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I replaced my car interior light with a Chinese LED strip about 80 cm long, the usual ones with 3 LEDs in series with a 75 ohm resistor. This is the circuit.

schematic

The diode helps drop some excess voltage, since the battery provides up to 14V and the LEDs are too bright.

As I marked it in the schematic, the ground are always connected to the car chassis. The long wires are about 1.5-2.5 metres long.

With the car in a dim location, whenever the windscreen wiper starts or stops, but also when I turn off and on the wiper lever without the wiper actually starting because it just completed a wipe, the LEDs briefly light up very dimly.

This happens both with the front wiper (connected to the battery via a lever switch and a fuse, about another couple of metres of wire) and the rear wiper (same as in the front wiper, but maybe 3-4 metres of wire). Both wipers motors are connected to the chassis as ground.

Since the circuit is open, how can that happen? do the wires act as capacitor and briefly charge and discharge through the LEDs after the inductive voltage surge, causing a brief emission of light?

Pᴀᴜʟsᴛᴇʀ2
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FarO
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3 Answers3

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With long wires you have an inductive coupling to stray noise. Put capacitors across each switch such as 0.01 -1uF ceramic.

Tony Stewart EE since 1975
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Need more information please. What year, make, model is the car? What is the part number of the diode strip, and a picture of the strip would be helpful.Stray capacitance can exist between any two conductors within close proximity to each other with some form of dielectric between them, and given that only a small current is needed to turn an LED on, I would say that it is a possibility that the door switches could possibly be poorly manufactured and even when "open" could be close enough to create a capacitor air dielectric effect and allow stray electrons to jump across essentially creating a capacitor.I would say that that it is more likely that there is more to this circuit than you may realize. Most newer cars use rectifier diodes in door switches to prevent feedback and to allow for door sensing circuitry to determine which door is open. You could install rectifier diodes in this circuit and that would most likely solve your problem, however, the year, make,and model of the car would be helpful because there may already be more to this circuit than just this LED strip, especially if the car is newer than a 2000. If the car has an alarm system, or the dashboard tells you when a door is open, this circuit is a lot larger than this. I have access to pull vehicle wiring diagrams. I will find out what else, if anything else is in this circuit for you.

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you can solve the problem by using a LED strip with constant current driving LED strip, the input voltage can be 9V~18V, then that may solve the problem.

bruce liang
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