At 04:28 PM 5/30/2011, Jason White wrote:
>Hello All. I'm currently working on a project that requires me to use
>2 1:16 multiplexers to interface with various peripherals due to
>limited I/O. I'm curious how much (RC?) filtering would be required to
>make a short fast pulse (eg. 1-10khz 1-5% duty cycle) a suitable
>signal to drive a relay.
>
>Thanks
>
>relay: odd Chinese 20A H-Bridge relay on hooked up via opto-coupler,
>No other specs
>multiplexers: CD74HC4067M
>MCU - 16f690

Use a signal diode feeding a parallel RC network (one end of RC=20
grounded, the other end to the cathode of the signal diode).  The=20
junction of the diode / R / C then feeds a CMOS or MOSFET input of=20
some sort, depending upon what you want to do.

If this is just driving the relay coil, a small MOSFET (2n7000 or=20
similar) is fine.  Gate to the D / R / C network, source to gnd,=20
drain to the bottom side of the relay coil, top side of the relay=20
coil goes to your relay supply (+12V?).  Don't forget the diode=20
across the relay coil.

Choose R to be somewhere between 10k - 100k, then pick C such that=20
the voltage across C remains well above the turn-on voltage of the=20
mosfet while you are generating pulses but decays quick enough so=20
that the relay turns OFF quickly.  R * C should be anywhere from 3x=20
to 5x longer than the period between your pulses.

As you can tell, most of this can be pretty loosey-goosey.  Use a=20
scope to monitor the voltage at the gate of the mosfet if you are concerned=
..

dwayne

--=20
Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
www.trinity-electronics.com
Custom Electronics Design and Manufacturing

--=20
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