HX711 module with Raspberry Pi Pico, raw values stay the same even if I add weight

[FrankHenckens]

Hello,

I am making a scale with a bending beam load cell, a HX711 and a Raspberry Pi Pico. I read the raw values of the loadcell and when I add weight, the raw value stays the same. It fluctuates but it also fluctuates without any added weight. All help is welcome.

[robert-hh]

If there is no change, either

1. the load cell is broken
2. the connection to the load cell is wrong or
3. you use the wrong input of the HX711.

The color scheme of the document is unusual. Usually, red and black ate V+ and V-, and green and white are the output wires. If you have a multimeter, you can check the impedance between the wires of the load cell. The impedance between V+ and V+ should be similar to the impedance between the output wires, and the impedance between V+ (or V-) and each output wire about half of the one between V+ and V-.
With a gain setting of 128 and 64 you have to use input A, for a gain of 32 input B.
The HX711 is quite noisy. Usually at least the lower 10 bits of the result vary.

[FrankHenckens]

I have your library and it works perfect, thank you!

[robert-hh]

Very good. Thank you for your feedback.

[FrankHenckens]

I have one more question, are the logic leven converters necessary in my circuit? I thought it was necessary between the HX711 modules and the pico, but on the internet I see people doing it without logic level converters.

[robert-hh]

Software usually does not change when not touched. Hardware can, if you move it. 0 as result is strange. It means that data or clock line do not work. Check the wires. It may be related to the 5V/3.3V difference.

Besides that I would recommend with an RP2 to use the pio version. The clock timing is more precise.

About the logic level converters, there are two aspects:

a) RP2 outgoing signals (CLK). The output level of the RP2 are 0 and about 3.2 V. The high level of the HX711 CLK input at 5V is not specified. For a CMOS circuitry the guaranteed high level is usually 3 - 3.5V. So it's in a gray zone, whether the high level of a clock is seen as such by the HX711 and may vary with the temperature.

b) RP2 input. The maximum input voltage at the GPIO pin is 3.6V for a 3.3V supply. The HX711 outputs about 5V with a limited current, which is not specified. That excess current has to be consumed by the surge protection diodes of the GPIO pad of the RP2, but they not designed for that. You could limit the current by inserting a series resistor in the data line. 4700 Ohm would limit the current to <300µA. Care must be taken to reduce the bandwidth of the connection.

But better use a level converter. Since it does not have to be open drain, something based e.g. on a TXB010x (like txb0102 or txb0104) would be the best. The offer better speed.

[robert-hh]

About levels: You could try to reduce the supply voltage of the HX711. It runs at 3.3V, only the input voltage range is reduced. At a GAIN of 128 it would be about 20mV. You may have to bypass the voltage regulator on the HX711 board.

[bidrohini]

I suggest cross-checking the loadcells in any other platform. If you have a full-fledged Raspberry Pi, you can try to test the load cells with that. Here is a detailed tutorial and sample code: https://www.theengineeringprojects.com/2023/06/interface-weight-sensor-hx711-with-raspberry-pi-4.html
If you have an Arduino, you can test the load cells with that too with relevant codes. If the cells work in other platforms, That will mean that problem the is in your pico setup or code.

[robert-hh]

@bidrohini The code in the example you've linked is wrong:

• It does not handle the sign bit properly. It inverts unconditionally the sign bit. From the data sheet: The output 24 bits of data is in 2’s complement format. The output range is 0x800000 to 0x0x7fffff. So if you invert the sign, for instance a value of 0x800000 (-8388608) gets 0. And that's truly wrong.
• It does not handle the conversion ready flag, causing spurious out-of-range results.
• It does not care for the clk pulse to be shorter than 50µs. If its longer, the device will fall into sleep until the next clock pulse and then wake up again for the remainder of the read transaction, which then is corrupted.

[FrankHenckens]

Hello Robert,

I would love your help with another project of mine if you have time. https://github.com/orgs/micropython/discussions/15947