This is a temperature-controlled box that is fairly easy to make, and will allow warm stratification of Australian native seeds. Also, it good for a biohackers cheap agar plate incubator.
Many plants will not germinate until after a warm period. This is an adaptation to prevent them from germinating in an unseasonable summer rain and then dying to subsequent lack of water. Storage of seeds at elevated temperatures is called hot stratification.
This device is:
- Low power ( it runs from a 9v power pack "wall wart")
- Inexpensive: My build cost was around $40
- Programmable: You can alternate between low and high (night and day) temperatures to cycle a summer of nights into just over a week !
- Near-unique. Other than a $2000 lab oven, there is nothing else like it
- Also useful as a cheap incubator for DIY Bio projects, such as incubating bacterial plates
Step 1: Is It for You?
- Grow or research arid or temperate seeds? This is especially true for some Australian natives, for examplehttps://www.publish.csiro.au/bt/BT06106 , or the Drosera glanduligula, the fastest plant in the world, which was my inspiration for this project.
- Incubate microbial plates at warm temperatures
- Hatch insect eggs in winter ready for spring
This may be the project you need!
Step 2: Overview of the Electronics
The system design is rather simple.
It consists of the following blocks
- Arduino microcontroller to "tell all the other bits what to do"
- Power supply regulator to supply energy to electronics and the heater
- 12 ohm 5 watt resistor to act as heater
- 3 push buttons for programming
- A DHT11 temperature/humidity sensor
- A LCD 1602 alphanumeric display to communicate with the user.
Circuit diagram walk-through. Power is supplied through a 2.1mm power jack. It is supplied to the input pin of a 7805 voltage regulated, where it is adjusted to 5 volts, and filtered by a 1000 uF capacitor. The regulated 5 volts is supplied to the LCD1602 display module, the arduino microcontroller, the heating resistor and the DHT11 temperature and humidity sensor. The arduino runs a program which senses when the push buttons are pressed, and sends prompts and information to the LCD. It also receives information from the temperature sensor, and turns on the heating resistor via the 3055VL MOSFET.
Step 3: Components
The following components are required for this project.
- Arduino uno or clone (not Wavgat or work-a-like)
- 1 x Veroboard or similar style 22 hole x 32 track prototyping board -main board (one of these has enough for both main and sensor board.https://www.ebay.com/itm/5-08mm-Pitch-2-Pin-Screw-...
- 1 x Veroboard 10 hole x 16 track for heater and sensor
- i2c/twi LCD 1602 module, such as https://www.ebay.com/itm/IIC-I2C-TWI-SP-I-Serial-I...
- 3 x pushbutton switches similar to https://www.ebay.com/itm/30x-Momentary-Tact-Tactil...
- DHT11 sensor such as https://www.ebay.com/itm/DHT11-DHT-11-Digital-Tem...
- MTD3055VL MOSFET, such as https://www.ebay.com/itm/10PCS-3055VL-MTD3055VL-MO...
- 1000uF electrolytic capacitor (10 volt or better)
- 1 x 1k ohm 1/4 watt resistor
- 2 x 10k ohm 1/4 watt resistor
- 1 x 5 watt 12 ohm ceramic resistor for heater - this style https://www.ebay.com/itm/1-Ohm-5-10-watt-Axial-Cer...
- 2.1mm barrel socket for power supply, such as https://www.ebay.com/itm/10Pcs-DC-005-Black-Power-...
- 8x female to female breadboard wires (note that there are 40x in this example)
- 20cm of speaker wire (to attach to the power resistor heater)
- 2 terminal pcb cable block like this https://www.ebay.com/itm/5-08mm-Pitch-2-Pin-Screw-..
- Big black bulldog clip for heatsink, or small TO220 - style heatsink for the voltage regulator
- Power supply - 9 volt 1amp "wall wart"
The following mechanical components are needed for this project.
- Rosin cored solder (prefer tin, but lead/tin is ok)
- tinned hookup wire or silver-coated copper jewelry wire
- 8x 8mm hex spacers to take M3 screws
- 2 x 50mm M3 brass spacers to hold heater and sensor board
- 8x M3 nylon screws, 5mm long
- 8x M3 8mm metal screws, 8mm long
- 2 x brass hex spacers 5cm long
- 10cm x 14cm 4mm thick plywood
- Polystyrene sheet
- 3 lots of 10cm x 10cm x 2cm
- 1x 10cm x 14cm 3mm thick plywood for lid
- 2 lots of 10cm x 8cm x 2cm
- "Bondcrete" glue
- Soldering iron
- small phillips head screwdriver
- pistol-grip clamp
- wire cutter
- 3mm drill bit and cordless dril
- very sharp scalpel
- small brush for glue
Step 4: Constructing the Electronics - Main Veroboard
Begin by constructing the circuit board.
Clean the board with a scouring pad soaked with a little alcohol.
