Soldering Tiny Temperature Sensors

RE: Soldering Tiny Temperature Sensors

shabaz — Fri, 27 Oct 2023 21:12:22 GMT

Just realized the STS40 temperature sensor has the same footprint, and same I2C commands as SHT40 (combined temperature and humidity sensor), which is useful in case one or the other is out of stock!

The code for it is really easy. To start a measurement, just write a single byte 0xFD to the sensor:

// start temperature measurement. Wait 10 msec before reading the result.
void start_temp_meas(void) {
    uint8_t buf;
    buf = 0xFD; // hi-res measurement command
    i2c_write_blocking(i2c_port, TEMP_SENSOR_I2C_ADDR, &buf, 1, false);
}

To read the temperature:

// read temperature measurement
float read_temp_degc(void) {
    uint8_t buf[3];
    uint16_t raw;
    float degc;
    i2c_read_blocking(i2c_port, TEMP_SENSOR_I2C_ADDR, buf, 3, false);
    raw = buf[0] << 8 | buf[1];
    degc = ((float)raw) / 65535.0;
    degc = degc * 175.0 - 45.0;
    return degc;
}

I've run some tests, and the measurements are super-granular. Each of these measurements were taken half a second apart.

Although all this is fine for board temperature measurements, measuring ambient temperature is a different kettle of fish. It really needs the sensor to be very far from any circuitry even if it's slightly warm, and there are the following guidelines in a PDF (screenshot below). My measurements are higher than room temperature by almost exactly 1.0 degree C, if the board is kept a couple of inches away from the microcontroller,but I think even conduction through pin headers is making an impact. If the board is closer to the microcontroller board (pi pico), then the measurements are 2.5 degrees C higher. Amazing what a difference that distance makes, considering the Pico isn't consuming a lot of power. This is without any enclosure so far.

I've decided to try to 'decouple from heat sources' (point 4 in the guidelines above) by making the traces even thinner and to narrow the PCB bottleneck a bit further.

I've tried to address points (1) and (3) by clearing up more space for any enclosure to be shaped around it and to provide decent access.

The STS40 could be powered down between readings, if it were powered from a GPIO pin rather than from 3.3V. However, I think a separate I2C interface (e.g. software I2C implementation) might be needed as well, to set the I2C bus low during power-down, which is no good on a single I2C bus if other peripherals need to be accessed. I'll consider that if the improvements described so far do not help as much as hoped.

It is definitely a bit of a challenge getting good ambient temperature measurements. Anyway, at least now I know that with first board layout and with no enclosure, the error is approximately 1 degree C provided the sensor is kept a couple of inches distant away from the Pico board.

RE: Soldering Tiny Temperature Sensors

beacon_dave — Wed, 25 Oct 2023 19:45:14 GMT

The thickness of the PCB in those close-up photos really illustrates the scale you are working with.

Interesting to see the different techniques as components keep getting smaller.

I was interested in Katie's mini hot plate technique in one of her element14 presents videos.

Looks like it could be a useful technique for some projects.

RE: Soldering Tiny Temperature Sensors

genebren — Wed, 25 Oct 2023 15:35:24 GMT

Nicely done!

I noticed that the silk screening on the PCB might be slightly lifting the component. I find that on some tiny parts, I either leave off the component outline or enlarge it sufficiently keep it from allowing the part to drop through and make good contact to the PCB.

RE: Soldering Tiny Temperature Sensors

cstanton — Tue, 24 Oct 2023 21:59:57 GMT

Wow they do be smol

RE: Soldering Tiny Temperature Sensors

misaz — Tue, 24 Oct 2023 20:40:02 GMT

Good job. Last year I did something similar with even smaller temperature sensor, but I used different technique.

[mention:541d694fc61b4231b244a9d5ad74bb83:f7d226abd59f475c9d224a79e3f0ec07]

RE: Soldering Tiny Temperature Sensors

DAB — Tue, 24 Oct 2023 20:03:55 GMT

Good job.

These SMD components are just too small for me to see, even with my flip down magnifying lenses.

RE: Soldering Tiny Temperature Sensors

Jan Cumps — Tue, 24 Oct 2023 19:59:27 GMT

I first thought that you should have put a landing pad for the centre pad. But that was a wrong assumption: