Motorcycle brake light, Again?

Discussion in 'The Projects Forum' started by tee2, Dec 2, 2009.

  1. tee2

    Thread Starter Active Member

    Nov 19, 2004
    I hate to do this again but I have done some homework first and use the search button with good results.

    Here is the circuit I'm going to use, it does exactly what I need.
    And I have most everything in stock, but the 3.3 UF cap, and the TIP117.

    I have three wires, one brake, one running light, and ground.

    I want to switch to LED's too. I'm thinking two sets of LEDs one for brake one for running. 3 or 4 to 1 ratio. I think it would be easier to design and build, and maybe more reliable. I'm open to sugestions.

    So here are a few questions.

    1. The circuit uses a TIP117. I am guessing as a switch. Is it just a high current transistor? Stick with that or is there something better for the LEDs?

    2. What type of LEDs?
    2a. Red or white? I'm going to use the stock red lens, the bulb is white does it matter?
    2b. I would think a wide angle would be best. I was looking at something like this...
    What about flux like this
    2c.3mm, 5mm, 10mm,? I have some of these in white and red I could try.
    2d. What about a 1w star, a bit pricey but, if I would need less of them?
    3. I was going to use one resistor per LED, if I have room on the pref board. I know I can go more but I have had problems before with a resistor going bad and blowing a whole string of LEDs.
    4. I know you can't answer this without knowing what LEDs I'm going to use, but how many LEDs should I start with? Say for a 5000mcd white super bright?

    I do know how to figure the resistor values and things like that. I don't want you to everything for me, I'll never learn that way. Just head me in the right direction.

  2. SgtWookie


    Jul 17, 2007
    Always appreciated. ;)

    You're in luck; Ken Moffett is a frequent poster here. :)

    I take issue with Tony van Roon's use of a polarized cap for C2, the 3.3uF that you're talking about. It would have to be a non-polarized type, as the polarity will change across the cap. Polarized electrolytic caps won't tolerate that; they would soon be destroyed.

    The TIP117 is a PNP Darlington transistor rated for 2A. You could elect to keep the PNP transistor, or change to a P-channel power MOSFET. A suitably sized power MOSFET would have a very low Vds (voltage drain to source), unlike the TIP117, which could have a Vce of 1v to 2.5v at maximum current.

    Well, you could use separate LEDs for brake and running lights - but that means you'd need to use more LEDs to get the same intensity for braking. Might as well just use the same LEDs for braking AND running lights.

    If you use some for just taillights, and some for brakelights, you won't be able to adjust the ratio without a soldering iron.

    But if you use PWM for taillights, and "full on" for brake lights, you can adjust the PWM to suit - with a screwdriver.

    You can stick with that, or change to a P-channel power MOSFET. The advantage of a MOSFET is that you will have just a very small voltage drop across it. The disadvantage is they are far more sensitive to static electricity. Once installed into a properly designed circuit, the static issue seldom arises.

    The US DOT specifies a red-orange color for brake lights. This is in a wavelength range of about 618nm-624nm. Your tail light lens is made with this color.

    If you use white LEDs, most of the light they emit will be filtered out by the tail light lens; only the narrow range of 618nm-624nm will make it through. You might think you're getting more light from a 20,000MCD white LED than a 5,000MCD red-orange LED, but if only the red-orange light gets through, what do you really have?

    White LEDs generally have a much higher Vf than the proper color LEDs do. This means you waste more power generating light in wavelengths that you won't be using.

    A white LED usually has a Vf of around 3.4 to 3.8v.
    Proper red-orange LEDs usually have a Vf of between 2.2 and 3v, nominally 2.6v.

    If your tail light lens becomes cracked or broken and you already have the prescribed wavelength LEDs for taillights, you will not likely get a traffic citation. However, if you use white LEDs, you will likely get cited.

    You might as well order the proper color LEDs to begin with. You will also have a much better idea of how bright your taillights will be.

    Have a look at Superflux LEDs. They have some 4-pin LEDs that would be very good candidates.
    HPWT-RH00-H400 - 44°x88°, 25°x68°, 5.0cd-15.0cd
    HPWT-MH00-H400 - 100°, 70°, 2.4cd-7.2cd
    HPWT-DH00-H400 - 70°, 40°, 6.0cd-18.0cd
    typ 2.6v@70mA

    Here's one worth looking at:
    Here's another; 10x brighter at 10x the current:
    Regulating the current may be a problem; you may need to use a BuckPuck.
    Heat dissipation and proper color become an issue.
    Resistors usually burn open. What most likely happened is you had an LED short out due to overcurrent, which cause the rest of the LEDs to short, and finally the resistor burned up.
    Change your thinking - go with the proper color LEDs first.

    Figure roughly 14v for your system voltage.

    Rlimit >= (14v - (total Vf_LED)) / Desired_LED_Current
    Last edited: Dec 3, 2009
  3. tee2

    Thread Starter Active Member

    Nov 19, 2004
    As always Sarge, great info.

    So for the 3.3 uf cap I would need a "Capacitor Tantalum Solid" type?

    I'll go with the MOSFET but wait until I know what LEDs I'm going to use.

    I have attempted exploring the PWM, but got frustrated and gave up.:( Too many ways different plans out there. Can you point me to a good solid circuit that will work for my app, and not be to hard to understand?

