How it works:
1. In the beginning the door is locked, cat is outside
2. Cat walks up to the gate, RF tag gets read within about 4 inches
3. Door unlocks and a light comes on.
4. Timer ensures that door stays unlocked long enough to give cat a chance to respond
5. Cat pushes open the flap and enters
6. Flap falls back, activates Hall effect sensor, and the door locks
7. Going from the inside out, an infrared proximity sensor detects the presence of an animal if within about 10 inches and unlocks the door.
Most animals will quickly learn to push the flap in response to the click (lock) and the light. Make sure that the edges of the door and the doorway are padded with soft material for when the tail gets caught! A few painful experiences may be enough to make the cat never want to go in there again.
The electronics are shown without enclosure, but it is advised to put an enclosure around the electronic parts once you're done. It is not further discussed in this Instructable however.
Step 1: Parts list
- 5V RF reader module (Seeed Studio 125Khz UART or equivalent. Most readers come with a small antenna. To use the custom made antenna discussed here, you need a reader that allows connecting an external antenna)
- RF tag (I used these 35mm disc shaped ones)
- About 86 feet of 24 gauge magnet wire (longer than the piece shown in the photograph)
- Sharp GP2D12 infrared proximity sensor
- Two 12V electronic cabinet locks (Nordson electronic). Solenoids, basically.
- One 12V DC power supply
- Two TIP120 Darlington transistors
- One Hall-effect sensor
- One or more magnets to activate Hall-effect sensor over a range of about 2cm
- One 100 Ohm resistor
- Four 2 KOhm resistors
- Two pushbuttons
- One LED
- One hinge (one that moves easily but without much lateral movement, for precision)
- One 6" x 9" piece of 1/8" thick Plexiglas and perhaps another piece of about 6" x 3" to mount the locks
- Hookup wire, including about 27" extra thin (26 AWG or thinner)
- Nuts, bolts and serrated washers
- Rubber isolation strip or other soft material for padding the edges of the door and door post
The system comprises several 'modules' (solenoids, RF reader, and so on). One of the pictures here shows the basic plan with the modules connected to the Arduino (the electronics). In the next steps the mechanical parts and each of the modules will be discussed in detail.