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- To: sigmusic-l@xxxxxxxxxxxx
- Subject: [SIGMusic] circuit2
- From: "Stephanie Bayani" <bayani@xxxxxxxx>
- Date: Sun, 25 Feb 2007 22:50:33 -0600
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i couldn't tell if everyone got the message.
I don't mean to butt in here, but I can't resist.
can't use the PIC to drive the solenoids directly, since it can only
drive 5V. You could use an open-collector buffer (7406) to handle the
voltage, but they can only handle maybe 20 mA of current. The solution
is to drive the solenoids with transistors. Connect the PIC output pin
to a 100 ohm resistor, and connect the other end of that to the base of
an NPN transistor. Connect the transistor emitter to ground and the
collector to the solenoid. The other end of the solenoid gets 24 V.
You have to put a reverse-biased diode across the solenoid as well (the
arrow points towards 24V) to keep it from destroying the transistor
with inductive voltage spikes.
The 2n3904 and
2n2222a are general-purpose all-around decent transistors. Find a copy
of Horowitz and Hill (Art of Electronics) for better suggestions.
There used to be a copy in 1110 Siebel.
24V->5V conversion for the PIC can be done with a 7805 voltage
regulator, but it's generally not a great idea to drop 19 V across a
regulator, since all that power gets dissipated as heat. If the
regulator gets too hot, you can buy simple switching (buck) dc-dc
converters that are almost drop-in replacements for the 7805, but
they'll cost $20 or so and be a bit larger.
I would have posted to the mailing list directly, but it thinks I'm at a different email address that I can't send from.
-Joel Jordan (former chair of SigArch)