W1MJ 5-Watt Fox V1.0
Eliot Mayer W1MJ
January 15, 2022
Updated March 19, 2022
Contents
Assembly,
MP3 Player & Interface Circuit
Appendix
1: UV-5R Interfacing Information
Appendix
2: MP3 Player Information
MP3
Player Measurements & Observations
Introduction
This document describes
the design of the W1MJ 5-Watt Fox, Version 1.0.
This type of fox is an automated radio transmitter used by the amateur
radio community in hidden
transmitter hunts, AKA fox hunts.
This fox uses an mp3
player to send voice messages to a Baofeng UV-5R
handheld transceiver via a homebrew interface.
The UV-5R has a VOX
feature, but it exhibited undesired dropouts even at maximum sensitivity. The dropouts could be eliminated by adding a
strong “sub-audible” tone to the recording.
However, the interface in this design contains its own VOX circuit,
which controls the radio’s PTT (transmit control) input. The sub-audible tone is therefore not needed.
Radio Battery Interface MP3 Player
Main Diagram
Notes:
·
The
PTT return is via the power supply returns (GND).
·
After
5VDC supply current was measured, it appears that a simpler linear regulator
could have been added to the Prototyping Board in instead of using the separate
DC-DC Converter. Impact on battery life
would be negligible.
·
In
the past, and in one instance with this fox, I have found the reliability of cigarette
lighter plug / socket mating to be so-so.
While the part of the Battery Eliminator that mates with the radio is
required, the regulator may be replaced in the future to improve reliability.
Top-Level Bill of Material
|
Transceiver |
|
|
Transceiver Battery
Eliminator |
|
|
Battery Eliminator
Adapter Cable |
+ 15A Powerpole connector |
|
Battery, LiFePO4
12.8V 30AH (Note 1) |
|
|
Power-Pole
Distribution Block, 4-Position |
Quicksilver PWR-BLOK
4 or
equivalent |
|
DC-DC Converter, 5V
3A Output |
|
|
Prototyping Board,
3.2” x 2.6” |
|
|
MP3 Player, 5VDC
Power |
|
|
Interface Circuit |
See BOM in Interface
Circuit section. |
|
Antenna |
Twinlead J-Pole; see Antenna section. |
|
Adapter, SMA-Female to UHF-Female |
From Quicksilver Radio |
|
Double Female SO-239 Bulkhead Mount, 2 inches
long |
From Quicksilver Radio |
|
Washers for Bulkhead Mount |
From Quicksilver Radio |
|
Antenna Pigtail Cable |
RG-58 w/ PL-259 connectors |
|
Shielded Cable w/ 3.5 mm TRS plug |
(1/2 junk box cable) |
|
Shielded Cable w/ 2.5 mm TRS plug (apparently not needed for PTT
return) |
From Amazon if needed (cut in half) |
|
Case |
|
|
Chain |
Schlage Weatherproof
Key Padlock with Flexible 3/8" Steel Looped Security Cable (padlock too small; see next item) |
|
Padlock |
Note 1: A smaller 12V or 12.8V battery may be used
with reduced operating time per charge.
The specified battery will run the fox for an entire weekend and had
already been purchased for other ham radio purposes.
Temperature Considerations
·
The
UV-5R transceiver has a specified operating temperature range of -20˚C
(-4˚F) to +60˚C (140˚F).
·
Bionenno Battery: According to Kevin at Bionenno,
the battery operates down to -10˚C (14˚F), but must be charged at
greater than 0˚C (32˚F)
·
DC-DC
Converter: 80˚C (176˚F)
maximum operating temperature; no minimum specified.
·
MP3
Player: Unfortunately, has no
specifications.
·
Interface: Using only through-hole parts for ease of
construction results in limitations. The
comparator IC is only rated for 0˚C (32˚F) to 70˚C (158˚F),
but I am taking an intelligent guess that it will be functional and performing
adequately below its rated range.
The overall operating
temperature range has to be determined by experimentation. See Final Assembly Tests.
Antenna
Twinlead J-Pole antenna:
·
Per
this article: https://www.qsl.net/wb3gck/jpole.htm
·
Feedline
= RG58, wrapped 3X around an Amidon FT140-43 ferrite core.
Related
article:
https://www.arrl.org/files/file/Public%20Service/TrainingModules/jpole-dual-band.pdf
Design, Interface Circuit
First schematic
from Digi-Key Scheme-It:
https://www.digikey.com/schemeit/project/fox-vox-interface-a084ee59940147349be0bb8107c3d642/
The
following simple circuit would suffice but requires a strong sub-audible tone
in the mp3 file in order to prevent UV-5R VOX drop-outs. The trimpot may not
be needed, based in on initial testing with an iPad as the mp3 player. The transformer is probably needed to prevent
undesired PTT keying if the mp3 player and radio battery eliminator share a
common ground (TBD).
To avoid the need for
the strong sub-audible tone, the circuit shown below is used. It is intended to be a better VOX. It keys the UV-5R PTT. The PTT return via the battery eliminator GND
appears to suffice, but J3 is shown in the diagram just in case.
