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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
    Commented Aug 12, 2017 at 8:44
  • \$\begingroup\$ simple led strip lights \$\endgroup\$
    – Ken
    Commented Aug 12, 2017 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
    Commented Aug 12, 2017 at 8:59
  • \$\begingroup\$ Changing to a DPDT Center Off switch is by far the simplest solution. \$\endgroup\$ Commented Aug 12, 2017 at 16:40

2 Answers 2

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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
    Commented Aug 12, 2017 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
    Commented Aug 12, 2017 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\$ Commented Aug 12, 2017 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
    Commented Aug 12, 2017 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
    Commented Aug 12, 2017 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\$
    – user131342
    Commented Aug 13, 2017 at 10:19

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