SparkFun Forums

[kenichi3432]

i wanna build a pet feeder but i need advice..
i am uncertain to what kinda transformer to use as i havent know what type of motor have a strong turning power. any fan will do .
where can i learn how to select the right component for my project?please direct/advice me on what to learn..
i am on my final year diploma in E&E engineering i seem to be able to use some theory to make a circuit for this project but if there's any other way besides using experience to select type of component please advice me ..
i tried surfing circuit design.. but most lecture videos on circuit design are about microprocessor which is unrelated to this practical project.
if there is a course /book/ video that i need to know to design this circuit please let me know.. this project also need to use an ic ...perhaps i can use 555 +capacitor+variable resistor to make delay but ... if you have any other ideas please share with me..

the condition that my lecturer gave me is that no programming is allowed..
and there will be a question session later on about "the reason why i choose this particular component why not something else?" what should i do so that i can design a practical circuit and answer all their question confidently.. if u guys have any suggestion on what i should learn please tell me and i will look it up...

[Renate]

I'd always suggest when using an IR signal detector in the open to use modulated IR signals.
This device has to work with the kitchen light on or off.
That will change the threshold quite a bit.

Use the same system as IR remotes.
There are already sensitive IR receiver modules for cheap.
Here is a typical one: http://www.radioshack.com/product/index ... Id=2049727

Then all you have to do is have an IR transmitter running at 38 kHz.
You could run a 555 timer at 38 kHz and drive the LED directly.
You don't need to drive the LED very hard for the distances involved here, 10 mA would be plenty.
As always, when checking IR LEDs, your cell phone camera is your friend (despite having IR cut filters in it).

I'd just get a pre-built switching 12 V supply.
It's easy, more efficient and your box doesn't have to have any high voltage in it.

[qwertymodo]

For the motor, I'd look into a stepper motor, or maybe a servo. From what I understand, you probably want more positional control than a typical DC motor affords.

I also second the suggestion to use a commercial "wall wart" power supply (unless that's against the assignment guidelines). Otherwise, you'd need to build a full power supply, which is more than just a transformer. As an EE, I'd think you should probably have covered PSU design, but if not, the basic stages are line voltage (110VAC) -> step-down transformer (~20ish VAC) -> rectifier -> regulator.

[yamitenshi]

As an EE, I'd think you should probably have covered PSU design, but if not, the basic stages are line voltage (110VAC) -> step-down transformer (~20ish VAC) -> rectifier -> regulator.

Just a quick question there from an amateur: why put the rectifier behind the step-down transformer? Is it just for the cost of the components (i.e. you need more heavy-duty diodes for 110VAC than you need for 20VAC), or is there an additional reason behind it?

[qwertymodo]

The transformer doesn't just step down the voltage, it also isolates the rectifier from the incoming AC, which is necessary for the rectifier to function properly (at least for a bridge rectifier, it needs a closed loop).

[Renate]

why put the rectifier behind the step-down transformer?

The short answer is that the transformer will not pass DC.
You have to put the diodes after the transformer.

The transformer also isolates the circuit from the house wiring.
In days of old some radios had no transformers in them.
It is physically possible to just put the rectifiers directly on the AC line.
No, it's not recommended.

With diodes (and some other components) the current rating is the cost-connected parameter.
Higher voltage rating can often be cheaply gotten.
For example a 1N4001 diode @ 50V is 1.6 cents in bulk.
A 1N4007 diode @ 1000V is 2.0 cents in bulk.