When the filament reel of a 3D printer begins to decrease, it becomes difficult to tell whether the remaining filament in the reel will be sufficient for the next print.

A similar scale to the vehicle's fuel gauge would be needed.

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Here is a weight scale that allows you to estimate how much filament remains in the printer roll and helps to predict whether it will be sufficient for the print you are about to undertake.
There are many other similar projects on the web at www.thingiverse.com and other sites dedicated to 3D printing.
The particularity of this realization is that it is housed between the support bracket of the filament roll and the 20x20 mm profile at the top of the Ender-3. The power supply is taken from the 24V rail from the nearby power supply. I added a rocker switch to the 24V input to turn off the scale when not needed, it is useless to waste even a small energy of a few watts. 

lt shows weight readings averaged over several seconds and therefore display is quite stable. Note that the force applied to the filament by the extruder generates a variation in the reading of some grams. I modified the pin that supports the roll by inserting a support with two ball bearings that allows rotation with less friction. See Thingiverse "Another Ball Bearing Filament Support Ender-3"

 

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A printed circuit board connects the pre-assembled economic modules together:

a 5kg load cell and an ADC HX711 module - M1, http://bit.ly/2TgD3bu ;
an ESP12E module -M4 , http://bit.ly/2TVHokU or ESP12F http://bit.ly/2TSCXHz ;
a graphic OLED display - M2, http://bit.ly/2TlLhyW ;
a button - S1, http://bit.ly/2TPz4Tx (the plastic cap is not used) ;
a power DC-DC 24V to 3V3 module -M3, http://bit.ly/2TqkHoI
a rocker switch, http://ebay.to/2uFnEYn ;

 

Directly on the printed circuit board there are few filter capacitors and some resistors.
A programming connector for the ESP-12 is made with a strip of aligned pads and is also used in the firmware development phase. In the development phase I soldered a female strip connector tilted inwards to easily connect the serial interface and programming switches.
The power supply wire added to the 24V is a double insulated plastic cable, since it runs outside the profile is black. I recovered it from a broken laptop power supply

Here: Circuit diagram and pcb layout pdf.

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To program in circuit the ESP-12 module I used the following adapter scheme:

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 I used this USB - serial adapter.

The FS101 software has been compiled using the Arduino environment for ESP8266 and the subsequent libraries:

- ESP8266_and_ESP32_Oled_Driver_for_SSD1306_display , https://github.com/squix78/esp8266-oled-ssd1306
- HX711 , https://github.com/aguegu/ardulibs/tree/master/hx711 ;
- OneButton , https://github.com/mathertel/OneButton ;
- eeprom , https://github.com/esp8266/Arduino/tree/master/libraries/EEPROM.

The current firmware ( FS101 version 1.0 )  does not activate the WIFI function of the ESP12 and the module is used as an economical micro-controller.

In order to configure the weight scale and modify some parameters I used a single button library user interface with 3 events: button pressed once, button pressed twice, timeout for unpressed button.  The parameters are saved in Flash (Eeprom). Among the parameters is selectable the density in grams per cm3 of the filament. It is used to calculate the estimation in meters of the filament assuming a diameter of 1.75mm.
The default calibration parameters can be modified into the ESP-12 module program by recompiling.

Hereafter a map of the user interface.

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I used my 3D printer (Ender-3) to make the weight scale box, the cover and some spacers, see https://www.thingiverse.com/thing:3537459

The box has dimensions of 215 x 32 x 20 mm and is fixed to the top profile of the Ender-3 using 2 bolts of 25mm and one of 8 mm that will be screwed in captive data to the groove of the profile.
I have used bolts and M5 data, however this requires drilling the two M5 holes in the load cell with a 5 mm drill bit to allow the bolts to pass freely and thus tighten the load cell at the base and at the profile.

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Two holes in the box allow the passage of the hex key and are also used to cool the electronic circuit and to pass with a small screwdriver to open the rear cover.

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Alternatively, M4 bolts and nuts can be used; drilling of the load cell is not necessary.
Be careful to drill holes in the load cell. Do not damage the wiring, and use a column drill and vice to secure the load cell securely. CAUTION: Do not pierce the cell by holding it temporarily - you would be seriously injured!
The roll support bracket has two 5mm fixing holes 20mm apart, while the load cell has two M4 threaded holes 15mm apart.
I designed a small spacer that fits the 5mm holes and has two 4mm holes.
It is necessary to mark and cut the second 4 mm hole in the original metal bracket.

Here you need a column drill and a vice to fix the bracket firmly. CAUTION: do not pierce by holding the bracket temporarily: you would be seriously injured!

I populated the pcb with only the power supply module and I connected it temporarily to a 24V power supply (the module holds up to 28 volts input).
I then adjusted the trimmer to have an output voltage of 3.35 volts. I decided to desolder the resistive trimmer from the module and replace it with a fixed resistor that in my case is about 19.6 kOhm (parallel of 20 kOhm and 1 Mohm). In this way the voltage cannot be changed by mistake.

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I soldered the other components on the printed circuit board. Be careful to install the spacer under the button, it serves to have the correct spacing towards the front.

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Before mounting the OLED module it is necessary to configure the I2C interface by installing on the back of the OLED module the resistors R1, R4, R6, R7 = 4k7, R8 = 0 Ohm.

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The FS101 pcb does not require any modification (fixing the modules when they do not exactly coincide in position may require tilting the connection pins a little and some adaptation).

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I have some spare pcb, if some maker wants one pcb can send me an email (ik1xpv AT gmail.com) with the full destination address and after my email confirmation of availability, send a Paypal donation to my email for expenses incurred (I think > 4 Euro).

 

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