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Nikko85

LED lights confusion- Tx help please

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Hi all

I've been using these Tx and Rx combos for my RC cars. They are cheap, take 4 x AAAs which I have many and have a fine range for me. They are also pretty cheap and small so I can take 2 cars with me in a rucksack to the park. They have a 3rd channel which is a single button.

They've worked perfectly, but I'm struggling with the 3rd channel and LED lights.

Some background:

When I've plugged a servo into the third channel the servo works, switching between left and then right with each press. There is also a "Rev" function which locks the servo in one position, but pressing the button will move it to the other side until you let go again. (I'm making a truck with detachable trailer, so this is perfect...)

However I'm struggling with LED lights. I bought these ones here.

When I plug them in they are always on, regardless of what I press on the Tx, either when the channel is reversed or not. Oddly enough when I plug them in backwards into the Rx they also work - this is strange to me as LED should only work in one direction as they are diodes I thought.

What's going on here? Pairs of LEDs going in both directions? Something that flips the current in the wiring? Something else?  And has anyone used this TX/RX with LED lights with success? Having lights that are always on isn't the end of the world - but there's something cool about being able to switch on and off with the Tx.

Thanks in advance with any help, particular LED experts or people who have used these Tx/Rx combos before.

Andrew

 

 

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A normal servo plug has 3 wires, white for signal, red for 5V, and black for negative. Signal is a pulse, so nothing 'useful', leaving 5V + and -.

When you plug in your LEDs, they get 5V through the red and black wire, and thus, will turn on. When you plug them in backwards, they still get 5V through the center red wire, and ground to the signal part of the receiver, which works because that uses 5V as a reference and thus, can act as ground for 'simple' stuff. (That's about the really really short version of it).

If you want to turn lights on and off remotely you'll need a separate switch. Our friends in the far east sell one (I have one on the Pajero, which is currently for sale, hint hint wink wink), and I'm sure that you can buy many of those unbranded switches on eBay. 

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Many thanks for that reply, So if I understand the red stays positive, and the white becomes the new ground, and that makes sense as to why flipping makes no difference. I had also assumed that servo wires worked like a motor ESC, where the polarity flips according to the Tx, but clearly not.

The bigger thing is that the third channel works for the servo as I need it to for one truck, and for the other I may look at that switch.

Thanks again,

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Well, white doesn’t become anything, it just happens to work as a ground in this situation. Maybe that’s just how your receiver works, maybe it’s common. Never tried it actually. :p

The slightly longer version of how a radio works is that it gives pulses of a certain duration, and that tells the servo to move to a certain position. the fact that an LED will light up on the white wire is because inbetween the pulses the white wire is at 0V (neutral), working as a ground to the +5V the receiver sees. The pulses themselves are +5V, and the only reason you’re not shorting stuff out is because the LED itself works as a diode, only letting current pass in one direction but blocking it in the other. 
 

I’m not sure what would happen if you plug in an incandescent light bulb the wrong way around, but I can imagine you’d short out your receiver and severely upset your BEC in the ESC. Don’t try that at home I’d say. 

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