A handy piece of test equipment to test FM transmitters

Kits are available from Talking Electronics.

This project is a handy piece of TEST EQUIPMENT.
You can never have enough assistance when designing FM bugs.
The first 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 of transmissions.
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 speaker.
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 diaphragm.
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.

MINI BUG DETECTOR - PARTS LIST

Buy kit

• 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

• 3mm dia coil

• 1 - 10cm 0.5mm wire for antenna

• 1 - Mini Bug Detector PC board

CONSTRUCTION

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 Artwork

IF IT DOESN’T WORK

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 the 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

FINALLY

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.