Context

Concerned by the global warming and as urban people I tried to find a manner to reduce my environmental impact. Like many French I am dependent on fossil and atomic energy. I wish I could do without it in the near future, but I am not able for the moment. In the meantime, I cycle, consume local products (where possible) and try to reduce my waste.

If it is easy to compost waste when you live in the countryside or just when you have a garden that allows it, it is more complicated when you live in town and have no garden.

Constraints

The worm composter is a beginning of solution because it allows to reduce significantly the volume of waste and it can be installed indoors. But there are several obstacles to its development:

- Psychological: the human knows how to generate waste but they must quickly disappear from his sight … with a worm composter the waste is degraded slowly and remain a moment in the home … and in addition there are worms that crawl under the rubbish

- Use: Start a worm composter is not necessarily easy. This requires observing the evolution of the waste and properly balancing the inputs. If this is not done correctly, smells, midges or other nuisances may appear.

- Drainage: when there is no garden where to value the compost, how to do?

Principle of worm composting

The worm composting (or vermicomposting) is a organic waste treatment technique. The process uses earthworms that feed on organic waste. Different varieties of worms can be used as Eisnia fetida, Eisenia hortensis, Eisenia andrei, or Perionyx excavatus. These worms eat the equivalent of their weight per day and reduce by 5 the initial volume that they have eaten. After digestion, the earthworms will reject a material devoid of odor, humus-like material called vermicompost.

Benefits of vermicompost

The vermicompost is particularly rich in organic matter so it’s a very good organic fertilizer. As explain Iyyanki V. Muralikrishna and Valli Manickam in Environmental Management the vermicompost has the following advantages over chemical fertilizers:

To learn more about you can read “Friend earthworm : practical application of a lifetime study of habits of the most important animal in the world” written by George Sheffield Oliver who experienced vermicomposting.

Operation of a worm composter

A worm composter is generally composed of a stack of trays in which the waste is placed in contact with the worms. The bottom of each trays is perforated to let the worms migrate from one tray to another. The waste is placed in the upper tray and the other trays contain the compost being matured. When the top tray is full, empty the tray with the oldest compost (normally ready if the composter is well sized) and place it at the top of the stack. In the lower part, a tank collects the worm juice produced during the composting process.

First sketch

GENERAL DESIGN OPERATION

Aims of the project

I started thinking about this project 2 years ago, but have not had the opportunity to spend time and energy on it. I want to seize the opportunity of the Fabacademy to progress in the realization of a first version of the worm composter.

I separated the objectives into 2 categories those I set for my Fabacademy final project and those in the long and mid-term objectives.

Objectives for my Fabacademy final project

Long and mid-term objectives

Community

The plus2vers website is a French web platform that references worm donors and share informations and advices about worm composting.

State of art

DIY solutions

There are many DIY solutions of different shapes and using different materials. I did not find a DIY project using digital fabrication.

Here are some DIY examples:

Polystyrene tank Plastic tray Wood tray
Pictures
Link Nature obsession Nature obsession Saint-Brieuc agglomération
Strengths

Good thermal insulation

Lowcost

Easy to build and maintain

Light in weight

Porous material for a good ventilation

Good thermal insulation
Weaknesses

Esthetic

Durability

Bad thermal insulation

Holds water

Rapid deterioration

heavy

Dries faster

Cost $ $ $ $ $ $

Commercial solutions

Commercial solutions exist at prices ranging from 65€ to 230€.

Lombric&Co Can-o-worms Wood tray
Pictures
Link Les gallinules Store Vers la terre
Strengths

Esthetic

Multifunctional

Good documentation

Wheels

Ventilation

Weaknesses On going development Bad thermal insulation Bad thermal insulation
Cost ? $ $ $ $ $ $ $ $

Functional prototype description

Starting a worm composter is not easy when you do not know. This requires being vigilant with inputs, especially at startup. If it is badly managed, nuisances can appear (odors, midges …). Worm composter is also vulnerable to environmental pressures, especially to temperature and humidity. To facilitate the composting process it’s necessary to provide optimum conditions.

Worm composting requirements

Features

For this prototype, I choose to respond primarily to the objectives mentioned above, so this one will integrate:

Parts

Here you will find the technical choices that I made to meet the objectives and achieve this prototype.

Materials

The choice of material for the tray is a complicated question because it must meet several requirements: - be resistant to a damp environment - innocuous for worms - machinable with the Fab Lab’s tools - esthetic (subjective !) - easily available

I made the choice to use machinable materials with laser cutting. This choice restricts the range of usable materials, but the laser cutting technique is more accessible for public who are not Fab Lab friendly than the CNC. In our Fab Lab we cut mainly poplar and okoume plywood and PMMA. Poplar is more easily cut with laser cutting and being a local species, so I chose this material.

