led help needed

Discussion in 'The Projects Forum' started by JeffSchaber, Mar 14, 2013.

  1. JeffSchaber

    JeffSchaber Thread Starter New Member

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    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.
  2. tracecom

    tracecom Well-Known Member

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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: Mar 14, 2013
  3. JeffSchaber

    JeffSchaber Thread Starter New Member

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    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.
  4. spinnaker

    spinnaker Senior Member

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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.
    takao21203 likes this.
  5. Bill_Marsden

    Bill_Marsden Moderator Staff Member

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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).
  6. JeffSchaber

    JeffSchaber Thread Starter New Member

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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?

    [​IMG]

    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.
  7. tracecom

    tracecom Well-Known Member

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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: Mar 15, 2013
  8. JeffSchaber

    JeffSchaber Thread Starter New Member

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    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
  9. JeffSchaber

    JeffSchaber Thread Starter New Member

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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:

    [​IMG]


    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:

    [​IMG]

    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: Mar 17, 2013
  10. tracecom

    tracecom Well-Known Member

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    Jeff,

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

    tracecom Well-Known Member

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    Here's the schematic. I have breadboarded it, and it seems to work, but of course, I don't have the LED strings that you are going to use, so I can't be sure.

    The driver transistors are rated for 1 A or .5 A, depending on which datasheet you believe. In either case, they should be sufficient to handle the 130 mA in the red and green LED strings. In addition, the transistors invert the signal from the PIC, but that inversion is not visibly apparent in the operation of the LEDs. There is also a very short period of time at the peak brightness of the red and green LEDs in which they go off. I don't know how long this is, but it is on the very periphery of my POV. In other words, I can just barely discern it. And with the white LED flashing at exactly that same instant, the short off period of the red and green LEDs is completely masked. I don't think it is a problem at all, but I wanted to mention it.

    If you decide you want to use this approach, I'll provide the code.

    BTW, if anyone sees any problems with the schematic, please point them out.

    Thanks.

    Attached Files:

    JeffSchaber likes this.
  12. JeffSchaber

    JeffSchaber Thread Starter New Member

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    Trace, Thanks for the Schematic. it looks pretty straight forward but there are a few things that I don't understand;

    There's a statement at the bottom of the schematic that says "Connect 12V ground to 5V ground" Am I correct is thinking that this circuit requires two voltage sources and the grounds from both should be connected together?

    The circuit itself looks like it could be made pretty small and light which is ideal for an RC flying machine but I have a 14.8V battery on board right now supplying most of the power, I could step down that voltage to either the 5V or 12V using a step down voltage regulator (this is what I have: http://www.pololu.com/catalog/product/2104) but would I need to add an additional power source to power the other input or can I use resistors to divide the 14.8V into 2 different voltages?

    Just looking to save as much weight as possible and adding another power source may put me over my weight limits. Is there an easy way to split a power source?
    Last edited: Mar 18, 2013
  13. JeffSchaber

    JeffSchaber Thread Starter New Member

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  14. tracecom

    tracecom Well-Known Member

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    Regarding the 12 VDC for the LEDs, a voltage divider is not a good idea because of the power it wastes. In addition, I am not sure it is really necessary; often a 12 VDC requirement has some leeway. It is possible that your LED strings will be fine with a 14.8 V power supply.

    In addition, the battery voltage will fall steadily as the battery discharges. If it turns out that the 14.8 V would be too high for the LEDs, then we might want to use a 1N4001 diode or three in series with the power lead to the LEDs. Each diode would drop about .7 V.

    Regarding the 5 VDC for the microcontroller, that's no problem. A small circuit using a LM78L05 will be easy to contruct and weigh very little.

    Let's see what other forum members think about this issue.
  15. tracecom

    tracecom Well-Known Member

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    Jeff,

    I just looked at the voltage regulator circuit you have. My only concern is that it (like all regulators) has a "drop out" voltage, which means that the input must be higher than the regulated output. (If that dropout voltage is posted, I have not found it.)

    As I previously posted, the voltage from your battery will drop as it is depleted. When it drops to the point that it is too low for the regulator to work properly, the regulator output will be unpredictable. Frankly, I don't know enough about the regulator and the characteristics of your battery to really determine how well they will work together. When the battery is fully charged, they will probably work well together, but when the battery voltage drops, there may be a problem.

