315 MHz ISM Band
315.00 MHz
Core Technical Specifications
Real-World Devices & Applications
Automotive & Access Control
- Car Key Fobs — GM, Ford, Toyota US/Asian models (pre-2015 fixed code, newer rolling code KeeLoq)
- Legacy Garage Door Openers — Genie, LiftMaster 315 MHz DIP-switch and Security+ units
- Wireless Tire Pressure Monitoring Systems (TPMS) — sensors broadcasting at 315 MHz in North American vehicles
- Gate remotes and barrier openers — residential and light commercial US-market units
Home Automation & Security
- Legacy 315 MHz wireless door/window alarm sensors and PIR motion detectors
- Budget weather station outdoor temperature/humidity transmitters
- Older wireless doorbells and driveway alert sensors (US market)
Industrial & Utility
- Wireless relay control modules for pump/motor switching
- Simple telemetry links for water level and tank monitoring
Reference Circuits & Schematics
Transmitter Circuit (TX) — SAW Resonator 315 MHz Module
A typical 315 MHz ASK/OOK transmitter uses a SAW (Surface Acoustic Wave) resonator to set the carrier frequency. The SYN115 or generic FS1000A module accepts a DATA input driven by an MCU GPIO pin. When DATA is HIGH, the oscillator runs and radiates RF energy through the antenna; when DATA is LOW, the oscillator is inhibited — this is On-Off Keying (OOK) modulation.
| Pin | Description |
|---|---|
VCC
|
Power supply input, 3V–12V DC (higher voltage = greater range, typical 5V) |
GND
|
Ground / 0V reference |
DATA
|
Digital input from MCU GPIO — HIGH activates carrier, LOW silences it |
ANT
|
Antenna connection — ¼ wave wire (22.8 cm) or helical coil |
Circuit Walk-Through
The MCU (ATmega328P, PIC, ESP32, etc.) encodes the payload into a binary bitstream using a protocol like EV1527 or PT2262. Each bit is represented as a specific pattern of HIGH/LOW timing on the DATA pin. The transmitter module’s SAW oscillator generates a 315 MHz carrier when DATA is HIGH. The antenna radiates this carrier as short RF bursts — the on/off pattern IS the data. No mixer or IF stage is needed because OOK is the simplest form of amplitude modulation.
Receiver Circuit (RX) — Superheterodyne 315 MHz Module
The RXB6 or SYN480R superheterodyne receiver module demodulates 315 MHz OOK signals back into a digital DATA OUT stream. Superheterodyne receivers offer far better sensitivity (−110 dBm typical) and selectivity than superregenerative types, reducing false triggers from nearby interferers.
| Pin | Description |
|---|---|
VCC
|
Power supply input, 5V DC (some modules accept 3.3V–5.5V) |
GND
|
Ground / 0V reference |
DATA OUT
|
Demodulated digital output — connect to MCU timer interrupt pin or hardware decoder |
ANT
|
Antenna connection — ¼ wave wire (22.8 cm) soldered to ANT pad |
Circuit Walk-Through
The receiver’s RF front-end amplifies the incoming 315 MHz signal, then mixes it down to an intermediate frequency (IF) using a local oscillator. The IF stage provides most of the gain and selectivity. A comparator/data slicer converts the demodulated analogue envelope back into clean digital HIGH/LOW transitions on the DATA OUT pin. The MCU reads these transitions (typically via interrupt-driven timing) to decode the protocol frame.
Signal Structure & Waveform Analysis
OOK Frame Structure — EV1527 / PT2262 Protocol
A typical 315 MHz fixed-code transmission uses the EV1527 encoder IC format. The transmitter sends the same frame repeatedly (usually 4–8 times) to ensure reliable reception.
Analysis & Capture Tips
- Use RTL-SDR at 315.000 MHz with URH (Universal Radio Hacker) to capture raw IQ and auto-detect the modulation/bitrate
- Logic analyser on the RX DATA OUT pin: set trigger to rising edge, zoom to see individual bit timing at ~350 µs resolution
- Flipper Zero: Sub-GHz → Read → 315.00 MHz, or use RAW capture mode for unknown protocols
- HackRF or tinySA: sweep 314.5–315.5 MHz to confirm carrier presence and measure signal strength
- For rolling code (KeeLoq HCS301): capture shows incrementing counter in encrypted payload — cannot replay
Bench Troubleshooting & Repair Guide
1. Power Check
Multimeter (DC Volts)Verify coin cell battery voltage under load with a multimeter. A CR2032 dropping below 2.8V under the brief TX current pulse (5–15 mA) causes the SAW oscillator frequency to drift off 315 MHz, dramatically reducing range. Replace batteries that measure below 2.9V even if they read 3.0V unloaded.
2. Carrier Detection
Spectrum Analyser / RTL-SDR / OscilloscopeWith a known-good battery installed, press and hold a button while probing the ANT pin with a near-field RF probe, oscilloscope, or spectrum analyser. You should see a 315 MHz burst appear on screen. No signal = dead oscillator, cracked SAW resonator, or broken MCU. Use an RTL-SDR dongle as a cheap spectrum analyser: tune to 315 MHz in SDR# or GQRX and watch for the carrier spike.
3. Button / Microswitch Testing
Multimeter (Continuity / Resistance)Check each button’s contact resistance with a multimeter in continuity mode. A good tactile switch shows <1Ω when pressed. Carbon-pill/rubber-dome contacts degrade over time — clean the PCB contact pads with IPA and re-coat with conductive paint if resistance exceeds 50Ω. Replace microswitches that feel mushy or fail to click.
4. Crystal / SAW Resonator Inspection
Magnifying Loupe / Soldering StationVisually inspect the 315.00 MHz SAW filter (small metal can or ceramic package marked ‘315M’ or ‘R315A’). Check for hairline cracks following drops, cold/fractured solder joints, or corrosion. Reflow the SAW resonator pads with fresh solder. If frequency has drifted, the SAW must be replaced — they are not adjustable.
5. Antenna Verification
Ruler / NanoVNAConfirm the antenna is intact and the correct length. A ¼ wave monopole for 315 MHz should be approximately 22.8 cm (≈ 9 inches). Broken, kinked, or incorrectly trimmed antennas drastically reduce range. For PCB helical antennas, check for cracked traces. Use a NanoVNA to verify antenna resonance is centred at 315 MHz with SWR < 2:1.
6. MCU / Encoder IC Check
Oscilloscope / Logic AnalyserIf the SAW oscillator fires but no valid data is received, the encoder IC (EV1527, PT2262, HCS301) may be faulty. Check VCC and GND to the IC with a multimeter. Verify the DATA output toggles when buttons are pressed using an oscilloscope or logic analyser. For DIP-switch coded remotes, verify all address switches make proper contact.