# need help with simple circuit

Discussion in 'The Projects Forum' started by tealcrystals, Dec 19, 2008.

1. ### tealcrystals Thread Starter New Member

Dec 19, 2008
4
0
ok ive got some gadgets in my car such as a digital tachometer, which run off the cars 12V supply. i have installed a toggle switch to allow me to switch between the 12V ignition power (on when the engine is on, off if the engine is turned off) and straight 12V battery power (on all the time). it works fine, but i want to install two LEDs, to indicate which way the switch is flipped i.e blue for ignition power and red for straight battery power. i dont want the LEDs to interfere with the supply voltage to the various gadgets i have installed because obviously the voltage drop will cause them to not function. ive experimented with a few configurations but cant get anything to work. as im only starting uni next year i havent learnt much about circuit theory beyond series and parallel circuits. could someone help me / give me a circuit schematic? this is what i have so far:

+12V bat----------o
⁬⁮⁬⁮⁬⁮⁬⁮⁬⁬⁮/o---------------supply +ve
+12V IGN----------o/

(the thing in the middle is meant to be a switch)

so i need to incorporate two LEDs, one to indicatate each of the 12v bat and 12v ign supply rails.

ahh..

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
Use a double pole switch with the indicator LED's running off one set of contacts.

3. ### gerty AAC Fanatic!

Aug 30, 2007
1,153
304
From your post it sounds like you may have wired the leds in series with the other loads. You shouldn't have a voltage drop just by adding leds.If you parallel the open contacts the led should light. It would show which side was open, not closed, but you get the idea.

OR

You could use a double pole double throw switch in place of your single throw switch. Just wire one set of poles the way you have it now and the other set of poles for your leds..

edit: beenthere you beat me by 2 min...

4. ### tealcrystals Thread Starter New Member

Dec 19, 2008
4
0
thanks guys. ill get the double pole switch tomorrow and try it out xD

5. ### Steve C Active Member

Nov 29, 2008
86
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dumb question, but why is the toggle needed to begin with? constant and IGN are shorted with the key in any position besides "off". Seems like the switch is redundant. an On/Off switch that only works with the car off?

6. ### tealcrystals Thread Starter New Member

Dec 19, 2008
4
0
argh i have another problem now.
i dont know how to power the leds to have them act how i want them to.
if i use the ignition power to power them, they will only light when the car is on which is useless, and if i use the always-power, one of them will be always on even when the cars turned off which will drain the battery.

A R G H !

this is my diagram with the circles representing the 6 pins of the switch:

IGN +ve -----------------O O--------------blue LED

output +ve---------------O O------------- power???

batt+ve ----------------- O O--------------red LED

basically i want the battery power to power the red LED and IGN power to power the blue LED. but how can this be accomplished? i see that if i put the seperate power supplies to the top and bottom of the right hand pins it would switch the power as it does with the other side of the switch, but where would the leds go? they would both be powered regardless of the position of the switch if they both came of the middle tag.. hmm *dies*
and i read ur blog beenthere and i know u like people to work things out themselves. i also know this is extremely simple to you. the question is, are you willing to help me, given the knowledge that i start uni next year and will indeed get to experience learning electronics first hand both at uni and at home. xD

Last edited: Dec 22, 2008
7. ### tealcrystals Thread Starter New Member

Dec 19, 2008
4
0
hmm i guess if i could find a switch with 8 pins, (the two middle pins from the above 6 pin switch replaced by 4 pins so the middle pins are not shared between the switch's two positions) i could make it work.

8. ### Steve C Active Member

Nov 29, 2008
86
0
I'm struggling here. Your devices are things like a digital Tachometer? Why do you need a switch to power these? Why didnt you power these devices from relay contacts? Where the contacts are connected straight to the battery via fuse, and the relay coil is connected to IGN. No switches, no LED status lights, no holes in the dash, no complexity, no thought. Things just work. Without your intervention.

What do you need to have personal control over? If you need a switch then:

-You want to turn something off when the car is running
-You want to turn something on when the car is off

Your example is a digital tach. NO reason to have this thing on when the car is off, or off when the car is on!

What are you really trying to do?