FM bug transmitter with 500 meters range | applied electronics engineering


FM bug transmitter with 500 meters range

By Applied Electronics - Friday, February 3, 2017 No Comments
In this tutorial we show a FM bug transmitter with 500 meters range. You can build this FM bug transmitter yourself at home. To make the transmitter range long it is powered by 6 Volt battery. By using 6V battery, the RF output power is larger and hence larger distance can be covered.

If you like radio transmitter and receiver circuit then see also How to make a simple one transistor radio at home?

Circuit Diagram of FM bug Transmitter with 500 meters range

 The circuit diagram is shown below.

FM bug transmitter with 500 meters range
Fig: FM bug transmitter with 500 meters range

 The circuit works as follows. At the input we have the electret microphone. It senses the sound from the environment and amplifies it(about 100 times). The sensed audio signal passes into the first stage amplifier made by the first 2N3904 transistor via the 22nF capacitor. The purpose of this amplifier is to amplify the signal. It amplifies the signal 70 to 100 times the input.

This amplified signal next goes into the RF amplifier stage. This RF amplifier made up of the 2nd 2N3904 transistor is designed to operated in the vicinity of 88MHz. The output power from this RF amplifier stage is 10mW. With this power you can reach several hundred meters.

To improve the efficiency an RFC(Radio Frequency Choke) is used. It is made by 8 turns coil and placed between the LC tank and the positive supply rail. The inductor in the LC tank is made up of the 5 turns of 0.5mm wire diameter and inductor diameter of 3mm. See Matlab Code for LC circuit for FM radio to see how the frequency can be calculated for given inductor dimension.

See also How AM modulation circuit works.

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