FM Transmitter with Digital Frequency Display using BH1417

Students study frequency modulation as equations — carrier, deviation, modulation index — but almost never transmit a real FM signal they can tune in on an ordinary radio. The classic student FM transmitter is a single-transistor free-running oscillator that drifts with temperature, hand capacitance and battery voltage, which teaches the wrong lesson about how broadcast FM actually works. This project does it properly: the Rohm BH1417F, a PLL-controlled stereo FM transmitter IC with a built-in stereo modulator, pre-emphasis, limiter and low-pass filter, holds the carrier rock-steady on a crystal reference. An ATtiny/Arduino-class controller programs the BH1417's frequency over its serial interface from a rotary encoder, and a 16×2 LCD shows the exact transmit frequency. The result is a stable, demonstrable stereo FM signal in the 88–108 MHz band that any nearby FM radio can receive — modulation theory made audible.

1. The controller reads the rotary encoder and programs the BH1417's PLL divider over its serial interface, locking the carrier to the selected frequency in 88–108 MHz.
2. Audio from the electret mic (via its preamplifier) or the AUX jack is selected and fed to the BH1417's audio inputs.
3. Inside the BH1417, the stereo multiplexer encodes left/right into the composite stereo signal with a 19 kHz pilot tone, applies pre-emphasis and limiting.
4. The PLL modulates the carrier with the composite signal, holding frequency stable against temperature and supply drift via the crystal reference.
5. The modulated RF passes through the matching network to the telescopic whip antenna and radiates as a standard FM broadcast signal.
6. Any ordinary FM radio tuned to the displayed frequency receives the transmission in stereo within the demonstration range.
7. The LCD continuously shows the carrier frequency so the receiver can be tuned exactly.

Tech stack:

• Rohm BH1417F PLL stereo FM transmitter IC
• ATmega328P / ATtiny controller (frequency programming)
• Rotary encoder with push switch
• 16×2 character LCD
• Electret microphone + preamplifier stage
• 3.5 mm stereo AUX input with level control
• Telescopic whip antenna + matching network
• Low-noise 5 V regulator, 9 V battery / DC adapter

• [BH1417F PLL transmitter IC] Rohm stereo FM transmitter with on-chip PLL, stereo modulator, pre-emphasis, limiter and low-pass filter — a genuine broadcast-band chip, not a drifting free-running oscillator.
• [Digital frequency setting] Rotary encoder selects the carrier anywhere in 88–108 MHz in 100 kHz steps; the controller programs the BH1417 over its 3-wire serial interface.
• [LCD frequency display] 16×2 LCD shows the exact transmit frequency plus input-source and stereo indicators.
• [Microphone input] Electret condenser microphone with preamplifier for live voice transmission.
• [AUX line input] 3.5 mm stereo jack accepts a phone or laptop as the program source, with an input level control.
• [Whip antenna with matching] Short telescopic antenna with a simple matching network, sized for classroom-scale demonstration range.
• [Regulated power] 9 V battery or DC adapter through a low-noise regulator, with reverse-polarity protection.
• [Labeled enclosure] Vented enclosure with a printed front panel: frequency display window, encoder, input jacks and power switch.

• Assembled transmitter PCB with BH1417F, controller, LCD and encoder
• Electret microphone, AUX cable and telescopic antenna
• Vented enclosure with labeled front panel
• Controller firmware source (frequency programming, LCD, encoder)
• Schematic, PCB layout notes and RF layout guidelines document
• Regulatory-compliance note for low-power educational transmission
• Project report PDF, PPT presentation and viva Q&A document