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The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

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Old 03-14-2013, 10:06 PM
JeffSchaber JeffSchaber is offline
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Default led help needed

Hello everyone,

I've got a little project trying to add LED lights to an RC Multicopter to help with orientation while flying and also for night flying. I need this to be as light as possible and my Google education has only confused me so here I am with my first post!

For Info; here's a video of the Tricopter: http://youtu.be/sAOdjS4m0b4

I read through Bill Marsden's blog (http://forum.allaboutcircuits.com/blog.php?b=378) "LEDs, 555s, PWM, Flashers, and Light Chasers" hoping I could figure it out my project on my own but didn't have any luck.

What I'm trying to do is take two of the 3528 SMD LED strips that you can buy just about anywhere (similar to these: http://tinyurl.com/d3qju48) and add a circuit that will alternately flash each strip similar to a train crossing signal or the red/green wing tip lights of an airplane. The flash should have about a 2 second interval but if I could vary that it would be great. I'm not dead set on these strips but they're already the right size, weight and convenient. (I plan to use red on the Port side, green on the Starboard side which will flash and a solid white LED for the tail which could be wired separately)

These strips already have resistors built in so I'm not sure how this would work. I'm cutting each strip to be about 12" long so the current should be less than their specification for a one meter piece (400mA, 24 watts/5 meter).

I have a 14.8v input source (3300mAh Lipo battery, 4S - 30C Continuous Discharge ) that I had planned to step down to 12v (since that's the voltage specified by these strips) using something like this: http://www.pololu.com/catalog/product/2104

Am I going about this all wrong and would it be easier to build the LED strips on my own rather than use the convenient prepackaged ones?

I have a little electrical experience but it's not my strong suit. I can follow a wiring diagram and assemble the parts easy enough but figuring out my own circuit is past my skill level without help.

I did look at the 555 circuit and can see the possibility there but need a bit of help trying to come up with something that's small and light.

If anyone has a little time to give me some guidance I would greatly appreciate it.
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Old 03-14-2013, 10:27 PM
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tracecom tracecom is offline
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A 555 based circuit would probably do what you want. A 555 can source or sink up to 200 mA (IIRC), and a 1 foot piece of your LED strip would draw about 130 mA. And considering that you only want the LEDs to flash, that should work.

So, you want the green to flash briefly (say 250 ms) pause 2 seconds and then the red to flash briefly, pause 2 seconds and then green to flash again, and so on... Or do I misunderstand the timing?

Maybe the red and green flash at the same time, every 2 seconds?

Last edited by tracecom; 03-14-2013 at 10:41 PM.
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Old 03-14-2013, 10:40 PM
JeffSchaber JeffSchaber is offline
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Quote:
Originally Posted by tracecom View Post
A 555 based circuit would probably do what you want. A 555 can source or sink up to 200 mA (IIRC), and a 1 foot piece of your LED strip would draw about 130 mA. And considering that you only want the LEDs to flash, that should work.

So, you want the green to flash briefly (say 250 ms) pause 2 seconds and then the red to flash briefly, pause 2 seconds and then green to flash again, and so on... Or do I misunderstand the timing?

It would be ideal if I could get the lamps to fade out as the other fades in and they should stay lit for approximately 2 seconds.

Otherwise, something like this: http://youtu.be/AyGOo8PLSLU would suffice if it were a tad slower.
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Old 03-14-2013, 11:05 PM
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You should consider using an mcu like a pic. You could use pwm to fade your lights and a timer to flash them. You options would only be limited by your imagination and how you have the lights configured.
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Old 03-14-2013, 11:11 PM
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Bill_Marsden Bill_Marsden is offline
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I will draw a schematic up and post it. This is a really simple circuit overall.

Pics are major overkill, especially for people new to the hobby. Some programming skills and extra equipment is required, a major over complication.

Glad to see you made it on board (the OP and I have been emailing back and forth).
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Old 03-15-2013, 03:15 PM
JeffSchaber JeffSchaber is offline
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I was looking at a typical 555 LED flashing circuit and I have a few questions about it. In the circuit below, why does the output from pin 3 (from the circuit) go to both the positive and negative sides of the 1st and 2nd LED? Am I correct in assuming that this is how they are made to alternate?



Also found an explanation with the circuit as follows:

* R1, R2, C1 and the supply voltage determine the flash rate. Using a regulated power supply will do much to insure a stable flash rate. For a variable flash rate, replace R1 with a 1 megohm pot in series with a 22k resistor. So, the values of these 2 resistors and one Cap will give me the rate I need, how is this figured? And, will it matter if there are multiple LEDs on bank one and two with resistors pre-wired?

* The duty cycle of the circuit (the percentage of the time LED 1 is on to the time it is off during each cycle) is deterimed by the ratio of R1 to R2. If the value of R1 is low in relationship to R2, the duty cycle will be near 50 percent. If you use both LEDs, you will probably want a 50 percent duty cycle. On the other hand, if R2 is low compared to R1, the duty cycle will be less than 50 percent. This is useful to conserve battery life, or to produce a strobe type effect, when only LED1 is used. So, "Low" being an ambiguous term, how do they arrive at 50% without any values? Or, do they just mean one value is lower than the other?

