Hi folks. I'm using the HX711 amplifier with the sample rate of 80Hz(I desoldered pin 15 of the HX711 and jumpered to Vcc to get the sample rate to 80Sa/sec). The bias for the sampling rate needs to be either 0V for 10Hz or 5V for 80Hz, however a short occurs when using the Vcc line as the Arduino 5V connection is being used for both the bias and the general voltage supply to the PCB.
What can I do to work at 80Hz?
Copia di hx711_english.pdf (314 KB)
Please post a drawing of your circuit. A photograph of a hand drawn circuit will do.
Put a 1K resistor in series with pin 15 before connecting it to 5V.
Hi,
OPs "circuit"
Where did you short pin15 to Vcc?
You may find that pin 15 is already connected to GND, when you connect pin15 to Vcc you are shorting out the supply.
Use a DMM to see if pin15 is connected to GND.
Please don't go back and edit a previous post, like adding attachments, it upsets the flow of the thread.
Thanks.. Tom.. 
Hi,
This is the circuit diagram.
Pin 15 is connected to gnd.
Tom..
The OP said
I desoldered pin 15 of the HX711 and jumpered to Vcc to get the sample rate to 80Sa/sec).
So it is not shorting the supply at all. It only shorts out the supply when you connect pin 15 to ground to get the other rate. That is why using a pull up resistor to 5V will prevent this from happening.
He also says
however a short occurs when using the Vcc line as the Arduino 5V connection is being used for both the bias
Which is nonsense. There is nothing stopping you using the 5V signal for biasing this pin and powering the Arduino.
I desoldered pins 15 of each of the 4 HX711 and jumped the 4 wires to a single 5K resistor in series before connecting it to 5V. No 80 Sa/sec though
Hi,
Can you post a picture of the board please?
Thanks.. Tom..
jumped the 4 wires to a single 5K resistor in series
Why have you done this? You made the resistor 5 times bigger instead of 5 times smaller!
No just try it on one HX711 with a 1K resistor like I told you to. If that does not work we can take it from there.
I tried it on one HX711 with a 1K resistor, and it worked. How can I do to use all the 4 HX711 at 80 Sa/sec using just one resistor?
Why do you want to use just one resistor?
You should use a 250R resistor if you want to try it but I don't think it is a good idea.
It is because I don't have a lot of space to use 4resistors. Anyway what is the best method in your opinion to reach this aim?I can try to find the place for 4 resistor.
Thanks very much for all your advices
aiss:
It is because I don't have a lot of space to use 4resistors. Anyway what is the best method in your opinion to reach this aim?I can try to find the place for 4 resistor.
Thanks very much for all your advices
Are you using the PCBs shown in your "circuit", if so each PCB will have more than enough room for a resistor.
Can you post a picture of your project please?
Thanks.. Tom...
aiss:
It is because I don't have a lot of space to use 4resistors.
You can always fit a resistor on a board. You can get tiny ones even smaller than those surface mount capacitors if you want.
Grumpy_Mike:
You should use a 250R resistor...
...to pull up four pins.
I know you don't recommend pulling up multiple pins with one resistor (why not?), but...
I'm curious to know why such a strong single pull-up is needed to pull up four pins. Leakage current?
And is the reason linear? That is, if you were pulling up eight pins with one resistor, would you use 125 ohms?
That is, if you were pulling up eight pins with one resistor, would you use 125 ohms?
Yes in theory. But only if they shared current equally, which seems unlikely.
I'm curious to know why such a strong single pull-up is needed to pull up four pins. Leakage current?
No not leakage current. It is something we are not being told. All we know is that 1K pulls up one input and 5K will not pull up four inputs.
As the OP has made no measurements of the voltage on the pins when they are pulled up then it is hard to say what is going on.
It was observed that operating a single driver circuit would allow the 80Hz frequency to be acheived, but when additional driver circuits were used, the sampling frequency would drop.
To test this, a set of experiments was conducted that would highlight the drop in sampling frequency with the introduction of more driver circuits.
Results
1 driver circuit with 1k 5V bias - 75Hz
2 driver circuits each with 1k 5V bias - 51Hz
3 driver circuits each with 1k 5V bias - 34Hz
4 driver circuits each with 1k 5V bias - 28Hz
As the sampling rate decrease with additional drivers (more data to transfer) there is an issue with the transmission buffer where this is being overloaded. The Arduino code appears to wait until there is space in this buffer before sending data, so until the buffer is cleared (at the bluetooth side possibly Using the BlueSMiRF - SparkFun Learn), then the sampling rate will be limited at the rate of the refresh.
It was observed that operating a single driver circuit would allow the 80Hz frequency to be acheived, but when additional driver circuits were used, the sampling frequency would drop.
I think that is a software issue. Given what you are doing for 1 driver circuit and just repeating it 4 times is going to lower the sampling rate as you add more units because the sampling rate is split between 4 sensors.
Do you have any delay in the sampling routines? How do you know they are ready to take the next sample?
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