A simple FM transmitter can have many interesting uses. This project is a wireless bug. It provides the ability to listen remotely when used with an FM receiver. Its range is not great, but it should perform well enough at least to impress your friends or enjoy some practical joking. Assembly is quite simple, using readily available parts. Once you complete this transmitter, you will have learnt some communication technology basics. You will then be ready to build a more complex transmitter.
- Skill level:
Things you need
- 74F13 integrated circuit (IC)
- Solderless breadboard
- 1nF capacitor
- Jumper wires
- 33pF variable capacitor
- 100nF capacitor
- 22K Ohm resistor
- Electret microphone
- 1-yard wire antenna
- 10uH coil
Examine the circuit diagram in Reference 2 under "FM Bug Transmitter."
Install the integrated circuit (IC) in the breadboard.
Select the power rail and ground rail. These are the isolated rows on the breadboard to either side.
Install the 1nF capacitor (C1) between the ground rail and a spare row.
Run jumpers between the lead of C1 in the spare row and Pins 1 through 6, Pin 9 and Pins 12 through 14.
Put a jumper between the junction of these connections and the power rail.
Place the 33pF variable capacitor (C2) between the same junction and ground.
Connect Pin 7 to ground with a jumper.
Connect a 100nF (C3) to the power connection of the IC.
Place a 22K resistor between C3 and the power rail.
Connect the wires of the electret mic to C3 and ground.
Attach the antenna to Pin 8.
Place the coil between the antenna and the power rail.
Insert the battery in its holder.
Connect the red battery wire to the power rail and the black wire to the ground rail.
Tips and warnings
- For real portability try building the transmitter into a small piece of veroboard.
- Never listen to anyone's conversation without his permission. This is an invasion of privacy and is illegal.
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