How to build a brake light?

stivensen

10 µW
Joined
Nov 9, 2018
Messages
5
My controller does not have a brake light output.
The idea is to send a singnal 5v or 12v to back light when the brake is activated. Which component is better to use? A transistor like a switch? A relay? Maybe someone have a example of existing circuit?
 
stivensen said:
My controller does not have a brake light output.
The idea is to send a singnal 5v or 12v to back light when the brake is activated. Which component is better to use? A transistor like a switch? A relay? Maybe someone have a example of existing circuit?

Brake signal wires don't have enough current to power the light, you can only draw 20-30mA safely, so ~80-120mW. So you need to control battery volts or a dc dc circuit from either a very low power relay or a transistor.
 
Let's say I will use a simple red LED that works from 3v. Can I use 5v output with a added resistor?
First event is shorting SL +5v with ground, this is activated brake. How to generate from this event a second event - send 3v to a Led?
Power source for the led might be the 12v to 5v regulator?
 
So, I need to build another circuit directly from battery to have enough power to power on a simple LED ?
 
stivensen said:
So, I need to build another circuit directly from battery to have enough power to power on a simple LED ?
Most likely.
The voltage regulator in the controller is very limited and already has some load on it. You might get away with 20mA.
If you want a brighter brake light, you will need some kind of dc-dc converter. There are all kinds out there. They make very inexpensive ones made to run off a bike pack and have a 5v USB output for charging a phone. These can supply at least 500mA.

Just search ebay for "72v USB Charger" and you will see a variety. Here's an example:
https://www.ebay.com/itm/5V-2A-Dual...h=item1a28260d3c:g:I-YAAOSwXy1brisE:rk:8:pf:0 $4usd

Most ebike controllers use the brake switches to pull the brake line to ground. Many have "low level" and "high level" brake inputs. The low level one gets pulled to ground. The high level one activates the brake function when it gets pulled above 5v or so and is intended to be connected to a brake light.

Unless the bike had brake lights to start with, you would normally need to rewire the brake switches to turn on the light. The switch contacts in the brake levers can't handle much either, but should be able to run a LED light.

If the controller does not have a high level brake input, you would need generate a low signal for the brake input when the light is on. This could be done with a single FET, an optocoupler or with a small relay.
 
Everything you need is detailed in the following most excellent post by teklektik:

https://endless-sphere.com/forums/viewtopic.php?t=75171
 
Alternately, you can madmax it like me. I run my brake lights directly off an old ebrake lever switch. (used to use a 1985 Honda scooter brake lever; the switch finally became unreliable after 30+ years of original + my use, and the hinge was floppy).

The way I do them is that I have LED taillights that run all the time at low brightness, and brighten during braking. I also have separate brake lights that run only during braking.

Simple dimming is a big power resistor (or several paralleled / seriesed, to get the necessary resistance and wattage), placed in series with the +12V (really +15V average) to the taillights, with the other end of the tailights to ground.

To keep the brake lights form coming on except during braking, and allow brightening of taillights for braking, there's a diode installed with it's stripe toward the taillights +V from the resistor, and the other end to the brake light +V wires. The other brake light wire goes to ground.

The brake switch is wired across these, so it allows full +V to both lights when the lever is pulled. I am presently using a separate lever for the lights (vs the controller ebrake signal), so there's two brake levers on the left side, but you could easily add a switch to any brake lever in a number of ways, or use a relay to engage brake lights and ebrake.




Because my camera autoadjusts for brightest stuff in scene, you can't directly compare the brightnesses of the lights, but if you look at the interior light of the cargo hold / dog carrier, or the red downlighting of the ground at the back, you can see how much dimmer it is in the "on" pic vs the "off" because of the camera dimming the whole image to compensate for the brake ligth brightness.

https://www.endless-sphere.com/forums/viewtopic.php?f=2&t=67833&start=775#p1424309

Brake lights off:
file.php


Brake lights on
file.php
 
Just to talk some numbers, I put 200mA load on controller's 5v rail and it dropped from 4.3v to 3.9v. The throttle and other sensors use ~30mA on average. So there you have it, safe load is 30-50mA. Anything else will mess with your throttle sensitivity/range and cook something long term.
 
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