rc car brake lights .

Discussion in 'The Projects Forum' started by dunkelly, Apr 17, 2015.

  1. dunkelly

    Thread Starter New Member

    Apr 17, 2015
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    hi there i am looking for some help with an ongoing rc car racing project . We race rc cars with cameras inside using video goggles to drive as from the car . we all have lights fitted and sometimes race at dusk and came up with the idea of fitting brake lights which would help enormously in racing . question is how to activate the leds , we have looked at reed switches and tilt switches so far but neither seem to fit the bill perfectly . Has anyone any suggestions as to how we might go about it . many thanks . Nige
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    What action are you using on the RC car to brake, or is this just when you remove power or decelerate the motor?
    Max.
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Would 'foot off the gas pedal' (i.e. zero throttle) do as a signal to turn on the brake lights?

    Edit: Max beat me to it, by a whisker :)
     
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Just a switch to turn them on and just leave them on.. Unless you really need lifelike brake light functions.. (like 2 levels.. on and braking)
     
  5. dunkelly

    Thread Starter New Member

    Apr 17, 2015
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    hi thanks guys , we are using 2 channel controllers so they would need to work as the power comes off the motor , ideally as close to the radio off and on signal as possible .it would be great to have them functioning as brake lights , they are separate leds so its additional lighting to the normal lights and they are superbright as opposed to standard for the normal lights .
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I am surprised only 2 functions the common RC Tx are 4 -6.
    Is there room in the car to put in a parallel receiver?
    Is this simple on off control or the normal PWM motor control signal?
    An inertia switch could be possible, but a bit tricky to set up.
    Max.
     
  7. ebeowulf17

    Active Member

    Aug 12, 2014
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    Seems like it wouldn't take too much work to tap into the wiring to the motor itself and use the voltage there to control a MOSFET, which would then switch the lights.

    Maybe.

    I often think things like this will be simpler than they turn out to be!
     
  8. dunkelly

    Thread Starter New Member

    Apr 17, 2015
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    hi , most cars still use only 2 functions , we are using 2.4 ghz so i am not sure if they still use ppm or pcm . sorry to ask what is a MOSFET. a parrellel receiver maybe a problem as the recievers are usually dedicated so theres no cross channel interference . an inertia switch sounds like a possibility , if a MOSFET detects voltage drop if it can be specified it may be more accurate ??
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I know the 2.4ghz RC planes use PWM for control, you could tap into this, but it gets a little invasive.
    Does your TX'r have more than 2 ch?
    Most RC that I am familiar with have either 4 or 6 at least.
    I am wondering if it is just a case of changing the RX as these are relatively cheap, two of the ch on the TX unit are usually toggle switches, and these operate things like Nav lights or any optional on/off function.
    Max.
     
  10. dunkelly

    Thread Starter New Member

    Apr 17, 2015
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    not easy to change as ours use a wheel to steer , sadly only 2 channels to play with. we all have plane and heli tx's as well but want to use the car tx's

    sorry meant to add , any thoughts about a diy inertia switch
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I see a couple of approaches and would both need a little experimentation.
    A common ice cube relay has a set of contacts that are suspended by the movable armature that has a spring to hold it biased on the off position, it may be possible to modify one of this relays by taking off the bias spring so the armature hangs loose.
    If hung vertically, the armature may be made to swing to the closed position when braking.
    As I said, it may take some experimenting.
    There is also the simple ball bearing in the plastic tube trick, the tube would be balanced at its centre point.
    The bearing would make two contacts on one end.
    Max.
     
  12. dunkelly

    Thread Starter New Member

    Apr 17, 2015
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    the bearing in a tube is worth trying first , the cars are 4wd but i think the inertia would still be enough to make it work .would like to follow up on the mosfet idea as well though ,will do some checking to see if there are any that may work with the voltages we are using and will test our voltages for variations . nige
     
  13. ebeowulf17

    Active Member

    Aug 12, 2014
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    Ok, so this may be overly complicated or impractical in this scenario, but I wanted to see if I could figure out the circuit I imagined earlier.

    This is assuming the motor is driven by pwm modulation of the full supply voltage.

    20150417_220014~2.jpg

    The way I think this would work is D1 prevents C1 from feeding back into motor circuit. C1 maintains charge on Q1's gate during pwm low cycles, discharging through R1 when the motor stops completely. When the motor is on and Q1 is active, that pulls down the gate of Q2, blocking current through the LED. When the motor stops, Q1 is inactive, R2 pulls up the gate on Q2, and the brakes light.

    I'm very new at this - does that all seem right?
     
  14. Søren

    Senior Member

    Sep 2, 2006
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    In the mid-eighties, I jazzed up my oldest sons very simple R/C toy car (18..20cm long) with KITT scanner, driving lights, turn flashers (L/R controlled by the direction motor) and brake lights coming on when the drive motor was coasting, which made for a quite natural looking brake light.

    It was all done in SMD to get it to fit, the logic was CMOS4k and it fed off the existing battery.

    While it's now just a vague memory, lost in the mists of time (along with the schematics I drew - think pen and paper, if not quill and parchment), I'll recreate the brake light part of it, if you like.
     
  15. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If instead of PWM control you have speed control via an ESC receiving old-school servo pulses of 1ms-2ms duration from the Rx, this should do the job. It would connect between the Rx and the ESC.
    Brakelights.gif
     
  16. dunkelly

    Thread Starter New Member

    Apr 17, 2015
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    hi , thanks guys all way over my head i'm afraid , i will go and do some more research into your diagrams as they do not mean that much to me at present , one quick question though are the components for these things both very small and readily available . Soeren if you can dig out the brake element of your conversion that would be great . Alec i am using an electronic esc but some are still using older versions with mechanical esc s ,would what you propose work with both ?
     
  17. Søren

    Senior Member

    Sep 2, 2006
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    Oh, I'm afraid it would be over your head as well then.
    What instruments do you own (or have access to) Multimeter, 'scope, other?
     
  18. Søren

    Senior Member

    Sep 2, 2006
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    When coasting, the motor will generate a voltage, so the LED won't kick in until it's nearly stopped - then the LED will stay on until the power is shut off or the car moves forward. If the car is reversing, the LED will be on as well.

    Why use MOSFETs here? BJT's will be more rugged in this application and likely much cheaper.
     
  19. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What's a mechanical esc? (ESC = Electronic Speed Control). If by mechanical sc you mean one where a standard servo moves a variable resistor (rheostat) then no, my circuit won't do for that, but you could perhaps arrange a microswitch to be operated at one end of travel of the servo.
     
  20. ebeowulf17

    Active Member

    Aug 12, 2014
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    For some reason I was thinking that the forward coasting voltage from the motor would be the opposite polarity of the forward drive voltage, in which case the diode would block it. Don't remember where I got that - might have just been a hopeful assumption? Certainly, if I had that wrong, this circuit would be useless in this scenario.

    As for the MOSFET choice, I mostly chose them so there would be minimal current requirements to drive them except when changing states (charging the gate vs constant current flow through the base.) Didn't realize bjts would be more rugged. That would probably outweigh any concerns over the minor current requirements for the bjts.

    Thanks for the feedback.
     
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