Thanks William,

Even half awake you can still give good advise:).
My circuits knowledge is limited so let me see if I understand you correctly.
As I understand the GPIO can drive a source at 3ma/10ma. This is not enough if I would like to have a transistor like 2N4401 reach full saturation.  Which mean that I won't be able to amplify the current to drive something that requires 400mA. Am I correct. However, it should be enough for a relay wouldn't it?

So you are recommending that I use a mosfet with a 2N4401in common-emitter inverted configuration so that i can achieve higher currents...right

On Sat, Apr 4, 2009 at 12:39 PM, William Hue <william@neodynsystems.com> wrote:
Please see clarification below.

William Hue wrote:
> Hi Joey, please see my comment below.
> Rizal Nor wrote:
>> Hi Dave,
>> Thank you for the quick reply again.
>>
>> On Fri, Apr 3, 2009 at 11:14 PM, Dave Hylands <dhylands@gmail.com
>> <mailto:dhylands@gmail.com>> wrote:
>>
>>     Hi Joey,
>>
>>      > I know i need a mosfet circuit to drive the relay. But what I'm
>>     not sure
>>      > whether i need
>>      > a relay in the first place. My device has a maximum load of
>>     400ma. According
>>      > to tutorial sites you gave me, the ULN2803 can drive up to 500ma.
>>      > Should I still get a relay, or should i just drive it directly
>>     from the
>>      > mosfet?
>>
>>     It really depends on the type of thing that you're controlling. Is it
>>     inductive? or resistive?
>>
>>
>> I want to control a satellite modem and a DO sensor.
>> I'm not sure if its inductive or resistive.
>>
>>
>>
>>     What voltage does your load require? Presumably your load needs DC
>>     rather than AC?
>>
>>
>> The DO sensor is using 12 volts.
>> And the satellite modem at 5 volts.
>>
>> As for now, the ULN2803 takes a while to get here. I'm going to use
>> 2N4401 that
>> i get from radioshack. It looks like this transistor can handle up to
>> 600mA.
>> I'm going to go for the relay switch option for now, unless you advise
>> otherwise.
>> Do you see any problems for using this transistor?
>
> To assure good saturation, you need 40 mA of base current at Ic=400 mA.
> I doubt the Gumstix GPIO pin can supply this directly, and even if it
> could, it would not be good for it.  I would use a MOSFET instead, but
> you need to either use a logic-level MOSFET or you can use the 2N4401 as
> an emitter-follower to provide approx. 12 V at the gate of the MOSFET.

Sorry, I am still sleepy this morning.  Emitter-follower configuration would only provide GPIO voltage minus VBE.  To provide more voltage at the gate of the MOSFET, the 2N4401 must be in common-emitter inverting configuration.  Sorry about that.

>
>>
>>
>>
>>     --
>>     Dave Hylands
>>     Shuswap, BC, Canada
>>     http://www.DaveHylands.com/
>>
>>
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>>
>>
>>
>>
>> --
>> Joey aka. Rizal
>> Home is where people think of you...
>>
>>
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