Turn the board so that the copper tracks are face upwards and cut the 3 copper tracks 9 holes in from the left and on tracks 4, 5, and 6. You can do this by placing a 5mm drill in the centre of the hole and turning it with your fingers until the copper track is completely cut.
The general principle from here on is to install the components in order of their height, from shortest to tallest. Print a copy of the layout diagram above, and as you install links and components, cross them out with a highlighter. This will help make sure that you have done everything without missing anything.
Turn the board over and install all of the copper wire links as shown on the surface. Once in place, solder one side of the link wire, then pull the other end tight with pliers so that the link is flush against the surface of the veroboard. Solder the other end to complete the link. Refer to the diagram above for placement of all the links. The following video may help you revise your veroboard soldering technique.
Next place the small 1/4 watt resistors in the appropriate place, as shown on the diagram.
Solder the connectors for the LCD and DHT11 in the board.
Attach and solder the push buttons.
Solder in the 1000 uF capacitor, and then the terminal connector that the heating resistor is attached to.
Prepare the board with a small drill to make sure that the legs of the power supply jack can be soldered to it. Solder in the 2.1mm barrel power supply jack. You may secure it to the board with glue, or by tying it to the board with wire or a cable tie.
Then solder The 7805 voltage regulator and MTB3055VL MOSFET in place, taking great care to ensure that they are placed in the correct way around (with their metal ends facing inwards towards the switches, as is shown on the diagram).
Step 5: Connecting the Main Veroboard to the Arduino.
Turn the top veroboard over so that the pcb tracks are facing upwards. Solder lengths of tinned wire (5cm is enough) to the veroboard at pins d2,d3,d4,d5,d7 and a4,a5 and gnd and 5v as shown on the diagram. This wire must be on the opposite side of the board as the components, and acts as "legs" to stand the board on the arduino. Trim off the excess wire on the component side of the board.
Now screw in the nylon hex spacers into the top of the Arduino board, using the 4mm 3M nylon screws. This will allow you to easily mount the veroboard and arduino onto the wooden lid.
Insert the other end of the wires into the arduino PCB and solder them in place. Now the veroboard is mounted on top of the arduino, as you can see from the photo at the top of this section.
Step 6: Making the Footer Board to Hold the Sensor and Heater
Take the 10 hole by 16 track veroboard. Align it against the template for the lid ( next section) and mark the two holes to take the 5mm brass spacers. Drill the marked holes with a 3mm drill.
Solder the power resistor to the veroboard so that the resistor is mounted on the normal side. You may need to widen the holes in the veroboard to fit the resistor.
Mount the DHT 11 sensor on the other side of the veroboard. This is important by using the veroboard as a "screen between the sensor and the heater, you get a more reliable measurement of the chambers temperature.
Solder the three wires to the board for the sensor, and the two thick wires to supply power to the heating resistor.
Check your work against the photo at the top of this section.
Step 7: Mounting the Components
Open and print the pdf template at this section (set your paper to A4 though, or the pdf may be scaled).
Cut a 10cm by 14cm sheet of 3mm plywood.
Stick the template to it and drill the holes. Note that this is a scan of mine. It works, but one of the LCD holes is about 1.5mm out. You should measure against your components and confirm the dimensions.
Mount the arduino and LCD by screwing 8mm screws through the plywood into the hex spacers.
Mount the heater/sensor board via the 5cm brass hex spacers, and feed the wires through the cutout on the template.
Attach the power wires, and plug in the sensor wires to the main veroboard.
Your setup should look like the photo in this section.
When you have completed the build, you are ready to program the arduino.
Step 8: Building the Box.
Cut out the 3 10 by 10 by 2cm polystyrene sheets. Use a sharp scalpel and a ruler. A hot nichrome wire may also work, but the cuts need to be very square.
Cut out the 2 10cm by 8cm by 2cm polystyrene sheets. These are the sides of the box.
Lay out the box on a table.
Glue the edges of the polystyrene sheets that will contact each other with bondcrete, then assemple the box, using trigger clamps to apply pressure to the joints.
Leave overnight, and in the morning (12 h later) your box is done.
Step 9: Programming the Arduino
Download the arduino sketch above.
Open it with the Arduino IDE.
Plug in a 9v 1 amp power pack into the power jack on the veroboard.
Plug in your Arduino to your computer via the usb cable.
Go to the tools menu, and select arduino uno as the board and the appropriate port.
Click the upload -> button, and after a minute the software will be on your arduino.
You can enter a fixed temperature by clicking left or right button and following the prompt, and you can enter and run a program by pressing the mode key. Instructions on how to use it are in the comments of the code.
If you are having difficulty finding a port for your arduino, it may be because you are using a clone. There is an Instructable for that ! https://www.instructables.com/id/How-to-fix-bad-Chinese-Arduino-clones/
Step 10: Final Thoughts
I hope that you find this build smooth and painless.
I love this box - it really helps me with plants.
If you want to adapt for larger petri dishes, you can increase the size of the polystyrene box.
It is not a fast heater, but a very reliable one.
I hope this has been of use to you.