For reference, see the
following:
·
Appendix 1: UV-5R Interfacing Information
·
Appendix 2: MP3 Player Information
Schematic,
draw with Digi-Key Scheme-It:
https://www.digikey.com/schemeit/project/w1mj-fox-v21-50fef9dbfa4f4964a210be8cecc17703/
Audio peaks from the MP3 player charge capacitor C2. R3 limits the loading on the player to
prevent distortion. When the voltage on
C2 exceeds the reference voltage at U1-3, comparator output U1-1 pulls the
radio PTT line (P1-Sleeve) to GND to key the transmitter. Hysteresis is provided by R7 to ensure clean
PTT switching. When the audio pauses, C2
discharges through R4, ending transmission after approximately 2 seconds. LED D2 indicates that PTT is active.
Potentiometer R2 is
used to set the audio level to the radio for proper modulation. Audio transformer T1 is needed to prevent the
audio circuit from keying the PTT.
Design calculations
(Excel file embedded in Word document):
Notes:
·
LM393
Comparator data sheet: https://www.ti.com/lit/ds/symlink/lm193.pdf
·
If an audio amp is needed for the vox, consider LM324: https://www.ti.com/lit/ds/symlink/lm324.pdf
Interface
BOM, downloaded from Digi-Key schematic project then edited:
Assembly, MP3 Player & Interface Circuit
The assembly drawing
is maintained in this Excel file embedded in the Word document:
Test Results
A successful test
with a solderless breadboard was done on 1/9/2022.
Breadboard is from REXQualis Electronics Component
Fun Kit.
Bench Measurements
12V supply: 1.1A
(TX=5W) / 0.6A (TX=1W), 0.0A (not transmitting)
12V supply current w/o radio: 36 mA
5V supply current:
62 mA during transmit, 60 mA when not transmitting.
The
comparator threshold measured close to expected levels per
Fox_Calculations.xlsx:
|
Vt,
Rising (VOX Not Tripped) |
370 |
mV |
|
Vt,
Falling (VOX Tripped) |
270 |
mV |
PTT voltage:
·
Receive: +3.3 V
·
Transmit: +80 mV
Comparator
input, solderless breadboard w/o transformer:
Final Assembly Tests
When the components
were transferred to the prototyping board, U1B was somehow damaged. Pin 6 had about 5 kohms
resistance to ground, which dragged down the comparator threshold voltage. The connection to this pin was then removed,
leaving the fox functional except for the Transmit LED (D2). The LED was just a nice-to-have feature so
changing U1 didn’t seem worth the trouble.
In hindsight, I should have used an IC socket.
Operating
Temperature: The fox ran continuously
from 10 AM to 10 PM on 1/14/2022, during which time the temperature dropped
down to -7˚C (20˚F). Battery
voltage 13.26.
Antenna Measurements
·
SWR,
away from other objects: Approximately 1.5
·
SWR,
right next to a tree trunk:
Approximately 3.0
MP3 Scripts
The audio
recordings were made with Reaper DAW. Each message is preceded with a hand clap
about ˝ second before the message proper.
This gives the VOX time to trigger and the transmitter time to start
transmitting, ensuring that the start of the message is heard. The claps themselves are not heard. Only one file is stored on the MP3 player USB
drive, and the player repeats the file.
CLAP
fox_20s_60s.mp3: File for 20 second transmission once every 60
seconds. This file was only used for
initial testing.
Welcome to
the hidden transmitter hunt. This is the
W1MJ 5-Watt fox. Please email your fox
hunting report to w1mj@arrl.net. Good
luck from W1MJ.
fox_12min_v1.mp3: This 12-minute file is normally used with the
fox. The following messages are
typically 20 seconds long and start roughly at 1 minute
intervals.
(1)
Welcome
to the hidden transmitter hunt. This is
the W1MJ 5-Watt fox. Please email your
fox hunting report to fox@w1mj.com. Good
luck from W1MJ.
(2)
This
fox uses a Baofeng UV-5R transceiver. The output power will normally be set to 5
watts but might be set to 1 watt on some hunts.
W1MJ
(3)
This
fox uses an MP3 player with a homebrew interface to the transceiver. Thanks go to K1MJC and K1PJW for their
contributions to the design. Photos and
technical details of this fox are available online at w1mj.com/fox.
(4)
I’m
considering a fancier fox design based around a Raspberry Pi. Features under consideration include:
·
limiting
transmissions to daylight hours, and
·
reporting
battery voltage.
If you have any suggestions, please let me know. Thanks.
W1MJ
(5)
This
fox has no logbook. Please email your
fox hunting report, your suggestions, or your complaints to fox@w1mj.com. Thank you from W1MJ.
(6)
Deployments
of this fox are announced on two groups.io groups: Northeast Massachusetts Fox Hunters and
Waltham Amateur Radio Association. W1MJ.