Poplar is not a wood species naturally protect from moisture. That’s why I thought about 2 ways to protect it, either a chemical protection or an inner protection plate in a resistant material.

At the beginning of the design phase I chose to cover the inner faces with PMMA plates. These plates were to serve as protection of the wood but also as a positioning guide and wrists. PMMA being a rather expensive material, I finally only kept the side plates (for the wrists and guide) and abandoned the full mechanical protection. I chose to protect the wood with a mixture of linseed oil and turpentine. I will have to observe how poplar evolve with this protection.

NEXT VERSION

For a next version I would like to test a solid wood to machine it with a CNC. I do not know yet which wood to choose, but preferably a local species (oak, chestnut, ash …?).

Mechanical parts BOM and budget

The design has been thought so that the composter can be made in a Fab Lab. The set consists of several parts.

BOM for a 955 x 500 x 390 mm vermicomposter

Components Quantity Average price
Poplar plywood 10mm 2 boards (600 x 900 mm) 15€
Poplar plywood 5mm 1 board (150 x 70 mm) 0,2€
PMMA (black) 3mm 1 board (770 x 500 mm) 41€
PMMA (transparent) 3mm 1 board (130 x 70 mm) 5€
PLA filament 2,85 30 gram 0,6€
Exterior wood glue 1 pot 8€
Turpentine oil 1 pot 6€
Linseed oil 1 pot 4€
Bottle 1 Free
Flexible pipe 1 1,5€
Screws (to fasten PCBs) 16 0,5€
Grid 1 10€

Total budget for mechanical parts = 81,5€

Trays

A stack of 3 boxes open on the top and bottom. Each is composed of 4 parts assembled by notches. A grid is placed at the bottom to hold the waste and allow the migration of worms. On the sides of each tray there are wrists to facilitate handling. To facilitate stacking the lower edges of the bins are bevelled and PMMA positioning guides are present on the sides.

Stand

For better ergonomics, a stand raises the stack of trays. In this configuration the worm juice tank can be positioning in the lower part. The feet also serve as a support for the lid.The stand holds the soil moisture sensor in the first tray.

Lid

The lid is mounted on the stand by means of a laser-cut hinge system. The inside face of the lid houses the electronic cards, temperature sensor and the fan. Openings on the upper face facilitate the flow of air.

Worm juice collector

In the lower part, below the 3 trays, a sloped waterproof tray can collect the worm juice. A hole is present in the lowest point to drive the juice into the tank.

Worm juice tank

For this part I use a simple glass bottle. The worm juice is recovered and stored in a tank located at the feet of the vermicomposter. A funnel is placed at the collector hole, and a pipe brings the juice into the tank. A cap closes the tank and holds the humidity sensor used as a level gauge.

Electronic parts BOM and budget

For the prototype I chose to use the PCBs designed and produced during the fabacademy. The electronics must be able to retrieve several information related to the composting environment:

It must also allow:

Here is the architecture:

Components Quantity Average price
Electrical Wires (3 colors) 1 coils 6€
Huzzah ESP8266 1 9€
12V Power supply 1 17€
Fan 1 12€
DHT 22 1 9€
Soil moisture sensor 1 3€
Polarized capacitor 470uF 25V 1 0,5€
Solder wire 1 3€

Total budget for electronic parts = 59,5€

Sensor board

This PCB is responsible for recovering data from the 3 sensors:

This data is sent in serial port to the other PCBs.

Components Quantity Average price
FR4 Epoxy PCB Board single side 1 boards (100 x 150 mm) 1,5€
ATTiny44 1 1,4€
Crystal 20Mhz 8pF SMD 1 0,5€
Resistor 10kohms SMD 1 0,1€
Resistor 1Mohms SMD 2 0,2€
Capacitor 1uF SMD 1 0,1€
Pin Header (ISP - 6pos) 1 0,2€
Pin Header (FTDI - 6pos) 1 0,15€
Pin Header (4pos*) 2 0,30€

Total budget for sensor board = 4,45€

Fan board

This PCB receives the temperature and humidity of the compost and controls the fan (starts if too hot or too wet). This board is powered by the 12v power supply and is equipped with a regulator that supplies the voltage of 5V to other components.

Components Quantity Average price
FR4 Epoxy PCB Board single side 1 boards (100 x 150 mm) 1,5€
ATTiny44 1 1,4€
Resistor 10kohms SMD 1 0,1€
Capacitor 10uF SMD 1 0,1€
Capacitor 1uF SMD 1 0,1€
Capacitor 0,1uF SMD 1 0,1€
Capacitor 100uF SMD 1 0,1€
5V regulator NCP1117ST50T3GOSCT-ND 1 0,5€
Motor driver A4953ELJTR-T 1 1,6€
Pin Header (ISP - 6pos) 1 0,2€
Pin Header (FTDI - 6pos) 1 0,15€
Pin Header (4pos*) 2 0,30€

Total budget for fan board = 6,05€

LED board

This PCB receives temperature, humidity of the compost and value of the soil moisture sensor used as gauge level and controls 3 LEDs (one for each data).

