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ARDF Fox Transmitter
Controller Project... Another wrong turn on the
information highway... |
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| Above left is the SAW III voice recorder
kids toy available from All Electronics for $1.85 each if you buy 10 or
more. The toy has a record button and a play button. It will approx 30
seconds of pretty good quality voice. The round button is REC and the
triangle button is play. The little hole above the REC button is the
microphone... which is a small electret. Above right is the front panel of the FOX transmitter controller that uses the SAW voice recorder board from the toy. |
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 FIG 3 |
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| More about the SAW Voice Recorder: http://blog.makezine.com/archive/2008/07/how-to-circuitbend-a-chea.html | ||||||||||
| FIG 4
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| FIG 4 above... is the
"functional schematic" for the hidden transmitter controller.
Important note: The SAW does not control the transmitter. The SAW is
simply part of the controller that provides the ham radio call sign ID and
whatever other audio message that the transmitter hunters will be listening
to. OOOOPS... Q3 is labeled wrong. Sub G for
S. I built this contraption little by
little in 2007. Some of it was experimented with on a proto bread
board... some was on the fly on the circuit board in the foto. So now in
2011, after not thinking about this for a few years... The OUTPUT going HIGH (12 volts) drives the three 2N7000 MOSFETs that are small signal switches into conducting. 12 volts thru the 100K ohm current limiting resistor connected to the G lead causes the transistor to conduct from S to G. When Q1's G is driven HIGH (12 volts on the G lead) the D lead is grounded. This affectively places a 2.2K ohm resistor from ground to the Tip lead P1 which is the microphone plug for the handy talky. The ICOM and Radio Shack handy talkies are keyed up by placing a 2.2K ohm resistor across the T and R of the microphone jack. When Q2's G is driven HIGH (12 volts on the G) the D lead is grounded. This affectively places a GND condition on the PLAY lead of the SAW circuit board. Q2 is acting just like the PLAY push button switch in the SAW toy. This action plays the message. The SAW audio amp is designed to operate into the load of the speaker. This design removes the speaker and subs a 10 ohm resistor in its place. The 10K ohm resistor and 1K ohm pot act as a voltage divider and volume control to adjust the audio output of the SAW to be the correct level for the handy talkies microphone input across the 2.2K resistor The wiper arm of the 1K ohm pot is connected to a 10 uF capacitor to isolate the voltage from the handy talky microphone circuit from the SAW audio output resistors. Audio travels thru this capacitor, but not the mic DC voltages from the HT. When Q3's G is driven HIGH (12 volts on the G) the D lead becomes grounded. 12 volts from the power source travels thru the LED, the 1000 ohm current limiting resistor, to the grounded D lead. This causes the LED to light up when the 555's output is HIGH. The LED simply tells the operator when the HT should be keyed up. This part of the circuit can be left out if the builder desires. The SAW toy is powered by two AA batteries which equals 3 volts. The LM317 voltage regulator reduces the power source voltage of 12 volts down to 3 volts for the SAW. In this design... I used the original microphone and RECORD push button switch. The microphone was relocated to the front panel of the project box and silicon bathtub sealer glued into place. The SAW RECORD button was left in its original place and an access hole was drilled for access. Note that the REC button is recessed so it will not get bumped. Bumping the REC button for the smallest amount of time will erase the memory. In the Fig 3 foto... I removed the SAW circuit boards PLAY push button switch and simply wired to the solder pad a lead from the 2N7000 Q2 signal path. When removing the PLAY PB... be careful to remove the correct switch. I caused myself some grief by mixing up PLAY and REC and spent hours figuring out why the SAW didn't work right. S1 shown in FIG 4 is the REC/PLAY switch. This switch simply interrupts the Q2 transistor from going into the PLAY mode while making a recording. The FIG 3 foto shows two sets of switch contacts being used. The second set interrupts the operation of PTT to the Handy Talky. This part was left out of the FIG 4 schematic. One of the big factors in making the SAW work as part of the transmitter controller is the timer's time ON must match the length of the SAW's recorded message. The values shown in FIG 5 will cause the handy talky to be ON for 17 or 35 seconds. When I record the SAW's message... I usually make two 15 second messages in a row. Each 15 second message contains the transmitter's ham radio call sign. So, If I want to make the transmitter more challenging to find... I set the timer to 17 seconds ON/OFF and only the first 15 second message goes out over the air. When set to 35 seconds... then the SAW plays both messages in a row. Simple math... right? |
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FIG 5
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| FIG 5 is the
schematic for the LM555 astable oscillator. Note SW adds an extra 47
mfd to double the on/off time. Pin 3 is the OUTPUT lead which alternates between GND and +12 volts. Here's my favorite website for 555 info: http://www.kpsec.freeuk.com/555timer.htm |
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| HANDY TALKIES...
I use various handy talkies in my rabbit transmitters. They have
two things in common: They are 2 meters and they are old and donated
to my ARDF cause. The ICOM 2AT is my favorite due to its simplicity-
nothing digital, no LCD display, no internal battery to remember memories
(that I know of). The ICOM and Radio Shacks (and maybe others) are put in the XMIT mode by placing a 2.2K ohm resistor across the 3/32" microphone jack. The SLeeve of the jack is grounded. Once current flows thru the 2.2K ohm key up resistor... then audio from your SAW or other controller audio source can be applied across the resistor, but thru a DC blocking capacitor. My experiments show that a very small amount of audio is needed to properly drive the HTs mic jack. 5 to 7 mV AC RMS is what's needed for about 2 KHz of FM deviation (according to my old Heathkit FM Dev meter). If the HT is over driven with the audio signal... it will limit the audio... but it will sound distorted and possibly over deviate the HTs XMTR. I used my Fluke VOM to measure the audio level across the mic jack. Any AC volt meter that will measure millivolts should work.
My various HTs use different battery voltages to power
them. The ICOM 2AT uses 7.2 volts. But I have powered them on 12 volts
and they work... but for how long?
I dont wanna take a chance of
destroying it. I have a Radio Shack HT that was made to operate
on 4 AA batts... so this one needs 5 to 6 volts. I have a Radio Shack
HT that has a 12 volt jack on it... so this one for sure will run on 12
volts... and so on. If you inherit a used HT... it is best to power it
with whatever voltage the battery that came with it was. Most are 7.2
volts. Beware that it is important to operate the old HT on LOW POWER... the high duty cycle of the controller, say 35 seconds, it a long time for a HT's XMTR to be on. I've overheated mine on HIGH POWER... and they get very hot and sound distorted. This cant be good for them or the airwaves.
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I remove the old cells and
charging circuit (the guts) from the HT's battery. This leaves lots of
room for the LM317 voltage regulator circuit. I drop a red and black
wire out of the battery housing at a convenient length and attach Anderson
Power Pole connectors for the convenience of swapping this XMTR into / out
of this enclosure. You can use clip leads or wire nuts if you
like... but I like the
No Muss / No Fuss of the
Anderson Power Poles.
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| But wait!!
There's more: KR7W's favorite Fox controller is NOT the SAW
project above...
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| Updated last: June 12, 2011... KR7W | ||||||||||
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Introducing... New Generation Fox XMTR... from
A R L
