Archive - Aug 14, 2006

Having an 18 pin PIC in my possession I decided to limit myself to this microchip and to build the entire project around no more than 18 pins. Initial experiments with the internal PIC oscillator were unsuccessful as it loses or gains too many seconds each day in an unpredictable way. With 2 pins used up for an external oscillator, 2 for power and 5 for the hbridge that left 9 pins for sensors and display. I wanted to use a 7 segment LED for cost, space and simplicity and so ended up with 7 pins for the LED, 1 pin for the feeder door sensor and 1 pin for an input button. I wondered if I might charlieplex the display somehow to get more display from the same number of pins but this seemed beyond me.

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When I set out with this project I bought a selection of PIC microcontrollers to play with. Mostly costing £1-£2 they are remarkable little devices. The original plan was just to operate the timer for the door with a small and simple device and so I chose the little 8 pin pic12f683 and used the internal oscillator to keep time. Unfortunately from experiments this lost or gained an unacceptable amount of seconds each day and despite fine tuning I realised I would have to use an external crystal oscillator for the desired accuracy. I also decided to use more pins to increase the project functionality. I managed to destroy a couple of pic16f648a and ended up with a pic16f88 which the project is based on. Just about any 18 pin pic would do (the pic16fxx would be a cheaper option) and converting the software to work on different pics is relatively easy.

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.