A handy piece of
test equipment to test FM transmitters
Kits are available from
project is a handy piece of TEST EQUIPMENT.
You can never have enough assistance when designing FM bugs.
thing you need to know after putting a bug together is the fact
it is transmitting.
This project gives you this indication.
Once you know a bug is transmitting, you can work on getting the best output
power and adjusting the frequency to a blank spot on the dial.
This Mini FM Bug Detector is basically a broad-band receiver. It
picks up the complete band from below 80Mhz to 150MHz and almost anything that transmits
in that band
will be detected. If a bug with a sensitive microphone is transmitting
nearby, the result will be a feedback whistle.
The Bug Detector will not determine the output power of a bug as it is detecting the
sensitivity of the microphone.
However it will prove the carrier section is working (this is the section
that produces the 88MHz frequency) and also the audio section.
We have other pieces of test equipment to test the output power:
LED power meter,
Field Strength Meter Mk I, and
Field Strength Meter Mk II.
We use all these items when testing bugs and that's why we designed them.
HOW THE CIRCUIT WORKS
The circuit is basically a broad-band receiver and will pick up all types
The quiescent current for the circuit is less than 2mA and the output is a mini piezo diaphragm.
Note: the circuit will not drive a
Each stage provides high gain and the signal is picked up by an un-tuned
front-end. You can see the front end is untuned as the inductor
does not have a capacitor across it.
You would think the 20 turn inductor would create a short-circuit to
ground. But signals at 100MHz will oscillate in the inductor and can be
picked off via the 470p capacitor. Signals of a low frequency will be
taken to ground.
The 100MHz signal is amplified by the first two stages and the audio
component is detected by the diodes on the base of the third transistor.
The fourth transistor is an audio amplifier to drive the piezo
The inductor across the piezo provides a load for the transistor and
creates a high voltage during part of each cycle to increase the volume.
The 22n capacitor removes the high frequency component of the signal.
The 47u across the power supply improves performance of the circuit by
preventing signals from the output stage passing to the front end.
1 - 1k
1 - 2k2
2 - 3k3
1 - 47k
1 - 220k
2 - 1M
1 - 470p
4 - 22n
1 - 47u 16v single ended electrolytic
2 - 1N 4148 signal diodes
1 - 2N 3563 transistor
3 - BC 547 transistors
1 - 3mm red LED
1 - mini piezo
1 - 10mH choke
1 - 22mm piezo diaphragm
1 - mini slide switch
1 - 3v lithium cell
1 - lithium cell holder
1 - 20turn 0.5mm enameled wire
1 - 10cm 0.5mm wire for antenna
1 - Mini Bug Detector PC board
All the components fit on the PC board supplied
in the kit. The length of the antenna is not critical and the dimensions
of the inductor at the front-end are not critical either. Both these items
are supplied in the kit.
Mini Bug Detector
IF IT DOESN'T
If the circuit doesn't work, you will need another piece of test equipment
to test it!
We have designed the LogicProbe/Pulser
and Mini Bench Amplifier
for this purpose.
These two pieces of test equipment will help you work on the audio (the
front end) of the circuit.
Additional details on locating a fault in a project can be found in the
two projects mentioned above.
USING THE MINI FM BUG DETECTOR
Switch the project ON and place it near the FM bug you are testing.
You will hear some background noise, called "white noise." Turn
bug ON and you will get a feed-back whistle.
If the background noise becomes very quiet but no feedback squeal is
heard, it will indicate the bug is producing a
carrier and the 88MHz section is working, but the microphone or one of the
audio stages is not working.
Mini Bug Detector Photo
This project is so simple, it can be put together in less than an hour and
provide a very valuable piece of equipment to help when building an FM transmitter.
You can also use it to find a transmitting bug.
Simply use the detector in a sweeping motion and when you are close, you
will hear a feedback whistle.