piclist 1999\09\15\193723a >
Thread: Avoiding relays on NE555 Timer cirucit
picon face BY : Mike Keitz email (remove spam text)

> >I am using a timer circuit to control an wiper motor on a car.

>> Each relay, therefore must
> control
> >about a 7amp 12VDC load. But mechanical relays are too loud.

Most cars still have a lot of relays, including one to start the wipers
intermittently.  They are mounted under the hood where you don't hear
them.  Maybe this idea is too simple or too complicated though.

I built an intermittent wiper circuit a long time ago based on a SCR.  In
that car, when the dashboard switch was "off", a switch at the wiper
motor would make it continue to run until the wipers returned to the down
position.  So all the delay unit had to do was bypass the switch for a
second to start the motor, causing the bypass switch to close, and it
would wipe once and stop at the down position.  A SCR works well for this
since the switch closing would shunt current away from the SCR and keep
it off.  That was an old car however.  I think the systems in newer cars
are much more complicated since they don't just stop at the bottom of a
cycle, they move the wipers further down to a "parked" position.  A
simple SCR approach may not be possible.

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See also: www.piclist.com/techref/io/motors.htm?key=relay
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