How to connect MQ135 to other sensors?

To connect an MQ135 gas sensor with other sensors to your microcontroller (e.g., STM32, Arduino, ESP32), you essentially build a multi-sensor system where each sensor's signal is read through different GPIO or ADC channels. The MQ135 is an analog gas sensor (sometimes with a digital threshold output), so it's easy to integrate with others.

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1. Understand the MQ135 Sensor

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2. Basic Wiring with Other Sensors

Example: MQ135 + DHT11 (temperature) + Hall Sensor

Wiring Example:

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[ MQ135 ]    --> A0 (analog input)
[ DHT11 ]    --> D2 (digital GPIO)
[ Hall ]     --> D3 (digital GPIO or interrupt)
[ GNDs ]     --> Common ground
[ VCCs ]     --> 5V (if all support it) or use level shifters

Important: If you're using a 3.3V microcontroller (e.g., ESP32, STM32), and the MQ135 is 5V-only, use a voltage divider or level shifter on the analog line.

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3. Power Considerations

• MQ135 consumes ~150 mA during warm-up – don’t power it from a weak GPIO pin.

• Ensure your 5V rail (e.g., from USB or regulator) can handle total current for all sensors.

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4. Reading Sensor Data in Code

Example in Arduino-like pseudocode:

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int mq135_value = analogRead(A0);
int hall_value = digitalRead(D3);
float temperature = dht.readTemperature();

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5. Timing & Sampling

• MQ135 needs a few minutes warm-up for stable readings.

• Avoid reading all sensors at once—use timers or delays to balance CPU load.

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Summary

• Use shared GND and 5V (or 3.3V) rails

• Don’t overload ADC channels — 1 analog sensor per ADC pin

• Use voltage dividers if interfacing 5V sensor with 3.3V MCU