Controlling a switch with a Balance

Using Arduino Project Guidance
#1

Hi,
First Up, I`m completely new to arduino and very sorry if my question sounds ridiculous, but I have a project I´m working on, that I think arduino would be perfect for (especially since I have an arduino nano since christmas :wink:

I would like to build an automatic filling machine for jam glasses, which is controlled by a balance. the filling would be done by a potentiostatically regulated extruder (that part is built and ready, so the arduino would only have to turn that one on and off)

So now to the problem/Question:
would it be possible to turn a relais on (and keep it on, for as long as a balance (one of those HX711 modules or similar) is above a certain value (empty container) and below another (targetet fill ammount plus empty container)

so the relais would have to have a vew functions

  1. tare the balance via switch
  2. display the current weight
  3. detect the empty glass weight and store it after pressing another button (maybe-5g so eventual variations are neglectable)
  4. be able to store the fill ammount (either via config from the pc, or even better via buttons on the system (can be in 1g or even bigger steps)
    5 turn the power to the filling machine (12V DC motor) on and off

if that is too weird, It would also be fine for the machine to start dosing after the push of a button until the target weight is reached.

I hope I dont sound completely incoherent, and the idea is somewhat clear... I would greatly appreciate all inputs, cheers.

#2

Welcome, Actually you picked a good project and it appears so far parts that are up to the task. For the motor I would suggest getting a part being sold as " Low High Voltage Control 12V 24V 36V Switch Mosfet Module for Arduino Connect" they are good upto 5A without a heat sink. I used a few and they work good. You can also just use a MOSFET and drive it directly from the Arduino. Hint: When the Arduino resets and until it is initialized the output pins are at an unknown or high-Z state. This can and does cause MOSFETs to momentarily turn on. Best solution use a resistor in the 10K range and connect it to the output pin and ground (pull down). that will keep the MOSFET off until you turn it on.

#4

Hi, thanks for the answer
would the MOSFET also be used to regualte the current? because thats not necessary, as I already have that built into the extruder with a hand turned potentiostat. The arduino wuld really only need to turn electricity to the dispensing machine on and off, depending on the scale, not control the voltage to the machine, as that is regulated there

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#5

I was just thinking of turning the power Off/On with the MOSFET.

#6

The point is that when you ask for assistance, you have to provide the details of your problem.

"12V DC motor" is not a useful specification. Sure it operates on 12 V, but how much current does it draw?

Notwithstanding you use a series rheostat, if the rheostat was turned to zero resistance and the motor was prevented from turning, how much current would it draw? This is referred to as the "stall current" which is also the current it draws at the moment of starting and defines the required rating of the power supply and switching device whether relay or MOSFET.

Here is one of the switching modules to which gilshultz refers:

It should cope with a 10 Amp motor which I should expect would be able to eject your jam clear across the room! :grin:

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#7

The Arduino could regulate the current, so that the rheostat is not needed. A rheostat does that by dropping some of the voltage, which creates waste heat. The Arduino would do it using PWM, which is more efficient, creating far less waste heat, and allows greater control of the motor speed, especially at low speeds. This means that the Arduino can begin filling each jar at a fast rate and then slow down as the target weight is approached, so that overfilling does not occur.

But to begin, you could use the MOSFET as a simple on/off switch with your rheostat to control the speed, and consider the more sophisticated approach later. If you want to be able to do that later, make sure you connect the MOSFET to a PWM capable Arduino pin.