# Dummy needs help!

Discussion in 'Electronic Design' started by [email protected], May 27, 2007.

1. ### Guest

Dummy needs Help!

I am trying to connect two LED 12 vdc lights to come on at intervals
on a 3 position rotary switch. . . Position 1 = Off. . . Position 2
= One light comes on. . . Position 3 = Both lights come on. . .
This doesn't seem too hard except that in Position 2, where only one
of the two lights are suppose to light, the power back feeds to the
second light through the wiring connected to Position 3 "where both
lights are wired together." I have considered using a diode to block
the travel of the (12 vdc) voltage in one direction, but I don't know
what size diode to get or if size even matters since I am only using 4
to 8 LED 12vdc lights here. I have also considered using a very
small 12 vdc relay here, but that seems like a lot of unnecessary work
and consumes a little more power and space than I want. Does anybody
have any ideas that will help?? Thanks. . .

2. ### JamieGuest

LED's draw very little.
I assume you are using LED's designed for 12V with a resistor already
inside of them ? If not I suggest you do.

You can use a 1N4000 series type that is good for .5 amps or more as
the back feed block.

3. ### Rich GriseGuest

Two - one per each LED.

Cheers!
Rich

4. ### ehsjrGuest

This will do it for you:

-----
+12 ---Vin|LM317|Vout---+
----- |
Adj [62R]
| |
+----------+---[LED]---+---[LED]---+
| | |
o1 | |
SW o2----------+ |
Gnd ----------------o-->o3----------------------+

The LM317 and 62 ohm resistor fix the current at ~20 mA.
For a different current, the resistor value must be changed.
The switch determines if the current goes through just the
resistor, or the resistor and the first LED, or the resistor
and both LEDs.

There is no such thing as a "12 volt LED" per se, but there
can be an LED/resitor combination that's rated at 12 volts.
The circuit above requires LEDs without resistors. If you
must use 12 volt LED assemblies (LED + resistor), use diodes,
as others have said:

o1

D1
o2--->|---+--[R]--[LED1]--+
| |
SW D2 | |
+12 ----------------o-->o3--->|---+ |
| |
| D3 |
+---->|------[R]--[LED2]--+
|
Gnd ----------------------------------------------+

The above uses 3 diodes so that LED1 will get the
same voltage in position 2 as it gets in position 3,
and so that LED2 gets the same voltage as LED1. You
could probably use a jumper in place of D1 and D3 and
not see any difference.

Ed

5. ### JasenGuest

Put a diode between 2 and 3 instead of the piece of wire you are using.
Or use a 2 pole switch.

1 2 3 C

0 0 0 0--- lamp2 --.
| |
0 0--0 0--- lamp1 --+-- 0V
|
|
12V

Bye.
Jasen

6. ### JasenGuest

12V -----[470R]-------------[LED]---+---[LED]---+
| |
o1 | |
SW o2--[81R]---+ |
Gnd ----------------o-->o3----------------------+

Bye.
Jasen

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