Two thumbs up on the relay approach, quick, cheap & robust! And
actually, no electronics are really needed (darn!)...voltage spikes on
12+ automobiles can be very nasty. Any circuit using active parts (PIC
controllers included) that is used on a car's electrical system will
have to have some kind of power conditioning to insure those little PN
junctions can survive the voltage swings...day in and day out. The only
"electronic" component needed here is a 2N4004 diode, reverse biased
across that relay's coil, without it you stand a good chance of damaging
the contacts on microswitchs. As the relay coil is de-energized the
resulting back-EMF voltage spike is more than enough to either weld the
contacts together or at least cause pitting of the contacts...that fail
later. The reversed biased diode will absorb enough of the back-EMF
energy to protect the microswitchs...one less thing to worry about.

And don't forget to include a fuse, physically located as near to the V+
supply bus (or battery) as possible! Current rating on the fuse should
be slightly over the combined current draws of motor(s) and relay coils
that pass through it. Keep it as small as possible. Chemical fuses
(Poly-fuse type) work well on things like this.

Now the hard part: the mechanical design! (ha ha)
The steering mechanism off of a remote control car should have
everything you need for the movement. Remote control cars are getting
cheap enough to justify buying one for the parts. It might also have
the microswitchs as well. A lot of those car motors are already rated
for 12VDC

For the actual lid assembly, a rotating "visor" approach should be
fairly easy to do. With a movement similiar to a motorcycle helmet's
visor. Small pins can be installed in the assembly to operate the
microswitchs.
Good Luck!
WestexJoe

ehsjr wrote:
Cableguy wrote:
Default, Thank You very much for you input, with it I learned the
"Zero Seeking" term...

What I want to acomplish is a movable faceplate for a car audio system
that would Act when the power was feed to the circuit and then return
when powered off.

Like this.

car key in On position go from A to B and Stop
car key in OFF position go from B to A and stop

I havent yet startred the face plate movement design because I havent
yet decided how far the movement should go, althought taht is not
important to the circuit design...

Once again Thank you for your input...


Paulo Gomes


A single DPDT relay should do it for you.

+12 Accessory ----[Ry1]----------+-- Gnd
|
Ry1-1 / |
+12 ---+---o-o---o o---o o---+
Batt | O_Lim | | C_Lim |
| | |-' |
| | |
| o |
| | |
| [Motor] |
| | |
| o |
| | |
| ,-| | |
| | | |
+---------+ +---------+
Ry1-2

Shown in the closed position with the C_Lim (close limit) switch
open. When the car key is turned on, the accessory line goes to
+12 and the relay is energized. +12 from the battery gets to the
top of the motor through the Ry1-1 open contact and the O-Lim
(open limit) switch. (-) gets to the bottom of the motor through
the Ry1-2 open contact. The motor spins until the O-Lim switch
opens when the limit is reached. The C-Lim switch closes as soon
as the face plate moves away from the closed position.

When the key is turned off, Ry-1 de-energizes. This time, Ry1-1
_closed_ contact brings (-) to the top of the motor through the
closed C_Lim switch, and the Ry1-2 closed contact brings (+) to
the bottom of the motor. The motor spins in the opposite direction
from the previous operation, and continues spinning until the close
limit is reached, which opens the C_Lim switch. The O_Lim switch
closes as soon as the face plate moves away from the open position.

Ed