# Need a circuit: 2 lights, 2 switches

1. Sep 17, 2009

### DaveC426913

Almost 20 years since I took Electronics in night school.

I have a 12V circuit with two lights and two switches.

I want:
- S1 to light up L1.
- S2 to light up both L1 AND L2.

How can I do this?

(In case it comes up, I do NOT want to run a second wire to one of the lights (like you might do in a three-way circuit) because the lights are distant and inaccessible.)

Here is some context in case it helps:

My boat runs off a car battery. I have 1] running lights and 2] a masthead-mounted anchor light that doubles as a steaming light (which is one of several running lights).

One switch will turn on the anchor light ONLY.
Another switch will turn on running lights which INCLUDES the anchor/steaming light.

I do not want to have to turn on two switches to get my running lights going.

Last edited: Sep 17, 2009
2. Sep 17, 2009

### Staff: Mentor

S1 is SPST, and S2 is DPST. That should do it, I think.

3. Sep 17, 2009

### Janus

Staff Emeritus
Do you need the switches to be able to both turn on and off the lights, or just turn them on? (IOW, do you need to be able to turn off L1 from either S1 or S2 no matter which one you used to turn it on?)

If you just need to able to turn them on :
I'd use 1 SPST switch (S1) wired in parallel to one pole of a a DPST switch(S2) to turn on L1 and use the second pole of the DPST to turn on L2.

Throwing S1 turns on L1.
Throwing S2 turns on L1 and L2.

The drawback is that if you turn on L1 with S1, you can not turn it off with S2 and vice-versa.

4. Sep 17, 2009

### DaveC426913

Ohhh. That's what double pole switches are for...

Hm. I've got all the switches, they are identical and SP. To do this, will I have to get a different switch? That would be a bummer. There's no user-facing reason for this switch to be of a different type.

The switches are independent, so:
S1 Off, S2 Off : L1 Off, L2 Off
S1 ON, S2 Off : L1 ON, L2 Off
S1 Off, S2 ON : L1 ON, L2 ON
S1 ON, S2 ON : L1 ON, L2 ON

I do not need to turn L1 Off using S2, no.

The point of S2 is to turn BOTH lights on; it will override S1's off state.

5. Sep 17, 2009

### Bob S

I would try a single DPDT polarity-reversing switch with a center-off position, and with a diode in series with light L2.
Bob S

6. Sep 17, 2009

### DaveC426913

I wish to have no confusing distractions for the user. Switches should all have same look and operation.

7. Sep 17, 2009

### Bob S

See attached thumbnail. I think you will need two DPDT switches and a diode in series with L2.
Bob S

#### Attached Files:

• ###### DPDT switches.jpg
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8. Sep 17, 2009

### DaveC426913

Perhaps I'm misinterpreting your schematic but I don't see how it does what I want. At first blush it looks like you have only one circuit going out to the lights. There'd have to be at least two circuits to operate the two lights differently.

9. Sep 17, 2009

### DaveC426913

I bought a 2A relay.

10. Sep 17, 2009

### Staff: Mentor

That will drain your battery quicker, and leave you stranded at sea... :tongue2:

You can get switches in SPST and DPST configurations that look identical to the user. The DP version has a body that is a little wider, but so what? The switch bodies are behind the panel that the switches are mounted on.

11. Sep 17, 2009

### DaveC426913

Yes. Drat.

Are all relays built the same? i.e. mechnaical magents? Or are some solid state? Might they use less current?

The one I got looks a lot like this but with 8 pins.
http://www.supplierlist.com/photo_images/74906/Communication_Relay.jpg [Broken]

It says 9908 and M4 12H and F.

Yes. If I had thought of that before I bought my switches, I could have made sure I got ones that came in both (or simply gotten them all DP).

Last edited by a moderator: May 4, 2017
12. Sep 17, 2009

### Staff: Mentor

Where did you get them from? Any chance you could talk them into exchanging them for DP versions? You'd pay a little more, so that would be incentive for the store to consider it.

13. Sep 17, 2009

### Integral

Staff Emeritus
I drew this real quick in paint, hope you can read it. I think it does what you want.

#### Attached Files:

• ###### Lights.JPG
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14. Sep 17, 2009

### DaveC426913

The first thing I did was go back and ask if they had them in DP. They don't.

15. Sep 17, 2009

### DaveC426913

Yeah. Once I realized what a DP switch was, the wiring was a no-brainer.

16. Sep 17, 2009

### Staff: Mentor

Time for a Digikey order...

Or, one other solution would be to use 3 SPST switches, but mechanically gang the toggles together on two of them. What do they look like mechanically? Any practical/aesthetic way to connect two of them in parallel mechanically?

17. Sep 17, 2009

### DaveC426913

I thought about that. I might get away with two if the bar overlapped but wasn't attached to the second switch (so it could push on but not off).

My switches are these (ironically, from Digikey :tongue2:):

18. Sep 17, 2009

### Integral

Staff Emeritus
IF you have 3 spst relays you can wire them to do what you need. You just need to wire the control circiuts to fire 2 at once. That should be easy to work out.

19. Sep 17, 2009

### Staff: Mentor

Relays certainly could work, but the extra current consumption is a disadvantage in a battery-powered application like this.

Dave, you mentioned 12V, but how much current are we talking about here for each circuit? If the relays only added 10% to the current draw, that might be okay...

20. Sep 17, 2009

### DaveC426913

L1 is a multi-LED light, like this:

The others (3) are small incandescent running lights.