foxhunt more fun for the fox.

One additional feature I'd like to add to the software is a time of 10 or 15 minutes from power on until the FHK started its transmission sequence. This would allow time to plant the hidden transmitter and turn it on, then to "escape" from the area before it started transmitting. This would also mean another input with a pushbutton switch to "test" it before arming it. Then this might lead to using some sort of display to show the time left before it goes "off" -- but now we are getting into a more complicated circuit and one which might be mistaken for a bomb since we all know bombs with timers have nice displays for some reason.

Since this unit is easily reprogrammed, I'll probably experiment with other time formats.

Ideas and suggestions are welcome.

Happy hunting! --AE5K