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i have a SPDT toggle switch and trying to power two lights when switch is up and only power one light when switch is down, both lights off when switch is in the middle (neutral), how can i achieve this? i am using simple led strip lights, dc powered.

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  • \$\begingroup\$ What sort of lights? \$\endgroup\$ – Finbarr Aug 12 '17 at 8:44
  • \$\begingroup\$ simple led strip lights \$\endgroup\$ – Ken Aug 12 '17 at 8:53
  • \$\begingroup\$ If you're feeding them DC, you could do this with diodes if you can afford the voltage drop. Otherwise you'll need a DPDT switch or a couple of relays. \$\endgroup\$ – Finbarr Aug 12 '17 at 8:59
  • \$\begingroup\$ Changing to a DPDT Center Off switch is by far the simplest solution. \$\endgroup\$ – Jack Creasey Aug 12 '17 at 16:40
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. If you are switching DC a diode will allow the UP switch to switch on both lights and prevent backfeed when DOWN is on. D1 and D2 represent the lamps with current control (resistors or CC PSU) not shown.

One problem with this is a 0.7 V drop across D3. The D1 lights may dim slightly when switching from down to up. This may not be a problem since you pass through the OFF position.

schematic

simulate this circuit

Figure 2. For AC the diode won't work. A relay solves the problem.

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  • \$\begingroup\$ Would the down-voter mind explaining how this fails to answer the question satisfactorily? \$\endgroup\$ – Transistor Aug 12 '17 at 9:36
  • \$\begingroup\$ I have the feeling that some who downvote may not always understand either the question or answer as perfectly good answers get targeted... and + from me... \$\endgroup\$ – Solar Mike Aug 12 '17 at 9:45
  • \$\begingroup\$ It's to be hoped the OP does not have a CC PSU, since this solution would overdrive the single LED string. \$\endgroup\$ – Jack Creasey Aug 12 '17 at 11:31
  • \$\begingroup\$ @JackCreasey I don't think that detail was available to Transistor when he gave his answer, hindsight is very accurate... All the question said then was lights. \$\endgroup\$ – Solar Mike Aug 12 '17 at 12:35
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If your lights are powered from DC, then a simple diode connection will work. Connect both lights through separate diodes to one side of the switch, connect one light through a diode to the other side of the switch; switch common is ground. Choose diodes to suit current loads.

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  • \$\begingroup\$ As Finbarr mentioned - what about the voltage drop across the diodes... \$\endgroup\$ – Solar Mike Aug 12 '17 at 9:53
  • \$\begingroup\$ As I understand it, "led strip lights" run off a supply of 12V (or more). The voltage drop caused by the diode is thus around 5%, which should be negligible unless maximum brightness is required. If this is indeed the case, only a relay solution will give zero voltage drop. \$\endgroup\$ – henros Aug 13 '17 at 10:19

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