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Hi I'm new to electronics and have little idea how electricity works. I have a motorcycle and wanted to add two LED lights. I want to have one switch in both LED but with different sources.

The first LED (which will be placed in front of the motorcycle) is from the battery, so when I turn on the switch the first LED will turn on. The second LED (which will be placed at the back of the motorcycle) is from the brake light, so whenever I press the brake and the switch is on the second LED will turn on but if I press the brake and the switch is off the second LED will not turn on. Is this possible?

diagram

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2 Answers 2

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Here's a schematic diagram which includes a double-pole single-throw (DPST) switch that should, as @Eugene commented, do what you need:

schematic

simulate this circuit – Schematic created using CircuitLab

When the switch is off, both LEDs will be off. When the switch is on, D1 will be on. D2 will also be on, but only if the brake is on.

I included the brake switch (BRAKE SW) only so you can see that D2 will only light when both the brake is applied as well as SW1.

I've added R1 and R2 which are current-limiting resistors so that the LEDs don't burn up. You can find lots of questions and answers on the site about how to select these:

If you're unfamiliar with switch wiring, check out Paul Nicholl's guide. For example, here's a DPST switch image from that source:

DPST switch

Vertically, each "column" of contacts is a pole. When the switch is off, the top and bottom contacts are not connected (said to be "open"). When the switch is on, they are connected (said to be "closed"). The left and right sides are separate and never connected to each other.

The dotted line in the schematic indicates that the two poles operate together, due to the physical actuation of the switch.

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  • \$\begingroup\$ Wow thank you so much for this very informative answer. I will try this \$\endgroup\$
    – Rak
    Commented May 6, 2022 at 11:14
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It's possible.

Here's the schematic.

enter image description here

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    \$\begingroup\$ An elegant solution - to drive the LED both from the side of the anode and cathode (the ground acts as a "chip select"). The left LED will be reverse biased through the brake lamp. Will it stand it? Maybe... if the total resistance is high enough… I have shown this trick in my question. \$\endgroup\$ Commented May 6, 2022 at 10:51
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    \$\begingroup\$ Thank you very much for your appreciation and also for opening my eyes to the depth to which one can go in analysing a circuit. I do hope the LED survives the reverse bias! Hat's off to you for recognizing, in this circuit, a similarity to the method you had devised years ago to easily observe signals on microcomputer buses and to kindle the interest of your students. I do appreciate your efforts to use analogies to motivate your students to think laterally and visualize abstract concepts. I am sure your efforts to promote your students learning and growth will never go in vain. \$\endgroup\$
    – vu2nan
    Commented May 6, 2022 at 15:13
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    \$\begingroup\$ Exactly! The right word is lateral thinking. I accumulated a lot of such simple but clever ideas not only in the laboratory but also in cars when we invented and mounted various devices in the 90's. For example, my favorite trick was to control the relay coil by the ground through a car lamp; when the lamp was on, the relay turned off. Another trick was to use the hysteresis of reed switches to make them act as latches ("magic switches"). The problem of creatively thinking people is the envy of conventionally thinking people who are trying to kill the new idea. There are many examples here. \$\endgroup\$ Commented May 6, 2022 at 17:22
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    \$\begingroup\$ I fully agree with you. \$\endgroup\$
    – vu2nan
    Commented May 7, 2022 at 0:55

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