Ultimate Chicken House - Phase II - building the hopper and feeding mechanism
This was perhaps going to be the trickiest part of the project. It seemed to make sense to use gravity to assist the dispensing mechanism and reduce the power consumption of the feeder. I thought of a number of different designs e.g. vertical screw, rotating drum (like a waterwheel), slider under a funnel, horizontal rotating chambers (I prototyped this with a Pringles can and yoghurt pots and it fell apart!) but all had problems. Essential to the design was minimal power use i.e. quick, low power motor or solenoid.
I discovered that it was easy for the feed to block a narrow tube unless you had a well engineered feeding mechanism. I wanted a simple design that even I could make! I ended up guestimating that a dispensing aperture of 4cmx5cm was big enough to avoid jamming and by slowly opening a trap door at the bottom it should be possible to control a decent amount of feed coming out in a relatively short space of time. By building this aperture in the form of a wooden box, any size hopper could sit above this. I took a trip to the model shop and got some 5mm ply and set about creating the design. I don't think the precise measurements/shape are important so long as the aperature is big enough and the hinged door fits snuggly at the bottom.
I'd like to thank my friend Paul for cutting the ply wood on his large and rather dangerous looking table circular saw and thus saving my fingers to complete the remainder of the project. The pieces were glued together using wood glue and small strips (offcuts) of about 6mmx6mm were glued along all of the corners (Paul's idea!). I added one further 'strut' across the middle of the long edges to reinforce it. Once dry I went around adding extra glue to seal the holes and also along the edges to further strengthen it. It wasn't necessary to use any nails in the end. Tip: If you don't have clamps to hold it together whilst the glue drys I discovered doing it in stages and lying it on the lawn useful as it sticks to the grass rather than a table and the grass can be easily pulled off once it's dry!
The hopper door was designed to fit inside the two side flanges to try to clear away any grain that might otherwise block the trap door. It was attached by 2 small plastic hinges I got from a model shop. At the end of the trap door is a metal eye that I found in a picture hanging set. On to this I attached some 6lb nylon fishing line which passed up through a further eye on the body of the hopper and on to the spindle of the motor. The motor was attached to the body of the hopper by part of an old window lock I had. Some motors would probably come with a mounting bracket. It was mounted at an upward angle to prevent the fishing line coming off the end of the spindle under strain.
overall picture here
It was difficult to know exactly what type of motor to get for this project in terms of torque, current draw, spindle size etc. I didn't have any old motors lying around and wanted to use something as cheap as possible. I bought a small 1:200 offset gear motor which is suitable for use in BEAM robots. After many wasted hours and intense frustration I discovered this motor to be very noisy (in the electronic EMF sense of the word) and it kept resetting my microntroller. I tried decoupling every part of the circuit and eventually discovered the need to put a capacitor across the motor terminals to mop up some of this interference. A 0.1uF non polarised capacitor is fine and solved many of my problems!
motor closeup here
With the unit assembled it was time to test it. My concerns were whether the motor had enough power to lift the trap door against the weight of the falling grain and whether the trap door might become jammed with grain getting caught. The motor coped admirably on both counts and in fact pulverised any trapped grain with ease. The trap door shut tightly but I left the power on too long and the door was ripped off its hinges. It's remarkable the power generated by this little motor!
rear hinge view here
To overcome this I reattached the hinges with small screws and added a contact switch made from two screws and some aluminium foil. The screw heads protrude about 6mm from the base of the hopper so hopefully the trap door doesn't have to do so much crushing before the contacts are closed and the motor turns off.
close up of screw sensors here
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