    Red it is! When I was typing what color to use I was thinking, this is a dumb question. But you explained it, and it makes total sense. And since this is a dual sport (on/off road) bike a broken lens is going to happen at some time.

    I'll look into the Superflux LEDs. The 10x ones look nice but for 20x the cost, I'll pass. :eek: Is there a difference between the one you linked to at Digikey and the ones from China on EBay? How do I connect them with four pins? I guess 2 "+" and 2 "-"?

    I guess my next step is to spend some time with a PWN and get to know that.:D

    Thanks again.
    Last edited: Dec 4, 2009
  4. tee2

    Thread Starter Active Member

    Nov 19, 2004
    Well I see why when ever I try to make a PWM I get discouraged and give up. I found some with Google and tried them with no luck. They all assume you know how it is supposed to work and give you half the info need. Because you should know to hook everything up. :(:confused:

    Well I found one that looked like I could follow using a 555. I built the circuit on a breadboard. I used one led 2v, 30ma, with a 390Ω. It does work kind of. I can see it dim some but very little.


    Any ideas what is wrong?

  5. SgtWookie


    Jul 17, 2007
    What NPN transistor did you use?
    Are you certain that you connected up the transistors' leads properly?
    Find a datasheet for your transistor, and make certain that you have it connected correctly. You may need to look at the images several times before it "clicks" as to the right connections.

    P1 is a linear (not log, not audio) taper potentiometer with a resistance of 50K Ohms.

    Is your power supply 12v?

    Calculating Rlimit for the LED:
    Rlimit >= (Vcc - Vf_LED) / DesiredCurrent
    So, if your Vcc=12v, your Vf_LED is 2, and you want 30mA current flow...
    Rlimit >= (12-2) / 30mA
    Rlimit >= 10 / 0.03
    Rlimit >= 333.333... Ohms
    Standard resistances:
    Closest standard value >= 333.333... Ohms is 360.
  6. spacewrench


    Oct 5, 2009
    I played with PWM on single LEDs using a microcontroller, so I had really good, linear control of the pulse width. I noticed that the visual brightness is not very linearly related to width. For example, you have to go to a certain width (maybe 10%) to see anything at all, then it goes up sort of linearly to 30%, then it gets brighter really fast, and by 50% or 60%, changing the pulse width doesn't really make any difference.

    If you only need two different brightnesses, you might want a circuit that allows you to switch between two different (and preferably both adjustable) pulse widths. I expect a 555-based circuit could do this, but I'm not enough of a 555 guru to design one.
  7. tee2

    Thread Starter Active Member

    Nov 19, 2004
    The transistor is a C547B, I checked that it was a NPN, and giid up to 100ma, so I thought it would work. Should I try some thing else? All the transistors I have came in a grab bag of parts, so they are probably odd-ball ones. ;)

    The bag that the pot says "Trim 50K" that is it. It is a small one to mount on a PCB.

    I didn't have a 360Ω in stock, so used the 390Ω. Does that matter? I thought higher would be better.

    I just checked the power supply, it says 12v 800ma. MM says 18.9 volts.:eek: I have a l7812cv regulator, should I use a 10uf on the in side and a 1 uf on the out side?

  8. tee2

    Thread Starter Active Member

    Nov 19, 2004
    The funny thing is I have a new Atmel STK500, AVR Starter Kit sitting here that I bought from some one I know. He had big plans to make great inventions and tons of money. He opened to box looked at the book and then sold it to me.

    I'm not much better but at least I did play with it a bit. :) I can upload code and things like that. My pea size brain is to small to figure out to do the coding:confused:.

    And I know for what I'm looking to do a TinyAVR would be great. I wounder if I could just cut and paste from some of the sample code that was on the sample chip? I might look into that.

    Thanks for the reply.

  9. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    Boy, it's has been a long time for that circuit. It's been in use since 1995 and moved through three different vehicles.

    I agree with SgtWookie, the 3.3uF cap should have been none polarized. That was my error, not Tony's. was and still is a 3.3uF/50V electrolytic...and has functioned for 15 years. :eek: If you are going to build it I would switch to a 1.0uF monolithic (more readily available) and change R6 to 200K. I also realized that along the way I had switched the TIP117 to a IRF5305 P-channel MOSFET...I think as an experiment when moving it between vehicles.

    Last edited: Dec 6, 2009
  10. SgtWookie


    Jul 17, 2007
    Well, when the specs say Ic=100mA maximum, figure they're good for 50mA.
    A 2N2222/PN2222 is rated for Ic=800mA, but it's good for up to around 500mA; providing you're supplying the base with Ic/10 current.

    That's OK.

    It's OK, you'll just have less current through the LED.

    Actually, 1uF on the input, 10uF on the output would be much better.

    You have to beware of unregulated supplies like "wall warts". They output within about 10% of their rated voltage when the rated load is applied. If the load is a lot less, the voltage can be considerably higher.
  11. tee2

    Thread Starter Active Member

    Nov 19, 2004
    Thanks for the reply guys.

    I'm putting this project on hold until I finish the others I'm working on. I ran into am odd wiring problem today (see my USB post) and thought I better just stick to one project at a time.