(7)
CW (18 WPM): I hope
you can find me DE W1MJ
(8)
Many
years ago, when most foxes were live humans, I put together an automated fox
with a reel-to-reel tape recorder. I
made the 2-hour tape sound like I was a live fox, and successfully fooled many
hunters. I openly admit that this fox is
automated. W1MJ
(9)
According
to the Transmitter Hunting article on Wikipedia, Transmitter Hunting is also
known as T-hunting, fox hunting, bunny hunting, and bunny chasing. Very interesting, but then again, one cannot
trust everything that one reads online.
W1MJ
(10)
Fox
hunting equipment can be very sophisticated but does not have to be. I hunt with only a walkie-talkie, a whip
antenna, and a 30 dB attenuator that I can put inline
with the antenna. I use the “body fade”
method and pay attention to signal strength.
W1MJ
(11)
I
usually fox hunt along with my dog Noah.
Once, Noah and I were just out for a hike in the Middlesex Fells
Reservation, and Noah suddenly broke into a run. I looked and saw that he was doing a more
traditional fox hunt. That fox got
away. W1MJ
(12)
CW (18 WPM): Good
luck finding the fox DE W1MJ
Appendix 1: UV-5R Interfacing Information
From http://www.miklor.com/COM/UV_Technical.php
From https://k0rx.com/blog/2017/11/baofeng.html
Measurements:
1.
On
UV-5R:
a.
All
3 connections to the 3.5 mm jack measure +3.3VDC against sleeve of 2.5 mm jack.
b.
All
connections to the 3.5 mm jack measure 0 VDC against each other.
c.
The
sleeve of the 2.5 mm jack is connected to Battery Eliminator (-) contact.
d.
Keying
radio with DMM from 3.5 mm jack sleeve to 2.5 jack sleeve, current measured
about -7 mA (unstable, but always negative {?}). See next item…
e.
Keying
radio with DMM from 3.5 mm jack sleeve to battery eliminator – (cigarette
lighter plug), current measurement depended upon setting of Radio Shack DMM; it
was either +0.5 mA or -1.5 mA.
2.
With
direct connection of iPad Ring/Sleeve to radio Ring/Sleeve:
a.
The
audio level seems reasonable (without attenuation).
b.
VOX
dropped out unexpectedly a few times with voice recording.
c.
VOX
stays on with a strong sub-audible tone added to the voice recording.
Appendix 2: MP3 Player Information
Description from Amazon
https://smile.amazon.com/gp/product/B07N1YMDK3
Use this DIY MP3 player/audio
decoder module to build your own MP3 player, Boom Box
or portable audio player. Connect directly to headphones, or pair the stereo
output of this player with an audio amplifier board to power your speakers.
Description:
- Plays MP3 audio files on a TF (MicroSD) card (the
memory card used in cell phones), or a USB drive in U disk playback mode.
- Superior sound quality, 3.5mm headphone jack to connect
headphones or to an external stereo amplifier. Onboard 2W mono amplifier
to directly connect a speaker (single channel only for monitoring, not
appropriate for stereo playback).
- Screw-type terminals to connect an external speaker (no
soldering).
- Can be powered with standard 5V USB power (a USB power
supply or power bank) through the Micro USB power connector. Can also be
powered by a 3.7V to 5.5V battery or external power supply (connected to
"Bat+" and "Bat-" terminals on either side of Micro
USB power input connector.
- Module designed to facilitate modification.
Product Overview:
- Supports MP3 format, automatically plays when TF card
or USB drive is inserted, onboard red LED indicates power and status
(solid red when power is on but not playing, flashing red when playing).
- U disk support (tested 32GB), TF card (tested 16GB)
Play mode. TF card is the power-on default mode, if
TF card is not inserted will play the USB drive. If both devices are
installed, you can manually set the playback mode, (see Button Operating
Instructions below).
- The buttons select Previous/Next tracks, Volume + -,
Pause / Play, Device Mode Switching (TF Card or USB Drive input), Repeat.
- Input power range: 3.7V to 5.5V
- Size: 45mm * 36mm (1.75" x 1.4")
- Double layer board design, large copper traces and
planes, stable and reliable.
Product information
|
Package
Dimensions |
6.5 x 4 x 1 inches |
|
Item Weight |
1.58 ounces |
|
Manufacturer |
Unbranded/Generic |
|
ASIN |
B07N1YMDK3 |
|
Customer Reviews |
|
|
Best Sellers
Rank |
#45,416 in Electronics (See
Top 100 in Electronics) |
|
Is Discontinued By Manufacturer |
No |
|
Date First
Available |
January 23, 2019 |
MP3 Player Measurements & Observations
·
Neither
Speaker terminal is connected to BAT- (GND).
·
The
sleeve of the 3.5 mm jack is connected to BAT- (GND).
·
When
playing an mp3, there is a blinking red LED.
·
Output:
o
Headphone
jack: Approximately 200 mV peak AC at
maximum volume, 0 VDC offset.
o
Speaker+
terminal: +2.5 VDC offset. Approximately 2V peak at maximum volume:
§ No output if plug
inserted in headphone jack.