November 25, 2014

Mini-quad headlights for FPV

Now that winter is coming (pun intended) we only have a few hours of sunlight during the day. This means that flying during weekdays are impossible. Even during weekends the time frame is limited.

What if you could put headlights configured correctly to be able to fly FPV during dark hours?

There are a lot of things to think about to get this right and I have no idea where to start.

  • Reflector angle, how to find one that is suitable for the camera I use
  • Light wavelength, what is best for the camera
  • Luminous flux, higher often means heavier gear and more expensive drivers
Since I have no idea where to start I'm going to build a first one just to see what happens. I started out with a headlight LED kit from HobbyKing. It's a bit short on specs but since I wanted to re-solder the wires I had to take it apart and found out a few interesting things.

Voltage step down diodes
PWM switch with voltage step down diodes for headlight system

The setup is specified to run at 6.5-8.4 V. Essentially it is made for 2S since it comes with a 2S connector. To achieve the voltage range they have placed the two LEDs in series and added two diodes to "step down" the voltage by about 1.4V. This is the very cheapest and ineffective way of doing this. As a result each LED run at 2.5-3.5 V. The setup is said to consume 500mA. This would give a per-LED power of about 1.2-1.5W. It's hard to guess but we can maybe expect 100lm to come out of each diode. But thats when the battery is fully charged and unloaded, it will probably be much less when we get close to 6.5V.

When re-soldering the LED wires I tugged one of them a bit hard and decided to the the reflector apart to check internal wiring. It's essentially a plastic reflector assembly with a LED emitter glued to it with a simple heat sink glued to the module. The emitter is a very typical 20mm star device like this one and can be found in a multitude of configurations, prices and qualities from Chinese vendors. If the emitter is not powerful enough it can easily be replaced by a more powerful one.

I have no intentions of using 2S to power the LED. In fact, I have no machines with 2S power systems. Instead I'll be using a cheap voltage regulator to provide the required voltage to the device. I will remove the two diodes and power the headlights using the switch. The switch itself is a quite useful device, it can be bought at hobbyking at almost the same price as the headlight kit. Anyone know where I can get one cheaper?

I'm building the headlight setup like a "module" that I can mount wherever I usually put the mobius. The mobius is useless in low light conditions and obviously will be of no use when I use the head lights. I'm using a glass fiber piece from the rotorbits series, very useful for prototyping.

Headlight LEDs mounted on a piece from the rotorbits

I will use a very cheap voltage regulator to drive the LED. Besides from cheap it is also very light and not current regulating. This is not recommended but I will give it a try to see how bad it can get. The regulator has a dial for setting the output voltage. I adjusted it to 6V to start with and will probably increase it to about 6.6V which is likely to be a suitable voltage for the LEDs.

Voltage regulator and electronic PWM switch
 Those cheap regulators are known to cause some noise which can effect the FPV equipment. This is often due to undersized capacitors. I added additional ceramic capacitors to both the input and output (output should be useless actually) to counter this right away.

Additional capacitors on the voltage regulator.

Once everything was put together I attached the to the plate with the LEDs. I tested the setup on the bench with ESC/Battery/receiver I could live without in case the non-current-regulatory property of the driver would cause imminent death of anything. But nothing happend, the LEDs worked and I increased the voltage a little bit as expected.

I attached a JST connector to source 3S from the power distribution and used the ReceiverPort in PPM+Outputs mode on the CC3D to connect channel 7 to the PWM switch. It took some fiddling with the transmitter and even more with the CC3D to get it working properly. But I now have the headlights on a switch on the transmitter.

Headlights mounted on the 250 mini-quad

Headlights in action (impossible to take a good photo of this)

I ran out of time but got to try them out flying line of sight. I recorded the results of course with a DVR to get the actual FPV view. It turns out the camera handles the low light conditions better than I expected and the headlights does a great job at lighting up branches that you wouldn't see otherwise.

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