Powering 5V rear light from BBSHD shift sensor cable?

[titusmc]

Does anyone know if you can safely draw <500mA of current from the 5V line of the BBSHD shift sensor cable? If this line is common with the display 5V supply, I suspect it may support 500mA since the DPC14 has a USB power port (assuming nothing plugged into this port simultaneously).

I'm hoping to make a custom cable to power my rear light and this seemed like an attractive option since that cable is currently unused on my ebike (I use the right hand ebrake cable for my gearsensor).

 

[flat tire]

Here's a better idea: just use a reflector, and ride under the notion that no motorist will ever see short you short of putting police lights all over your bike (and even then...)

 

[flat tire]

wtf, double post

 

[titusmc]

Sorry, but I'm quite happy with my light vs. a reflector since it has accelerometer-based brake light functionality (LucidBrake).

 

[flat tire]

That would be a useful feature if you could count on people paying attention. They don't. Actually, it wouldn't be useful because it wouldn't be NECESSARY if people paid attention. And you already know they don't. So why waste your time with a unnecessary light that serves essentially no purpose?

Just get a reflector so you're in compliance with the law, and boom, done.

 

[crazymanc]

I had mediocre lights and reflectors on my bike and people didn't see me. Then I upgrade to something more significant. 4 CREE XPE red LEDs mounted around the reflector and two MKR white at the front, all with suitable lenses. These aren't driven particularly hard for a total power consumption of about 10 watts. But now people see me.

It's easy to see why, I've seen when others ride the bike, to try what an ebike is like, that the bike stands out even at considerable distance.

Obviously this cannot be powered from a 5V 500mA line but as you've got a whacking big battery on an ebike there's no logical reason to go around with poor lighting.

Yes I'm an engineer and yes I designed/built it all myself, but there are more capable lights out there for ebikes.

As to your question about the sensor... I don't know. It depends how it works. Usually a break/switch would have a pull up resistor on the input pin of the microcontroller. Then, when activated, the switch would connect the input pin to ground. Normally the input pin is held high by the pull up (and this connects to the power supply) but then goes low when the switch activates.

This effectively connects the power supply to ground​, but does.it through a high value resistor. The resistor inherently limits the current to a mA or a few hundred micro amps depending on the value. Clearly you cannot power a lamp from this.

Alternatively, if hall effect sensors are used for the breaks then there are three lines. Power, signal and ground. Typically the signal will also use a pull up resistor, but power and ground could theoretically be used by something else. A word of warning though, the power line could be protected by a series resistor. This would prevent the system from suffering potential damage in the case of a fault condition. Hall sensors don't need much current, so there could be a 500-1k ohm resistor in series with its power feed. If this is the case then you will not be able to power a lamp from it.

 

[Voltron]

Around here (California) a tail light is required at night, not just a reflector.

 

[amberwolf]

Regarding the OP's actual question, I don't know that most ebike controllers could spare 500mA on the 5v line; the ones I've opened up often only have a teeny tiny 5v regulator, whose entire current budget might be around 100-200mA.

I have not seen inside a BBSxx type though.

If your controller is not potted you could open it up (probably a PITA if it's mounted to teh bike already) and check what actual part is used for the 5v regulator--but it's probably either an SMT version, or a TO92 case, for the LM7805, so it's total power dissipation probably has to stay pretty low.


But if you're using a main bike battery that starts out near 60v fully charged, or higher, you could try connecting the ac prongs of a 5v phone charger to the battery (doesn't usually matter which polarity), and then connect the 5v out of that to your light.




Regarding lighting in general...I don't think any of you are thinking outside the box enough. (note that these pics don't show my KIA headlight turned on just the Grin Tech bicycle headlight. Also doesn' show the turn signals or the center brake light, or the brightening of taillights during braking).

Without the downlighting on, and just the bike headlight and MC taillights, it's practically invisible

 

[rcelegans]

just to answer the OP question. It doesn't work. I used a Y splitter thinking I could draw 5V from the Brake sensor cable to charge a USB battery and although it does supply 5V as soon as you start riding and the motor kicks in the motor slows down and no power goes to the motor.