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More SquawkBox Information


Major Features
Specifications
Description
Applications
General Profile
The Voice Chip
The PIC Micro
The PLL Chip
More Information

 

Major Features


  • Miniature VHF/ FM hunt transmitter : 146.565 MHz, 50 mW RF out
  • SMT components and small size : 0.9 x 2.4 inches
  • 60 second voice record/playback chip
  • Built-in electret mike and RECORD button
  • Operates from 7-15 VDC, 60 mA while transmitting
  • Strap selectable repeat times, 15 seconds to Ĺ hour
  • PLL synthesized transmitter with CPU crystals and strap frequency select
  • AUX outputs for PTT and message audio ( for an external transmitter )
  • Power-down retention of voice message ( non-volatile message storage )


    
    Specifications
  • Physical Dimensions : 0.9 x 2.4 inches, SMT components
  • DC Input Power: 7-15 VDC, 60 mA transmitting, 5 mA idle

  • RF Output Power: 50 mW into 50 ohms. Wire pads provided for RF output

  • AUX Outputs : PTT ( active = gnd ) and message audio
  • Frequency Range : 144-148 MHz, strap / crystal selectable ( with some limitations, see text )
  • Message Duration : 60 seconds maximum record / playback capacity Actual TX time = message duration + 1 second of silence
  • Repeat Intervals : Strap selectable, 15 seconds to Ĺ hour in 16 steps
  • Chips : Microchip PIC 16F84 micro, for supervision ISD Corp. ISD2560 voice record / playback chip ICST Corp. ICS525 PLL CPU clock generator chip 78L05 voltage regulator
  •  

    Description


    The SquawkBox T-hunt transmitter is a unique low - power transmitter, intended for service in recreational 2-meter VHF amateur T-hunts. The SquawkBox transmitter is so named because it contains a voice record / playback chip and an integral electret microphone ( with a record button ) which allows a verbal "hidden T" message to be recorded ( or re-recorded ) at any time.

    Designed to promote T-hunting activities by clubs and organizations, the SquawkBox provides an alternative to "standard" T - hunt transmitters, which must be adapted from expensive / bulky mobile or portable transceivers that were never really intended for T - hunt service. In addition to the voice record / playback abilities, it is much smaller and lighter than "adapted" hunt transmitters, and offers the possibility of some very interesting and unusual hunts... run one up a flagpole, tie one to a kite, etc.


    The use of a voice record / playback chip to make ( hidden - T ) "announcements" means that the control operator for the hidden - T does not have to approach the transmitter to ( personally ) make these announcements, and can "stand off" a short distance from the transmitter, while the hunt is in progress. The transmitter will ( therefore ) remain concealed from nearby hunters, even if the control operator is discovered. ( in most hunts, the objective is to find the transmitter, not the operator )

    The ability to record new messages ( at any time ) also means that "hints" or "clues" can be provided as the hunt progresses, if the hunt proves to be too difficult for the participants. The time interval between transmissions can also be reduced as the hunt progresses, ( with some external switches) to further accelerate the hunt.

     

    Applications


    The SquawkBox is very simple to use, but it is also very agile... it can be modified by the end-user ( with suitable skills ) for a variety of applications that are not directly related to T-hunts :

    
    	1.  Auxiliary outputs are provided for the PTT and message audio signals, so that an external 
                               transmitter can be used with it, to allow operation at higher RF power levels, or on other 
                               frequencies or bands.
    	2.  It is also suitable for a "CQ Contest" message generator, to preserve the operatorís 
                               voice in ( 48 hour "marathon" ) amateur contests.
    	3.  It can also serve as a low-power simplex repeater. ( some PIC code changes may be required )
    	4.  It can serve as an "announcement" transmitter at conventions or amateur festivals, to announce 
                               venue and/or schedule changes, lost-and-found items, special events, raffle results, etc.
    	5.  It can serve as a repeater "cuckoo clock" or as a remotely re-programmable ( voice ) 
                               repeater identification / announcement generator. ( some PIC code changes may be required )
    

    There are probably several other applications, limited mostly by the userís imagination. For example, one fellow ripped the battery pack out of a Polaroid film pack ( = flat, and only 1/8 inch thick ) and taped the whole transmitter, battery and antenna to a plastic rail fence, using plastic tape with identical color as the fence railsÖ virtually invisible, even standing directly next to it.

    In summary, the SquawkBox transmitter is a really slick, very sophisticated little transmitter, with a truly unique combination of features and virtues. In the spirit of social responsibility, they will be sold only to operators holding a valid amateur license, and shipped only to the "address of public record" for that operator. For those inclined to more ambitious adaptations, technical information will be provided ( to users ) who wish to adapt the SquawkBox to some other task.

     

    General Profile


    The ( standard ) SquawkBox transmitter generates 50 milliwatts of RF power on 146.565 MHz, ( U.S. National T-hunt frequency ) and measures 0.9 x 2.4 inches. Operation on other frequencies ( with some limitations ) is possible by changing 16 programming straps for the PLL chip and / or changing the PLL crystal. ( more information later )

    Current draw is 60 mA transmitting and 5 ma "idle". A fresh 9V alkaline battery will provide reliable operation ranging from 10 hours to several days, depending on the message duration and the transmit repeat interval. The transmitter is constructed entirely with SMT components on a high - quality, commercially manufactured PC board with through plated holes and a tin/lead reflow finish.


