I2C enabled mosfet switch

[bryce1]

Hi guys,

I designed a low side n-mosfet switch that is controlled by an attiny48 (i2c slave). The idea was to have a couple of relatively generic relais alternatives that can also do pwm with 12V loads in a couple of different places.

It works well, but I keep wondering if there is a better way. Do you know of any i2c enabled ICs that do this kind of stuff already and are cheaper or better suited?

Thanks!

[abaxas]

http://www.adafruit.com/products/732

or pick your favorite i/o expander of choice.

[Precipice]

or pick your favorite i/o expander of choice.

And perhaps an optocoupler?
i2c, multi-board design, currents high enough to be switched by relays, with FET-speed edges - not always a happy combination. 
Certainly take care.

[bryce1]

that is some valuable advice. can you elaborate?

the attiny costs less then one dollar. this is more expensive and does less.

[abaxas]

that is some valuable advice. can you elaborate?

the attiny costs less then one dollar. this is more expensive and does less.

Time to write software is not 'free'.

[bryce1]

that is true.but writing software scales better.

[bryce1]

with FET-speed edges - not always a happy combination. 

how do I take properly take care of fet speed edges?

[Precipice]

how do I take properly take care of fet speed edges?

'carefully'

Slow them down with resistors, but not so much that unlucky circumstances (high load, edge-case FET) is at risk from thermal damage as it turns on or off. (I seem to recall you were doing relay-style switching, so is should just be individual edges you care about, rather than sustained heat from PWM turning it on and off all the time).
You can also use current limiting, but that's harder.

You might also want to put some capacitance in parallel with the gate, since you can't really trust the gate capacitance to be the same from transistor to transistor, batch to batch.

Or/and you could also put a bit of resistance under the Source, to limit the current the load can take, especially at switch-on. Have a search for source degeneration.