Components Quantity Average price
FR4 Epoxy PCB Board single side 1 boards (100 x 150 mm) 1,5€
ATTiny44 1 1,4€
Resistor 10kohms SMD 1 0,1€
Resistor 1kohms SMD 9 0,9€
Resistor 0 ohms SMD (jumpers) 5 0,5€
Capacitor 1uF SMD 1 0,1€
Pin Header (ISP - 6pos) 1 0,2€
Pin Header (4pos) 6 0,90€
RGB LEDs 3 0,50€

Total budget for LED board = 6,10€

Huzzah ESP8266

Huzzah receives temperature, humidity of the compost and value of the soil moisture sensor used as gauge level and send data to a webserver.

DHT 22

This sensor is only used to monitor air temperature.

Temperature range: -40°C to 80°C

Technical details

Soil moisture sensor

This sensor is used as a gauge of the worm juice tank.

Technical details

DIY compost moisture sensor

Following Neil’s advice, I created a humidity sensor based on a capacitive circuit with 2 electrodes.

Components Quantity Average price
FR4 Epoxy PCB Board single side 1 boards (100 x 150 mm) 1,5€
PLA 4grams 0,10€

Total budget for DIY compost moisture sensor = 1,6€

Fan

I use a 12v fan placed on the lid to force ventilation especially when the temperature or humidity are too high.

12V Power supply

I chose this power supply for its size and because it is sufficient to power all the electronic components.

Technical details

Total budget

Components Average price
Mechanical parts 91,5€
Electronic parts 59,9€
Sensor board 4,45€
Fan board 6,05€
LED board 6,10€
DIY compost moisture sensor 1,6€
TOTAL 159,6€

!!! note “COST REDUCTION” For the next version I would like to check if it is necessary to keep the fan. The use of a solid and moisture-resistant wood could help to remove the PMMA side panels. So the total cost could go down to 110€.

DIY moisture sensor calibration

On Neil’s advice I made a capacitive sensor to measure the moisture tux in the compost. To know more about the operation see here.

The objective of the measurement is to obtain a value between 0% (for dry medium) and 100% (for medium saturated with water).

To calibrate it, I made measurements in a dry soil and in a water saturated soil and I compared the values given by a commercial soil moisture sensor

Dry soil
Water saturated soil

I got the following final values:

Soil Value DIY sensor Value commercial sensor
Dry soil -24 000 0%
Water saturated soil -19 900 100%
Air -27 800 0%

!!! note “CALIBRATION” To obtain roughly stable values, it took 10 to 15 minutes !! To improve the response of the sensor and to adapt it to the use, it would be necessary to vary the delay between the pulsations.

Use of the DHT22 without library on the ATTiny44

The library for using the DHT22 temperature probe takes too much memory to be used on an ATTiny44. Needing only the temperature and not the humidity I integrated the reading of the probe directly in a function of the code for the INPUT board. So, I did not need the library anymore.

int temp ()
{
  int val = 0;
  if (read())
  {
    int f;
    f = (int)(data[2] & 0x7F);
    f *= 256;
    f += (int) data[3];
    f /= 10;
    if (data[2] & 0x80) f *= -1;
    val = f ;
  }
  return val ;
}

boolean read(void) {
  uint8_t laststate = HIGH;
  uint8_t counter = 0;
  uint8_t j = 0, i;
  unsigned long currenttime;

  // pull the pin high and wait 250 milliseconds
  digitalWrite(7, HIGH);
  delay(250);

  currenttime = millis();
  if (currenttime < _lastreadtime) {
    // ie there was a rollover
    _lastreadtime = 0;
  }
  if (!firstreading && ((currenttime - _lastreadtime) < 2000)) {
    return true; // return last correct measurement
    //delay(2000 - (currenttime - _lastreadtime));
  }
  firstreading = false;
  //
    //Serial.print("Currtime: "); Serial.print(currenttime);
    //Serial.print(" Lasttime: "); Serial.print(_lastreadtime);

  _lastreadtime = millis();

  data[0] = data[1] = data[2] = data[3] = data[4] = 0;

  // now pull it low for ~20 milliseconds
  pinMode(7, OUTPUT);
  digitalWrite(7, LOW);
  delay(20);
  cli();
  digitalWrite(7, HIGH);
  delayMicroseconds(40);
  pinMode(7, INPUT);

