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hidden unit. Amateur roc ketee rs should find
this circuit ideal for use in larger-sized
rockets. A miniaturized Beeper located in
th e nose-eone of a downed, lost ro cket
would aid grea tly in fi nding it.
The Beeper emit s a tone-mo du lated six
met er carrier which cycles off an d on . The
heart of the unit is th e trans mitter which
consists of a 2N706 overtone oscilla to r
followed by a 2r\708 a mplifier. Th e transistors
used in t he transmitter are not critical.
Literally any NPN rf transistor can be used
with good results. I tried an epo xy transistor
from a scrapped uhf tuner in the final. The
outp ut increase d 25 %. The output is normally
100 milliwatt s or better.
Even though the duty cycle on the
transistors is light in th e Beeper due to th e rf
cycling, th e transistors do get quite warm if
th e flasher circ uit is jumped out of the
circ uit. In an y case, a simple heatsink can be
made for the amplifier wit h a pair of tin snips
and some aluminum stripping. The crystal
oscillator uses a 50 .385 MHz overtone crystal.
I was able to get several from a surp lus
cry stal outlet for $ 1.60 each . The cry stal I
used doesn't use a socket , bu t is wired into
th e circuit. Leave at least 1/2 inch of wire on
each lead and use heat sinks when soldering .
Excessive heat will ruin these crystals and an
ounce of care may save you $1.60 plus
postage. Sockets were used for t he transistors
in my unit since I wanted to try th e
several kinds I had on han d to find which
would give the best results.
The modulat or is a mult ivibr ator cou pled
int o the base of the 2N7 06 oscillator. The
modulation was judged to be in the neighborhood
of 75 %. Unfortunately, I was
u nable to get enough rf to give a rea dable
pattern on a scope. However, if the signal is
stro ng enough to move th e S-met er, it can be
heard.
Both th e multivibrat or and t he t ran s­
mitter der ive th eir operating volta ge from
th e output of a light flasher circuit developed
by RCA. The flasher circuit consists of
transistors Q I , Q2, Q3, Q4, and t heir asso ciat
ed parts. Notice th at while the flash er
circuit uses PNP trans istors, th e transmitter
and modulator use NPN's. Since the ou tp ut
of the flasher is nega tive, the flasher mu st
brea k the negative grou nd return of th e
modulator and transmitt er. The rate of th e
flasher can be 'changed by adjusting th e
values of Cl and C2 until th e desired rate is
found . An increase in capacitance will
increase the time inte rval betwee n beeps.
The length of the beep itself may be changed
by changing the value of C2. If you desire to
vary the ton e of th e beep , changing the
values of C4 and CS will do the trick. A
word of caution - mult ivibrators are inherently
rich in har monics and , if you _are not
careful, you can get a pretty wide carrier ou t
of -the Beeper. With the values shown, the
Beeper will be modulated by a to ne of about
.5 kHz with a bandwidt h of less t han 25 kHz.
Transformer T I tends to limit the high
frequency response elim inating serious spurious
sidebands.
The prototype was constru cted on a
springboard -to aid in designin g and par t
su bstitution. The finishe d unit was built on a
small piece of Vector board. Care should be
taken to provide a good ground fo r the
trans mitt er stages. They are slightl y regenerative
and poor grou nding may set up a
feedback loo p. The best way to build the
unit would be for the more industri ou s ham
to make his own circ uit board. Since mo st of
us are weekendexperimenters this approach
may not be too practical.
To facil itate tuning up the transmitter,
the flash er circ uit sho uld be bypassed .
Connect a number 47 pilot light to the rf
output jack; this will serve as a dummy load
and rf indicat or. For th e initi al tuning, no
more th an 9V ~ho u ld be used . First tune the
oscillator (CI O) for max imum as indi cat ed
on a nearby receiver, grid dipper (in t he
wavemeter mo de) , or a sensitive field
strength met er. As you increase the capa citance
, the output will become greate r and
then suddenly quit. Che ck this on a receiver,
as th e oscillator may break into spurious
oscillation. When the poi nt is found where
the outp ut IS great est, tu rn the capa citor
back 1/8 turn; thi s will insure t hat th e
oscillator will start after the voltage is
removed and turned on again .
Aft er the oscillat or is tuned, pro ceed to
adjust C8 and C9 for maxim um outp ut as indicated
on the 47 bulb. Th e final t uning may
int eract with the oscillator so you may have
to adjust both until you get the desired
58
73 MAGAZ IN E