This project is car hands free that retransmits the audio signal from a cell phone to the FM broadcast band. By placing the cell phone’s speaker near the microphone, the user can use the phone as a hands-free device while driving.
- Power supply: +2.7V to +5.5V stab – 2.1mA
- RF power: -10dBm
- Impedance: 50-75 ohm
- Frequency range: 70 – 150 MHz
- Integrated Varactor for Tuning
This project is based on MAXIM’s App Note 5123
This is a FM transmitter circuit with a direct audio input that eliminates background noise from the microphone. The electret microphone is biased with R1. The AC-coupled microphone audio signal modulates the VCO frequency by changing VTUNE. The output frequency of the VCO will follow the volume or amplitude from the microphone.
An audio signal with approximately 20mVRMS is adequate for functionality.
Do not over-modulate the VCO, or the resulting audio from the FM receiver will be distorted.
Over-modulating the VCO also weakens the carrier signal by emitting power onto the unused spectrum by the receiver.
In Figure, the microphone and R1 are replaced with a direct connection to the audio output port of a cell phone.
The cell phone’s volume needs to be adjusted accordingly for optimal performance.
A direct connection to the cell phone will eliminate background noise from the microphone.
Thanks Maxim for samples – by spilkos.
3D PCB rendering