Modena wrote:
> testing the circuit with an LED works fine, but when I hook up my
> actual 12v valve, it must be causing interference or something because
> pressing fire and the circuit switches on, and stays on indefinitely,
> hence the valve stays open.
>
> anyone have any ideas how I might filter this?
You need to isolate the load from the logic. There are three basic
solutions: transistor, logic-level FET or opto-isolator/relay.
If the load isn't too big and you like ruining lots of circuitry trying
to make things reliable, a simple transistor can be used. There are
deceptive devices, however, and when you think you've got them working
properly, they bite you. There are dozens of circuits on the web, which
tells you just how many people have tried to design the "perfect"
transistor switching circuit.
The new kids on the block are logic-level FETs (I think they have a more
official name) have been designed to be fool-proof, allowing logic
circuits to directly drive a high current FET. They have all kinds of
safeguards built into them (designed to eliminate the need for all those
fancy transistor circuits mentioned above), but I have yet to find one
with good old-fashioned solder leads ... they all seem to be tiny little
surface mount packages. If you find one, let me know, because I'll use
them for everything.
The approach that I've used the most (along with Dr. Sommer) involves an
opto-isolator IC and a relay. The logic triggers the opto-isolator (5v,
very low current), the opto-isolator triggers the relay (usually 5v or
12v, 100ma or so) and the relay triggers the load (any voltage, any
load). It sounds like a lot, but it's quite simple once you've wired it
a couple times and it is one of the most reliable switching approaches.
Frank P.
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