Hi all!
I have the following problem: I want my arduino to switch on/off a 5V 800mA load.
Since the arduino and the load are powered from batteries, I would like to have an efficent way to do it. I was searching around and I have seen TTL Mosfets. Are they a good solution for my problem? Do you have any suggestions?
Thanks a lot!
Yes, Logic Level, Low Rds, N-channel MOSFET, use it to disconnect the Ground from the load.
Here's a good tutorial: http://bildr.org/2012/03/rfp30n06le-arduino/
You should also add a 100 Ohm resistor in series between the mosfet gate and the arduino pin that you are using to drive the mosfet gate. That isn't included in the tutorial, but it's good policy.
Sometimes its more sensible to switch on the high-side (so grounds stay in common), in which case a logic-level
p-channel MOSFET is the solution. 800mA is a bit high for many bipolar transistors (there are some though that
can switch 800mA from an Arduino pin, modern "superbeta" switching transistor is the type.
For reasonably low loss (0.1V max at 0.8A) I'd suggest a MOSFET with Rds(on) of 0.1ohm or less. For low-voltage
switching go for a low-voltage MOSFET, they have lower on-resistance than say a 100V device. A 20V or 30V
device is adequate for 5V load!
Thank you all for the replies!
MarkT, it is not clear for me the advantage of switching from the high side. What do you mean with "sometime its more sensible"?
Uhm I forgot to tell an important detail. My load is a Raspbberry Pi with and a usb hub. So probably what MarkT is proposing is the right way to do it; i.e. the common ground reference is important. Am I right?
ACRay:
Uhm I forgot to tell an important detail. My load is a Raspbberry Pi with and a usb hub. So probably what MarkT is proposing is the right way to do it; i.e. the common ground reference is important. Am I right?
Yes it is important as you might find your self switching it on due to peripherals having a common ground with the arduino.
why are you using an arduino to switch power to a raspberry. Can't you use the one processor to do both tasks (that of the arduino and that of the raspberry)
Good question 
I'm trying to build a solar powered "drone". It has 2 servo motors, 2 cameras, 1 wifi dongle, backup batteries and solar panels. The main idea is that when there is enough energy in the batteries, everything is running. When the batteries are going down, the drone automatically shuts down the pi, cameras and dongle and puts itself in recharging mode. In recharging mode, only the arduino is powered and it's most of the time in sleep mode. From time to time, it wakes up checking the batteries level. If at some moment in time there are is enough energy, it wakes up the pi again.
Yes I know, it's a bit crazy 
Hi,
I found this one doing a great job:
It's a relais, that you can trigger with a simple digital out from your Arduino, Attiny or whatever. Added bonus: since it's a relais, you can trigger currents that are either higher or lower than the signal coming from your Arduino. So no need to find the right transistor or mosfet. And the circuit you are switching is physically seperated from the circuit that powers your Arduino.
Good luck,
David
There's loads of these reed relays on eBay:
http://www.ebay.co.uk/itm/R2-2A-5V-DC-Reed-Relay-2PST-0-5A-125VAC-2-Circuits-DIP-Packaking-/180818418891?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item2a199e1ccb
The coils only draw 10mA so you can drive them directly from an arduino pin.