    The voice chip ( IDS2560 ) has a 60 second recording capacity, and a "non - volatile" message memory, so recorded messages will remain intact even if power is interrupted for prolonged periods. ( according to the spec sheet, 100 years ! )

    A PIC 16F84 microcomputer supervises the operation of the voice chip and transmitter. The PIC uses a simple RC oscillator, ( in the "standard" transmitter ) but crystal control for the PIC is available if "high precision" repeat intervals are ( for any reason ) necessary.

    A fully integrated ( crystal controlled ) PLL synthesizer chip ( ICS525 ) generates the RF signal, and a type Pi - L output network ensures good spectral purity. An on-board 78L05 voltage regulator allows reliable operation down to 7 volts of input power.

    Repeat time intervals are selected with 4 straps, yielding 16 different time intervals, ranging from 15 seconds to 30 minutes. Transmit time equals the duration of the voice message, plus one second. if the message time "overruns" into the interval for the next transmission, the message is immediately repeated upon completion of the prior message. ( no message truncation, no "skipped" transmissions )

    Auxiliary outputs are provided for the PTT and voice signals, to allow operation on other frequencies, ( or bands ) or at higher RF power levels, with an external transmitter.

     

    The Voice Chip


    The voice chip ( ISD2560 ) has a very effective ALC circuit. The electret microphone has very good frequency response and sensitivity, and the audio quality is surprisingly good ! Different versions of this voice chip have recording capacities as high as 2 minutes, and are "drop-in" replacements for the one employed in this design.

    According to the ISD2560 datasheet, multiple voice chips can be "cascaded" to achieve messages with longer durations. This could ( probably ) be achieved by simply "stacking" multiple chips on top of the original one, and tying most of the pins ( but not all ) directly to the pins on the original chip.

    These chips also allow record / playback of multiple ( independent ) messages, but that feature is not exploited in this design. In fact, there are a LOT of features in the ISD2560 that are not exploited in this design... it is intended primarily for service in a telephone answering machine, or a "voice mail" telephone system.

    For people with technical skills, these features could be exploited with some "re-work" of the SquawkBox transmitter... Utilizing these features would require some study of the ISD2560 datasheet, probably some changes in the PIC code, and possibly some cuts / jumpers on the PC board.

     

    The PIC Micro


    The PIC 16F84 micro is very popular with hobbyists, and lots of folks know how to use them. It employs a FLASH program memory, which can be electrically re-programmed without the need for a U/V EPROM eraser. In fact, it can even be re-programmed without removing it from the PC board... we have provided five programming "pads" on the PC board, for this purpose. ( thatís how we program them... )

    These five "ICSP" pads ( In-Circuit Serial Programming ) allows direct connection of the PIC chip to an external PIC programmer. This method also eliminates the need to buy a fairly expensive chip "adapter" for the ( SOIC footprint ) PIC chip. ( typically over $50 for the adapter )

    An "adapter cable" can simply be made with 5 wires, which run from the ICSP pads on the PC board to an 18 pin DIP plug. The DIP plug serves to emulate the DIP "footprint" of a conventional 16F84 micro, for insertion into the programmerís regular DIP socket.

    Furthermore, the ICSP pads are arranged in a straight line with exactly 0.1-inch separation between adjacent pads. This allows a simple ICSP "tool" to be fabricated, which ( if employed ) will eliminate the need to directly solder the 5 ICSP wires to the PC board.


    The ICSP "tool" can be made with 0.1-inch "perf board", and ordinary sewing / safety pins or needles. The pins / needles can be bent / formed to create 5 spring-loaded contact "fingers", which are then anchored to the 0.1-inch perf board. Pressing the 5 contact fingertips into the mating holes ( in the ICSP pads ) will allow quick connect / disconnect of the ICSP cable.

     

    The PLL Chip


    The ICS525 PLL chip is actually designed for service as a "clock generator" for todayís high - speed computer CPU chips. As such, it is "adapted" for service as a radio transmitter, and has some limitations, mostly in its channel capacity... not all ( possible ) radio channels can be achieved with a particular crystal, simply by changing the programming straps.

    In many cases, a different ( standard - value ) "CPU" crystal can be used, which will put a particular ( desired ) radio channel within range of the PLL chipís programming straps. We have done computer studies of this question, and can recommend a suitable ( standard value ) CPU crystal ( and straps ) for about 80 percent of the available 2 - meter VHF channels.

    The PC board has pads to accept either SMT or "cylinder - type" crystals, to allow the use of a wider variety of crystals. All components in the oscillator circuit are selected for zero - temperature coefficient, and independent tests from one of our users indicates the temperature drift is less than 500 Hz across a range of 100 degrees Farenheit.

     

    More Information


    Clicking on the link below will trigger a download of a ZIP file. ( 78 KBytes ) It contains several technical documents for the SquawkBox transmitter :

    SquawkBox Technical Information

    The zip file contains :

  • Schematic Diagram
  • Board Pictorial
  • External connections
  • Frequency straps
  • Timing straps
  • Crystal / PLL strap List

    Further inquiries can be directed by e-mail to Bob Simmons.