Device assembly

bcMeter V1 Assembly Documentation (v02, January 2025)

LED and Sensor Brackets

The orientation of both the 3D printed infrared LED and sensor assemblies has been simplified to reduce orientation mismatches.

The infrared LEDs are housed in clear-looking parts while the infrared sensors are in dark parts. Each must be correctly oriented – note that the spherical lens on each part should point toward the shorter end of the bracket, ensuring both point toward the filter paper.

Glueing: Use plastics glue (avoid "super glue" as it can leave residue that affects LED and phototransistor sensitivity for days or weeks). Place a droplet of glue on each leg to close the hole.

Assembly:
- Insert the LED and sensor bracket assembly into the case.
- Pre-treat both counterparts with plastics glue before insertion to create airtight channels.
- Ensure the middle section between the sample and sensor areas is completely covered with glue to prevent air leakage.
Fixing: Prior to insertion, secure the LEDs and sensors with plastic or hot glue (avoid tilting; check orientation).
Mounting: Once inserted, mount the brackets to the case and verify that the LED and sensor lenses are centered.

Reference & Sample Parts

On the circuit board, the LEDs must be connected in series. The negative pin of the first LED serves as the positive pin for the second LED. Use a standard 4-pin 2.54 mm JST connector and cable, soldering the connections in the order:

red – black – yellow – white - connect one by one. If your cable has different coloring, maintain the same electrical order.

The infrared sensors (for measuring attenuation) are connected in parallel. Their connections follow a different order:

black – red – white – yellow

This completes the most crucial part of the bcMeter assembly. LED-to-sensor orientation and air leakage directly affect accuracy. Minor misalignments can be compensated by the Calibration function.

Sealing

Continue case assembly by adding a rubber seal to the bottom part:

Ensure the seal's diameter is precisely known — critical for accuracy.
Check for air leakage; if needed, apply a second seal to the top part and ensure holes overlap.
To verify diameter, let the paper run black and examine the sample spot.
Add M3 screw threads to the poles and use superglue to secure them.

Pump Control Circuit

The bcMeter is versatile with pump selection. Any pump running at 5V will work. Two low-cost types:

Membrane pumps — Recommended.
Propeller pumps — Not recommended for this use case.

Membrane pumps can use a PWM controller (Type A) or operate without one (Type B). For PWM-controlled pumps, remove Q4 from the PCB.

Airflow Meter Setup

If an airflow meter is used:

Connect the pump's input to the airflow sensor's output.
Connect the airflow sensor's input to the air sampling connector.
Connect the pump's output to the bcMeter's air output.

Finalizing the Build

Insert filter paper and screw the top part of the case by hand.
Connect a stronger 12V pump and cover the air inlet.
If noise is heard (indicating leakage), use glue to seal the gap.

Description of the Components (V1 PCB)

U1: MCP3428 4-Channel Analogue/Digital Converter
D1 / D2: IR LED connector for sensor and reference
Q1 / Q2: IR Phototransistor for sensor and reference (ADC Channels 1 and 2)
J1: Omron D6F Airflow Sensor (ADC Channel 4)
J2: SSD1306 Display (optional)
J7: Heater or Fan (optional)
J12: SPI Header (optional)
J5: Air Pump
J4 / J9: PWM + Power for Air Pump (remove Q4 for PWM pump)
Additional: Several headers (I2C, ADC, Power) for expandability