Moteino Mega and Software serial

[gregcope]

Many thanks.  I can get those with free shipping;

http://m.rs-online.com/h5/mobile/uk/catalog?url=%2Fweb%2Fp%2Fproducts%2F5429478%2F?cm_mmc=UK-PPC-0914-_-google-_-3_MPN_Phrase-_-1380f9f1-0ad9-aac8-dbd9-00001b3cce34&gclid=CIfG1fCc0sUCFcZ02wodMg8ASg

One quick Q;

Why that one?  What is the difference (in summary) between that one and the others so that I might learn something.

One challenge in this area is that I have read quite allot, and there seems no consensus on parts ...

[TomWS]

Many thanks.  I can get those with free shipping;

http://m.rs-online.com/h5/mobile/uk/catalog?url=%2Fweb%2Fp%2Fproducts%2F5429478%2F?cm_mmc=UK-PPC-0914-_-google-_-3_MPN_Phrase-_-1380f9f1-0ad9-aac8-dbd9-00001b3cce34&gclid=CIfG1fCc0sUCFcZ02wodMg8ASg

One quick Q;

Why that one?  What is the difference (in summary) between that one and the others so that I might learn something.

One challenge in this area is that I have read quite allot, and there seems no consensus on parts ...

Unfortunately, in the Arduino world, a lot of information is based on 5V Arduinos.  The transistors you read about were fine if driven from a 5V processor but they aren't really low enough Vgsth for a 3.3V processor (which is much harder to accomplish on a power FET).  I was able to use the 2N7000 simply because this circuit draws very little power from the transistor so, even though it's operating in the weak part of its 'knee', it's still good enough for this app.

Secondly, the power transistor you have is an 'N' channel part and, for this circuit, you need a 'P' channel part because you are switching the positive rail rather than ground.  Also the power transistor you have has 4 times the gate capacitance of the one I recommended - largely due to its increased current capacity (which you don't need for this case).

However, the primary driver to select this particular transistor is that you wanted a through-hole transistor.  I could find a zillion surface mount (well, maybe not that many) transistors that are low threshold  and support currents >500mA without going to a multi-digit Amp capacity.  It wasn't so easy finding an through hole transistor that would switch >500mA and not end up support tens of amps. 

The other criteria was that it had to be something you could get on your side of the planet.  After that, it came down to price 

As an example of the 'knee' I mentioned earlier, I've attached two clips from two datasheets, both N channel transistors.  The first is from the 2N7000 data sheet, the second is from a transistor I'm using on a project.  In the case of the 2N7000, it doesn't even think about conducting current until the Vgs gets around 3V, take a look at the second one, it starts conducting a significant amount of current at 1.25V...  Another useful curve, which I trust more, but isn't included in the 2N7000 datasheet is RdsOn vs Vgs.  This really shows where the 'knee' is.

Tom

[gregcope]

Thanks Tom.  Those P-Mosfets are cheaper than the others!

Last Q... (sorry);

What would you recommend (from the two N Mosfets and the P-Mosfet) for switching/powering on/off from a digital pin a 25mA GPS or a (upto) 1A GPRS modem?

[TomWS]

Thanks Tom.  Those P-Mosfets are cheaper than the others!

Last Q... (sorry);

What would you recommend (from the two N Mosfets and the P-Mosfet) for switching/powering on/off from a digital pin a 25mA GPS or a (upto) 1A GPRS modem?

For your GRPS modem, since it draws so much current, I would use the same circuit (from 12V) to a linear voltage regulator to drop down to the GPRS modem voltage.  I think switching regulators, unless they are VERY GOOD regulators, will be too noisy and I don't think you have it on that often to worry too much about wasted power.  Doesn't that GPRS radio have an enable pin that drops power to virtually nothing?

For the GPS receiver, I don't think you have a proper MOSFET in your 'inventory'.  I'd use a PNP bipolar transistor (PN2907 or equivalent).  Emitter to 3.3V, base driven through a 2.7K resistor from a Moteino pin (low to turn ON), and collector driving your GPS device.

You might want to look into getting some SOT23 breakout boards. Sparkfun and your very good friend Felix sells these.  This will allow you to use these very handy surface mount MOSFETs without having too much breadboarding difficulty. 

Tom

[gregcope]

Sorry Tom.  You've lost me.

Looking at the spec of the XXX Do I need the 2N7000 to drive it's gate as I assume this bit;

Gate-Source Threshold Voltage VGS(th) VDS = VGS, ID = - 250 μA - 2.0 - - 4.0 V

I assume that means that a 3.3v Digital pin on a Moteino should be able to drive that Gate easily?

From here; http://docs-europe.electrocomponents.com/webdocs/0791/0900766b8079172b.pdf

For simplicity can I not just use this everywhere?

[TomWS]

Sorry Tom.  You've lost me.

Looking at the spec of the XXX Do I need the 2N7000 to drive it's gate as I assume this bit;

Gate-Source Threshold Voltage VGS(th) VDS = VGS, ID = - 250 μA - 2.0 - - 4.0 V

I assume that means that a 3.3v Digital pin on a Moteino should be able to drive that Gate easily?

From here; http://docs-europe.electrocomponents.com/webdocs/0791/0900766b8079172b.pdf

For simplicity can I not just use this everywhere?

I'm not sure what you're struggling with.  There were two places that the circuit using this transistor was used, both of them require the 2N7000 to provide the voltage shift so that the P-Channel MOSFET's gate can be pulled up to 12V.

What am I missing?

[gregcope]

My own stupidity/inexperience.