*The NE555 timer chip can be damaged by reverse polarity voltage being applied to it. You can make the circuit goof proof by placing a diode in series with one of the supply leads. So, anywhere inline with the Vcc pin 8?

*The purpose of R3 and R4 is to limit current through the LEDs to the maximum they can handle (usually 20 milliamps). You should select the value of these according to the supply voltage. 470 ohms works well with a supply voltage of 9-12 volts. You will need to reduce the value for lower supply voltages. If I have a 14.8V battery would the correct resistors in this location effectively step the voltage down to a 12V output?

Like I said in my first post, I'm trying to google educate myself as I actually love learning this stuff! I could go out and buy the parts and guess with a trial and error approach but without having the background probably wouldn't be able to know what I would need to change if it wasn't right.
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Old 03-15-2013, 07:27 PM
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In the circuit you posted, the LEDs alternate because the output of the 555 is alternating between the source current mode and the sink current mode. It would be suitable for your purpose if all you want is to flash the green and then flash the red, but if you want to fade between them, it would require additional components.

With regard to your other questions, they are all good, but I am at a Staples store on a demo computer, so I won't try to answer them now. Besides that, Bill_Marsden is the resident expert on 555 circuits and will be back soon, I am sure.

FYI, I got interested in your project, and because I am currently trying to learn PIC programming, I took a swing at implementing your requirements in a PIC. I actually got it working first thing this morning. The red and green LEDs fade back and forth, and I have a white strobe once per second. It looks pretty cool. http://youtu.be/Qyw4IgKuQN4 The frame capture rate makes the fades look choppy on the video, but they are really smooth live.

However, as Bill said, a PIC is overkill for this project unless you already have the programming hardware and software. In addition, the output from the PIC would not drive your LED strings directly and would require a couple of additional transistors to act as switches.

I should be home later and I'll check the thread again, then.

BTW, the tri-rotor video is super; is that you flying? I want one.

Last edited by tracecom; 03-15-2013 at 11:07 PM.
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Old 03-15-2013, 11:12 PM
JeffSchaber JeffSchaber is offline
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Quote:
Originally Posted by tracecom View Post
It looks pretty cool. http://youtu.be/Qyw4IgKuQN4

BTW, the tr-rotor video is super; is that you flying? I want one.
That is exactly what I'm trying to accomplish! The tail, white strobing effect makes it perfect.

Not sure if I mentioned this in my original posts but the copter is actually controlled by a Arduino based board Atmega328 I believe. there's a ton you can do with these and I plan to add a GoPro camera to it so I can do video similar to this: http://youtu.be/RSYSao0W258. This guy actually is having the video feed transmitted back to goggles he's wearing as he's flying it.

I wish that was me flying but unfortunately it's not, when I saw the video I said the same thing as you, "I want one!" So, I'm in the process of building it now.

These are the specs for the flight controller, I think I have open channels left that could be utilized but I'm still in the process of learning this as well so not sure:

ATmega328
MPU6050 (Inversense) 6 axis gyro/accelerometer with Motion Processing Unit
CP2102 ( USB to UART Data Transfer)
Acro and Self-Level modes; 2 Aux inputs

- All MWC Firmwares,High extend capacity
- Up to 8-axis motor output (have to use S-PPM Receiver)
- 2 Servo output for camera (only available when using 4 motor or less)
- 2 status LED
- 1x serial port for bluetooth.
- On board Micro USB port
- I2C socket for extend sensor,I2C LCD/OLED display or any I2C devices
- Separate 3.3V and 5V LDO voltage regulators
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Old 03-17-2013, 03:20 PM
JeffSchaber JeffSchaber is offline
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My LED strips came in yesterday so I thought I would add what info about them I could figure out. It was hard to see the traces but the image below looks to be how these are laid out:




You can cut the strip between every 3 LEDs. Each of the LEDs are SMD style but I have no idea what their value is. I measured the length of strip that I need for each (red, green, and white) and I will be using 4 of the above segments or 12 LEDs and 4 Resistors like this:

L1 + L2 + R1 +L3 + L4 + L5 + R2 +L6 + L7 + L8 + R3 + L9 + L10 + L11 + R4 + L12


The vendor for these had a sticker on the reel pointing me to their site http://tinyurl.com/bn4f9ub but I couldn't find any specs on the strips.

As you can see from the diagram above, this is the layout of the strips and each cut point is on either end of the above diagram.

The resistor had the number 151 stamped on top of it so I'm assuming it's a 151 ohm resistor?

Here's a close up of the strip:



Not sure if any of this helps but thought I would provide as much info as possible.

I downloaded DesignSpark PCB software to play around with and hopefully once a schematic is figured out I could have it printed to a circuit board.

Last edited by JeffSchaber; 03-17-2013 at 03:26 PM.
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Old 03-17-2013, 05:40 PM
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tracecom tracecom is offline
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Jeff,

I'll try to post a schematic using a μC tonight.
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