[Soekris] How to drive external relays?

[Christopher Hoover]

> Has anyone ever used a soekris box to drive a set of small relays?  
> I have a project in mind where I would have to control a small 
> series of relays and be able to turn them on at pre-scheduled times
> and keep them on for anywhere from 1 to 45 minutes.  The relays
> would be used to send a larger voltage down the line to control 
> end devices.

If you are switching DC and your load potential and switching current is
reasonable, consider using a semiconductor switch instead of a relay.

You can build such a circuit from discrete parts around a transistor or
a MOSFET.   (See a standard reference like Horowitz & Hill for details.)
But what I recommend these days is an integrated circuit.

ST makes a very nice line of such things.  They call them "Intelligent
Power Switches".  

More here:

<http://www.st.com/stonline/products/families/analog_and_mixed_signal/in
telligent_power_switch/intelligent_power_switch.htm>

Other vendors sell similar IC's.

These parts are highly robust and have standard CMOS level control
inputs.  Basically you drop the load across two pins (high side or low
side, depending on the part) and you drive the control input with a
GPIO.  That's it.

I've used these parts in several projects (personal and professional).

-ch