PIR Sensor Output Interfaces: Digital, Analog, Open-Drain Compared

Introduction

PIR sensors and modules offer different output interfaces. Choosing the right one and interfacing correctly is essential for reliable operation.

Output Interface Types

Push-Pull Digital Output

Output is actively driven HIGH to VCC and LOW to GND. Can source and sink current.

Examples: HC-SR501, HC-SR505, AM312

Pros: Simple, no external components, clean levels

Cons: May not be 5V/3.3V compatible without level shifting

Open-Drain / Open-Collector Output

Output transistor switches to GND when active; otherwise floats (high impedance). Requires external pull-up resistor to VCC.

Examples: Panasonic EKMB series, some industrial sensors

Pros: Easy level shifting (pull-up to any voltage ≤ max rating), wired-AND capability

Cons: Requires pull-up resistor, slower rise time

Analog Output

Continuous voltage proportional to IR signal. Requires ADC for reading.

Examples: Murata IRA series, some Panasonic variants

Pros: Rich information, enables advanced processing

Cons: Requires ADC, more complex firmware

Interface Characteristics

LOW (0V)

HIGH (VCC)

No

High-Z (pull-up HIGH)

LOW (0V)

Yes

Mid-scale

Varies

No

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Interfacing to Microcontrollers

Push-Pull Digital

Connect directly to GPIO if voltage levels match. For 5V sensor to 3.3V MCU, use level shifter or voltage divider.

Open-Drain Digital

Add pull-up resistor (typically 4.7k-10k) from output to MCU VCC. Then connect to GPIO. This automatically level-shifts because pull-up is to MCU voltage.

// Open-drain wiring
// Sensor OUT ────┬──── GPIO (input)
//                 └──── 10k resistor ──── 3.3V

Analog Output

Connect directly to ADC input. Ensure ADC voltage range matches sensor output range. Add RC filter if noise is present.

Voltage Level Considerations

5V Push-Pull to 3.3V MCU

Risk: May exceed MCU maximum input voltage (typically 3.6V).

Solutions:

• Voltage divider (2 resistors)
• Level shifter IC
• Choose 3.3V-compatible sensor (AM312)

Open-Drain Level Shifting

Open-drain is ideal for mixed-voltage systems. Pull-up to MCU voltage, and sensor’s open-drain transistor can handle higher voltage (check absolute maximum ratings).

Pull-Up Resistor Calculation

For open-drain outputs: R = (VCC – VOL) / I_sink. Typical values: 4.7k to 10k for 3.3V, 10k to 20k for 5V.

Special Cases

Panasonic EKMB Series

Digital versions have open-drain output requiring pull-up. Some variants offer analog output for raw signal access.

Excelitas PYD 2597

Features one-wire digital interface, which is a bidirectional communication protocol, not a simple output.

Troubleshooting Interface Issues

Open-drain without pull-up

Push-pull but reading wrong polarity

No pull-up on open-drain, or floating input

5V sensor connected directly to 3.3V pin

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Conclusion

Understanding output interface types is crucial for proper sensor integration. Open-drain outputs offer flexibility for mixed-voltage systems, push-pull is simplest for same-voltage systems, and analog provides maximum information.