# LED brake lights - detecting voltage drop?

I am trying to light up some LEDs on my Rustler VXL RC car only when braking/reversing. I found out that on my receiver I have three wires: black(-), red(+), white(+). The one that interests me is the white (signal wire). This wire tells the speed controller to either turn the motor clockwise or anticlockwise.

White wire voltage is as follows. - no throttle: 0.55V - full throttle forward: 0.71V - full throttle reverse: 0.39V

So, when I brake/reverse the voltage drops from 0.55V to some value but no less than 0.39V.

How can I design a system to catch that voltage drop and activate a switch to light up the LED? What parts would I need to buy? And, what's the difficulty level for this?

Thanks

• Is there any fixed voltage source you can use as power? What's the voltage on the red wire? How complex should it be? Commented Dec 27, 2012 at 20:43
• Look into op-amp comparator circuits. Difficulty level... maybe 3 out of 10 to get something decent. Commented Dec 27, 2012 at 20:43

## 2 Answers

It's probably not that simple. How did you measure the voltage level, with a multimeter I expect? It is probably a pulse stream with the information actually encoded in the length of the duration of the positive pulses. Probably the pulse varies from 1 to 2 ms. This looks like a voltage signal only because the meter averaged it over time.

If all the above is correct, then to use this signal you have to decode the pulse width. There are various ways to do that with most microcontrollers. The micro measures the pulse width, decides whether that indicates reverse or not, and turns on a output accordingly.

A simpler way to do this is to let your existing decoder decode the signal and see what it decides. Ultimately it's going to control the motor. You could have a transistor turn on whenever a negative voltage is applied to the motor. Probably the controller is pulsing the motor between 0 and full supply voltage when going forwards and 0 and negative supply voltage when going backwards. The fraction of time it spends on the supply voltage versus zero is how hard it drives the motor. By detecting negative voltage accross the motor, you should be able to tell when the controller is trying to back up.

• You are right. It is pulse based. Commented Dec 27, 2012 at 21:10

Do it in the RC car with a simple circuit. It can reasonably easy be adopted for bipolar use with an extra bridge rectifier. Difficulty level ... magrinal ;o)

When a voltage is applied to the motor, C2 will be charged through D1. Whenever the voltage on the motor drops, C2 will want to discharge but is forced to do this through the LED's and R.

$R = \frac{U_B-2×U_{LED}}{I_{LED}}$

• Bad circuit. The motor is probably driven by PWM, so the large cap accross it could really stress the H bridge driver. The ripple current spec of the cap will likely be exceeded. Also, since the cap is polarized, it is guaranteed to be out of spec for one of the drive directions, whether the drive is PWM or not. Commented Dec 27, 2012 at 21:02
• @OlinLathrop Agree for the PWM, didn't think of that. Commented Dec 27, 2012 at 21:04
• looks like Elektor style schematic Commented Jan 22, 2013 at 9:37
• @ChetanBhargava if you look closely you can see it was drawn by hand. Dates from 1973'ish Commented Jan 22, 2013 at 15:22
• @jippie I meant "style". The component symbols are same as what I used to see in Elektor. Commented Jan 22, 2013 at 19:06