I’ve got the Beefcake relay control from SF, http://www.sparkfun.com/products/11042 . I would like to use it with my Mini, an am wondering if I can grab 5V from the USB and use AN3 (out) with a transistor as a switch for the 5V. Good idea, bad idea? What else do I need (like resistors)? Am I smokin’ too much hemp?
The coil at 5v takes 200mA ± 10%, USB max current is 500mA / port. The maths is easy, but would I do it, well if you pass the hemp I might give it a try. Really depends on what ever else you power up with it.
Whatever else = a DHT11 temp sensor and an XBee, which collectively draw about 50-100 mA depending on what the XBee is doing at any given time. So, I’ve got current to spare. Sound doable? Thanks for posting any math (hint)
@ Davef - own a gadgeteer mainboard? If so, I’ll barter one of my breakout modules for a schematic. The breakout is also good for plumbing gadgeteer modules to any uC.
My hardware skill level is good enough to get me in trouble, so I often check with the experts here to help prevent any magic smoke. I don’t want to hurt my one and only mini. I could hook the relay to an Arduino, but have other plans for those…
There are a few comments I can add,
When I said port I mean a computer port. A hub shares the 500mA. If you are using a hub use a powered one.
If your mini suffers from resets when the relay is turned on, it will be due to a switch on current transient due to the inductive load (the coil). In this case you will need to add a capacitor across the 5v and GND. You could try something like 220uF or even X10 that value. Dont confuse the switch on transient with back EMF that is taken care of with a diode across the coil. Its very difficult to give you a precise value because I dont know what the current transient is ( it would need to be measured). Given a Farad is defined as 1Amp second, the maths is not complicated. You need to know the half amplitude width and the amplitude of the transient. You can then work out from the Farad definition the precise capacitor value you need.
I like the idea of bartering, but dont have the tools to supply you with a schematic. I’m dont work in that arena anymore.
Thanks for the tips. I’m soliciting information here to compare with my own research to “double check” the concept. I’m not powering the relay directly, and the board it is on has some protection circuitry. I’ve successfully breadboarded the idea, and will soon try it with the mini.
I’m happy to send a MakeBread module your way for the effort. Email your mailing address to my forum name at gmail. If you’re not in the US, I may hit you up for a little bit of postage.
Thanks for the offer, but the information I give is free of charge, otherwise a charge would have been up front. ;D