  // read in timings
  for ( i = 0; i < 85; i++) {
    counter = 0;
    while (digitalRead(7) == laststate) {
      counter++;
      delayMicroseconds(1);
      if (counter == 255) {
        break;
      }
    }
    laststate = digitalRead(7);

    if (counter == 255) break;

    // ignore first 3 transitions
    if ((i >= 4) && (i % 2 == 0)) {
      // shove each bit into the storage bytes
      data[j / 8] <<= 1;
      if (counter > 6)
        data[j / 8] |= 1;
      j++;
    }

  }

  sei();

  // check we read 40 bits and that the checksum matches
  if ((j >= 40) &&
      (data[4] == ((data[0] + data[1] + data[2] + data[3]) & 0xFF)) ) {
    return true;
  }


  return false;

}

Fabrication process

Mechanical Parts

1st. step: to set the dimensions

2nd. step: production

Download and laser cut or print this parts (or use your own files created previously from the parametric model):

Parts Material Tool
Stand / Trays / Lid (1)(SVG, PDF, DXF) Poplar 10mm Plywood Laser cutting
Stand / Trays / Lid (2)(SVG, PDF, DXF) Poplar 10mm Plywood Laser cutting
Cap’s tank (SVG, PDF, DXF) Poplar 5mm Plywood Laser cutting
Inner PMMA side plates, guides and LEDs sign plate (SVG, PDF, DXF) 3mm black PMMA Laser cutting
LEDs sign plate (SVG, PDF, DXF) 3mm transparent PMMA Laser cutting
DIY_soil_moisture (SVG, PDF, DXF) FR4 Epoxy PCB board Milling machine
Funnel (STL) PLA 3D printer (FDM)
Soil moisture holder (STL) PLA 3D printer (FDM)
Worm juice collector Molding products, fibers, solid wood CNC

You should have all this parts:

3rd. step: post-processing

To facilitate stacking of the trays the lower edges of the bins must be bevelled. To do you can use a spindle moulder machine with a 90° Vbit.

To allow penetration of the screw heads it is necessary to make conical holes in the PMMA guides and side panels.

4th. step: the grid

A grid is placed at the bottom of the trays to hold the waste and allow the migration of worms. I used 6,4mm mesh grid.

Cut the grid with (+10 mm offset) Fold the grid on each side

After assembling the trays, the grids will be stapled in the bottom on each side.

5th. step: assembly

To facilitate the assembly and especially to ensure the squareness of the trays, it is advisable to create a jig.

Place the panels and test the assembly without glue Glue the notches
Put the panels together Use clamps to tighten

Use the same technique for the lid and the stand.

6th. step: chemical protection

Apply two coats of a 50/50 mixture of turpentine and linseed oil on every wood parts.

7th. step: fastening the grid

Staple the grids inside each tray.

8th. step: fastening the inner PMMA side plates

Fasten the inner PMMA side plates with screws. The edges of the grid is thus blocked under the PMMA plates.

9th. step: make the worm juice collector

10th. step: instal the worm juice collector

As I did not know what the collector would look like at the time of the design, the fastening system is not integrated in the 3D model. I simply used plywood brackets screwed onto the stand to hold the collector. It is important to respect the slope of the collector during installation.

11th. step: fastening the inner PMMA guides on the stand

Fasten the PMMA guides with screws.

12th. step: fastening the LED sign plates

13th. step: install the pipe and the tank’s cap on the Bottle

Electronic Parts

1st. step: make the PCB’s

2nd. step: upload code to each microcontroller - Sensor board code - Fan board - LED board code

3rd. step: fasten the PCBs to the Lid

Use small screws and make sure you do not short circuit !

4th. step: fasten the FAN to the Lid

Use cyanolit glue to fasten the FAN.

5th. step: wiring

Fan board:

!!! note “POWER SUPPLY” You need to add a 470uF capacitor between V+ and V- of the power supply.

Sensor board:

!!! note “DHT22” You need to add a 10kohm resistor between VCC and signal of the DHT22.

LED board:

HUZZAH: - VCC -> VCC LED board - GND -> GND LED board - Rx * -> PA2 (Tx) LED board

!!! note ” * Rx Tx” the name of rx and tx is reversed between the huzzah FTDI and the FTDI of the produced PCBs.

Final result

Conclusion

The work done allowed me to reach almost all the objectives that I had set for this first version of the vermicomposteur realized as part of the fabacademy.

However, as design and manufacturing progressed, I realized several areas for improvement.

Overall, I’m happy with the result and I hope this project will inspire other worm breeders!

Downloads

Fabrication files (.zip)

Softwares (.zip)

Links

Environmental Management

Advantages of the vermicomposting

Book: “Friend earthworm”

plus2vers website

Sensor board

Fan board

LED board

Worm juice collector