Step 1 - Drill Holes
There is a total of 5 holes you need to drill. Two on the bottom and three on the top. Drill two 3/16 wide holes 1/2 an inch from each other on the top and bottom. Next drill a hole 7/64" wide in the center of the top side. The two larger holes on the top will serve for the red and blue led. The smaller hole on the top will serve for the power line. The two holes on the bottom are for the screws that will be used to sense the moisture of the dirt.
Step 2 - Add the Sensor
Tread one nut onto each screw. Then tread the screws through the holes in the bottom of the box. The heads of the screws should be on the inside of the box. Now thread the washers onto the screws and the remaining nuts. The overall lineup should be screw head, nut, project box, washer, nut. Before tightening it down add a wire between the screw head and first nut. This will create a secure electoral connection.
Step 4 - Make the Circuit on a SOLDER-LESS Breadboard
In order for the program to work you need to know the resistance of dry soil. To do this you need to build the circuit for the soil moisture circuit on a solder-less breadboard and use an Arduino Uno to see the resistance. To do this connect digital pin 6 to a 100 ohm resistor, and connect that resistor to one of the screws. Now connect digital pin 7 to a 1k ohm resistor and connect the other end of the resistor to the other screw. Now connect analog pin 0 to the resistor, the same side that connect with the screw. Now upload the voltage flip code and record the value of the avg when the screws are in moist dirt.
STep 3 - Cut the CIrcuit Board
Right now the circuit board does not fit in the project box. To make it fit cut of about 1/2 an inch down from the top and 1/4 an inch from the side. I used a Dremel to cut it off but anything should work. Now the PCB should fit into the project box without a problem.
Step 5 - Program the ATtiny chip
After adding the cutoff resistance (the resistance that you found in the previous step) to the program below upload the program to the attiny. To do this you can use the Arduino as a programer or you can buy a programing stick. Either way use this Tutorial.
Step 6 - Solder the Circuit
Now you need to make the circuit. Solder the dip socket into the middle of the board. It's easier to solder the resistors first.