The "Good Buddy" Car Bug

tracking/audio transmitter for 27 MHz

The "Good Buddy" Car Bug is a small low powered transmitter capable of transmitting a pulsed tone or audio on the CB radio frequency band. It was designed to help locate stolen trailers, cars, and boats which would be transported across the highway system. The owner of the stolen vehicle could notify other CB'ers to be on alert for the signal and to notify them if they heard it. Because of it's low power the active range would be limited to the general vicinity of the transmitter and therefore would not be a nuisance to normal radio traffic. Additionally, the transmitter can be switched between "pulsed tone" and "audio". If switched to "audio" the transmitter will transmit any sound in it's immediate area. In this configuration the transmitter can be used to monitor activity in other remote areas such as attached trailers, campers, or any areas that need some form of security surveillance.

Transmitter range is a solid 100 meters and with a good receiver and antenna system may reach a mile. The transmitter will overcome the highest "squelch" at 20 to 30 meters making it both noticeable and memorable to anyone passing the transmitter on the highway. Given that many commercial trucks crisscross the country several times every month the odds are very good that the transmitter will be located, especially if a reward is offered.

Theory of operation: (please refer to schematic)

The "Good Buddy" Car Bug is made up of three sections; a pulsed tone generator, an audio amplifier, and a transmitter.

The pulsed tone for the tracking transmitter mode is made from a single 4049 Hex/Inverter (IC1) integrated circuit. Two of the inverters are configured to set the pulse rate which can be adjusted from a few tenths of a second to several seconds by a 1 megohm potentiometer (R1). The pulse rate stage is connected to the tone generating stage by a diode (D1). The tone stage consists of two more inverters whose frequency is set by a 500 ohm resistor (R3) and 1mf capacitor (C2). The final two inverters of the IC are paralleled together to provide extra power to the transmitter. A 2.2 mf capacitor (C3) is attached to the output of the inverters for wave shaping. This capacitor produces a wave similar to a sine wave which modulates the transmission.

The audio stage is composed of an Electrax mike (M1), transistor (T1) and LM386 audio amp (IC2). The mike element and transistor provide preamplification to the LM386 which drives the transmitter.

The transmitter is a simple one transistor design. The frequency is selected by selecting the desired CB transmit crystal . Only one coil needs to be hand wound and consists of approximately 1.5 turns at 1/8 inch diameter. Any small diameter (23 to 26 gage) insulated wire can be used. The other coils are miniature epoxy covered chokes. The antenna is 8 inches and is tuned by a 12-100 pf capacitor (C8) in series with a 6.8 microhenry choke (L3). Longer antennas can be used and tuned by adjusting the 12-100 pf capacitor. An eight inch wire is connected at the junction of C8 and L3 and folded into a rectangle and positioned at 90 degrees to the circuit board opposite the antenna. The other end off the wire is attached to ground. This wire provides a ground plane for the antenna and greatly increases the range of the transmitter.

The crystal is a CB transmit crystal (RS 21-12xx). If you are interested in enlisting the help of truck drivers and other travelers then a channel 19 crystal would probably be the best choice because it is the most commonly used channel on the highways. If range is the consideration then channels1 to 5 probably have the least noise and provide the best low level signal detection.

Switch S1 is DPDT and is used to select "tone" mode or "audio" mode. Switch S2 is the "on/off" switch. Other types of motion switches can be substituted for S2 if desired. A panel mounted phone jack (RS274-292) makes an excellent switch by using the open frame closed contact terminals. When the plug is inserted into the phone jack the contacts are held open and when the plug is removed the contacts close. Other types of switches can be used depending on your application including magnetic door and window switches and "lever" switches. Any normally open switch that can be configured to close when the protected object is moved will work.


Please refer to the schematic and parts layout diagrams. The circuit can be easily constructed on a modular printed circuit board like RS 276-174, which can then be easily mount in a small plastic box. Constructing the circuit as described works well but is not the only possible configuration. It, for instance, could be made significantly smaller by compacting the parts placement and using 1/8 watt resistors and small capacitors.

1. Construct the Pulsed tone portion of the circuit as shown in the diagram.

2. Test the pulsed tone portion by connecting a 9volt battery to the wire from pin 1 of IC1 and pin 8 of IC1. Measure the pulse rate and frequency with an oscilloscope or hook a small 8 ohm speaker between pin 6 of IC1 and negative (-) battery. You could, also, use a radio headset and some test leads in place of the oscilloscope if you like. You should observe a pulsed audio tone whose interval varies by adjusting R1.

3. Build the transmitter portion next as shown in the diagram. The antenna can be any piece of stiff 8 inch wire. One approach is to mount a banana jack (RS 274-725) on the outside of the enclosure containing the circuit and solder a 7 inch antenna into a banana plug. Connect the antenna lead from the circuit board to the banana jack and then plug the banana plug and antenna into the jack. This allows for the enclosure to be water resistant. Be sure to connect the 8 inch wire "ground plane" as shown and position it 90 degrees to the circuit board and on the opposite side of the antenna.

4. Connect the battery connector and switches as shown. You can fashion a homemade "slip switch" by overlapping to stiff wires for Switch S2 and slipping a small flat piece of plastic between the wires . You can also, use any number of SPST manufactured switches.

5. Using your CB receiver, tune the 12-100 pf capacitor for maximum signal (you will have to disconnect your receiver 's antenna for this adjustment).

6. After confirming operation, disconnect the battery and install the audio portion of the circuit as shown.

7. Position the circuit in a small box to protect it from the elements.

8.. This completes the construction of The "Good Buddy" Car Bug.

Locating parts:

The part are available from many sources. Here are a few:

Radio Shack................. just about everywhere
DC Electronics............. 1-800-467-7736
Mouser electronics....... 1-800-346-6873

Mouser has the coils (part # 43LS106 and 43LS686) and the electret mike element (part#25LMO44) as well as all the other parts needed except the CB crystal.

DC Electronics has the coils (same part number as Mouser) plus they don't have a minium order.

The crystals are from Radio Shack

If you want to wind your own coils you might try the following formula.

L = a2n2/(9a+10b)
L = inductance in microhenrys
a = coil radius in inches
b = coil length in inches
n = number of turns



Pars list:
circuit board... RS 276-174
IC1....4049 Hex Inverter/driver
IC2.... LM386 Audfio amp
R1.....1Megohm Pot
R2..... 1Kohm
R3..... 500 ohm
R4..... 1K ohm
R6..... 680 ohm
R7.....10Kohm pot
R9..... 3.3Kohm
R10.... 100 ohm
R11.... 470Kohm
C1..... 4.7 mf
C2.....1 mf
C3..... 2.2mf
C4..... 2.2mf
C5..... 0.1mf
C6..... 4.7mf
C7..... 0.1mf
C8..... 12 - 100 pf adjustable
C9..... 6.8pf
L1..... 1 to 2 turns 26 wire 1/8" dia
L2..... 1 microhenry choke
L3..... 6.8 microhenry choke
T1.....MPS2n4401 RS 276-2058
T2.....MPS2n4401 RS 276-2058
M1..... Electret mike element RS 276-092
XTAL. CB radio transmit crystal RS 21-12xx
D1..... 1n914 or 1n34A (just about any diode will work)
S1..... DPDT switch RS 275-626
S2..... just about any SPST switch

return to home page

E-Mail to:

Copyright ©2001, Jerry Baumeister
Revised - 1/27/2001