Re: Design Help Needed

gecono wrote:
On Thu, 29 Mar 2007 07:24:49 -0500, "Anthony Fremont"
<spam-not@xxxxxxxxxxx> wrote:

gecono wrote:

Okay according to the usb-uart module data*** regarding power I
will have 5V/500mA available from the usb bus. For me I think it
would be easier to tie into that power for the pic.

Below is a relay circuit I found online. I translated it to the
ascii you see below. It contains the transistor mentioned above.
Hopefully it makes sense...


vcc
| \
|__ o o
| )|
1N4001 - )| relay
^ )|
|__)|
|
|
|/
[PIN]----/\/\/\--| NPN
10K |
|>
|
|
GND


I will have six of these one for each button. Each one will connect
to a io port on the pic.

I'm about ready to order the parts. If anyone sees anything wrong or
something missing please let me know. (Feel free to give your nod of
approval as well ;)

That looks like it should work. How much current do the relays
need? The PIC can supply about 25mA out, but a driver transistor is
still not a bad idea. A 2n7000 should be an ok substitute for the
NPN transistor above, letting you skip the base resistor.

The 16F88 is a good general purpose PIC. My favorite 18 pinner
anyway. ;-) The 8 pin 12F683 is nice when 6 i/o pins is enough. I
haven't used the 16F688 yet but it should be similar, just in 14 pin
package.

I've been maintaing a project over on the Mouser website (nice website
for a beginner, shipping stinks). The 2n7000 you mentioned is
currently backordered. They cross-referenced another one of their
products the Fairchild Semiconductor BS170. Do you think that will
work?

Wow, I just got a bunch from them. I musta cleaned them out. ;-) I haven't
really had any complaints on the shipping, but I live in houston so I just
have it shipped ground. I always get it the next day. :-) It's usually
$4.50 or so for the typical under 1lb shipment. I try not to order until I
need a bunch of stuff. That's one nice thing about their site, your
shopping cart doesn't die when you close the browser.

If I stick with the npn transistor how do I choose the correct one?
Mouser lists over 6500.

You have to check the specs on each one. :-O Not really....you just need a
general purpose switching transistor like a 2n3904 or 2n2222 depending upon
the current requirements of your relays. You might even be able to get
those at radio shack still. You'd have to check the datasheets to be sure,
but those should be good for 100 - 150mA. If you use an SSR, you won't need
the diode either. It's for a magnetic coil type relay. It won't hurt
anything, you just don't really need it.


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