This FM wireless tranJmitter produces a beep that can be tuned on any dead
point of the FM broadcast band. The circuit can be used as a localizer when
attached to objects or persons. You also can use it as a wireless alarm.
The signal can be tuned on common FM radios in a range up to 150 feet.
Alterations for a far-reaching version are given below.
The circuit produces an intermittent beep of about 1 Hz. An audio tone is
generated by lClb (pins 5, 6 and 4) and its frequency is determined by R2
and C2. You can easily alter this frequency by adjusting R2. Resistor values
between 22,OOO and 100,000 ohms can be experimented with or, if you
prefer, 100,000 ohms in series with 10,000 ohms can replace R1.
Repetition rate is per R1 and C1. Values are variable. For C1, values between 0.15 and 1 uF can be tried. lt is important to test for the ideal value of
this component, depending on the intended application.
The schematic diagram of the wireless beeper is shown in Figure 1. Three
gates of a 4093 lG are used to generate the low-frequency signals, and one
RF transistor is used to generated the high-frequency signal.
Components placement on a homemade printed-circuit board is shown in
Figure 2. Placement is critical for the high-frequency stage. Capacitors in this
stage should be ceramic.
L1 consists of four turns of No. 18 or 20 wire on a1l2 in. form without aferrite
core.
The antenna is formed by a piece of bare wire between 5and 10 inches long.
It can be connected to any tap on the coil. Experiment to get better performance.
The power supply current drain is about 10mA.You can use four M-cells or
a 9V battery to power the transmitter.
For a wireless alarm, 51 can be replaced by the contacts of a relay. The relay
is used to interface the alarm with the transmitter in this application.
Another way to interface an alarm with the transmitter is to connect relay
contacts in series with C3 or between points A and B in the schematic. See
more detals in ldeas to Explore.
Parts List -Wireless Beeper
lC1 - 4093 CMOS Integrated Circuit
Q1 - 8F494 or equivalent NPN RF silicon transistor
L1 - coil - see text
CV - trimmer capacitor - 20 to 40 pF maximum
R1 - 2,200,000 ohm, 114W,5% resistor
R2 - 39,000 ohm, 1/4W, 5% resistor
R3 - 8,200 ohm, 114W,5% resistor
R4 - 6,800 ohm, 114W,57o resistor
R5 - 47 ohm, 1/4W 5% resistor
C1 - 0.22 uF ceramic or metal film capacitor
C2 - 0.047 uF ceramic or metal film capacitor
Cg - 0.01 uF ceramic or metal film capacitor
C4 - 4,700 pF ceramic capacitor
C5 - 4.7 pF ceramic capacitor
CG - 0.1 uF ceramic capacitor
A - antenna - see text
S1 - SPST slide or toggle switch
81 - 6 to 9V - four AA cells or 9V battery
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