Ridiculously stupid question. DPST switch

Thread Starter

CRNUT

Joined Feb 7, 2008
8
This would have been easy for me back in the day but I am lost right now. I want to control 2 12V light bulbs with a SPDT switch. The switch needs to light one bulb in one position and both bulbs in the other position. Every diagram I can think of will light both bulbs in both positions. Can't be that hard maybe just a diode somewhere. Can anyone draw me up a simple diagram?
 

nomurphy

Joined Aug 8, 2005
567
No, with just a switch you will need a DPDT. Otherwise, it will require a logic circuit.
 

Thread Starter

CRNUT

Joined Feb 7, 2008
8
So no easy way around this even if I control the bulbs with relays?
 

kubeek

Joined Sep 20, 2005
5,587
Well if one of the bulbs is always on, you could just switch the other on and off.
 

nomurphy

Joined Aug 8, 2005
567
Yes, you could use a relay or two to make a logic circuit. But why not just use a different switch?
 

SgtWookie

Joined Jul 17, 2007
22,201
This is 12V DC, right?

Is the switch simply SPST, or does it have a center off position as well?

See the attached - it uses a single diode in addition to your two bulbs and switch.

The diode will have about an 0.7v drop across itself, leaving one bulb slightly less bright.
 

Attachments

Thread Starter

CRNUT

Joined Feb 7, 2008
8
Correct 12V DC, with a center off.
I want
middle---nothing lit,
bottom--one lit,
top------both lit

Two bulbs and preferably two basic 4-5 pin relays (automotive type)
 

Thread Starter

CRNUT

Joined Feb 7, 2008
8
I should be about to do the same thing as your diagram but replace the bulbs with the primary relay coils and I should be ok?

I don't think the switch is rated to handle the bulbs thats why I wanted to use relays...
 

nomurphy

Joined Aug 8, 2005
567
Ummm, the 1N400x series is only good for 1 amp which may not be sufficient. Make sure the switch and the diode are rated at least 2x above the currrent of the lights.

I = W/V
2.1A = 25W/12V
 

SgtWookie

Joined Jul 17, 2007
22,201
Ahh, ok. I was thinking you were working on radio indicator lights or something.

Sure, in the schematic I posted, just replace the lamps with the relay coils.
You'll need two SPST relays in addition to the diode.

Also, use diodes in reverse bias across the relay's coils to absorb the reverse EMF when the current is interrupted, otherwise your switch contacts will have a short lifespan due to arcing.
 

Thread Starter

CRNUT

Joined Feb 7, 2008
8
I am actually installing HID headlights on my four wheeler and trying to use the factory mounted switch built into the handle bar controls haha
 

SgtWookie

Joined Jul 17, 2007
22,201
Oh, OK! Yeah, the current draw of HID headlamps would've cooked the switch pretty quick!
 
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