    However, the 5 VDC is no problem. The current requirement is very low, and we can get that with a LM78L05 circuit that is small and light weight.

    I also need to try to understand the LED strings better, and need to do some calculations in that regard. In the worst case, you might need to set the output from the Pololu regulator somewhat lower than 12 V, which would mean the LEDs wouldn't be quite as bright, but still might be ok.
    Last edited: Mar 18, 2013
  16. JeffSchaber

    JeffSchaber Thread Starter New Member

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    I personally would rather not use the Pololu regulator and would prefer to have everything in one circuit if possible.

    I can mail you a piece of the LED strip if that helps. I have extra so it's not an issue.

    In addition to the LEDs I'm running with the 14.8V battery, I'm also powering 3 750Kv motors plus the receiver, a servo and the control board so these will also draw power away from the LED circuit.

    My ultimate goal is to make a Power Distribution Board (PDB) that the battery plugs into and includes the LED circuit with pads for the external components (radio receiver, control board, servo and motor ESCs).

    This is the current PDB that I have:

    [​IMG]

    I want to expand on this board by adding the LED circuit and adding a Lipo "Battery Low" alarm similar to these: http://tinyurl.com/cq233z7

    I have a couple of these alarms right now and you can pick them up for about $2. The problem with this hobby is that everyone makes all of these small aftermarket items but no one has an all in one solution. I don't plan on selling these but if people want to help me cover the cost of getting the PCB boards made I'd prefer to have that all in one solution.
  17. tracecom

    tracecom Well-Known Member

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    From the photo, it looks like the LED strings are directly connected to the battery power bus without any voltage regulation at all. If the LED strips you have are identical to the ones used in the photo, that would mean that they can be directly connected as well. Of course, the control circuit to make them flash would be inserted in series with the positive voltage lead. Then, we could build a small 5 V regulator that would run the PIC. That, and the PIC circuit I previously posted would fit on a PCB about 1.5 inches square, and the LED strings would connect to that PCB.

    Are the LED strings that you have identical to the ones in the photo in terms of voltage required? And I see three strings in the photo; are they one string of red, one string of green, and one string of white?
  18. JeffSchaber

    JeffSchaber Thread Starter New Member

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    Bill and Charles, I really do appreciate all the help and it's nice to know there I have options.

    Bill, I know this thread has gone past what we were talking about but I'm still open to a 555 solution if you think it's a good option? I'm so new at this that I wouldn't know what would be the best route to take but given my anal background in research (I'm an aircraft technical writer), I love to debate the merits of the options presented to me.

    There are many routes that can be taken with electronics and I really wished I would have more knowledge in this field but I don't so, thank you gentlemen for being so gracious with your help. I don't want to waste anyone's time either so I'm Ok if one of you says use the others solution.
  19. JeffSchaber

    JeffSchaber Thread Starter New Member

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    The LED strings I have are identical and in this photo they are directly attached but this board was set up to use an 11.1V lipo (this is not my board). I went with the bigger battery because I had more components to drive and the motors are more efficient on a larger battery. Right now I just have the bare board with nothing soldered to it. The board does have provisions for two battery sources so two independent circuits.

    For that matter, I guess I could add another battery on the other circuit but weight might be an issue.
  20. tracecom

    tracecom Well-Known Member

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    My guess is that even though there are provisions for two batteries, both batteries are in parallel and must be the same voltage. Without a schematic of the PDB, I can't be sure of that, but it seems logical. To be sure, you could use a DMM to check the continuity between all the + (positive) pads, and between all the - (negative) pads on the PDB.

    I will do some research on the LED strings, and we'll know which way to go. However, the fact that the LED strings will work on an 11.1 volt battery means that we could regulate the 14.8 volts down to at least that voltage.

    ETA: The LED operating voltage is an issue regardless of whether you ultimately use a 555 based or a minicontroller based solution. Of course, whereas a uC requires a 5 V source, the 555 will operate quite happily on 14.8 volts.
    Last edited: Mar 18, 2013
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