Bicycle LED Headlight –

I got a cheap bicycle headlight a while ago. The battery-operated 3-LED headlight with 80dB multitone bicycle horn is suitable for the universal normal bicycle handlebar. It is a rather amazing gizmo. I thought it would be interesting to look at its internal electronics to see how things are done. And yes, I do have a note at the very end explaining hacking the bicycle headlight through a few simple steps.

This bicycle headlight has a simple UI with two pushbuttons, three different light modes, four different horn sounds, and a detachable handlebar mounting clamp (with a thumbscrew and a rubber grip strip). Simple enough!

It runs off a 4.5V battery (1.5V AAAx3). Based on my quick observation, the maximum current taken by the headlight circuitry is slightly under 120mA, which is about 360mA for the horn part. This doesn’t allow for great outputs or runtimes, in my opinion.

It takes four clicks to cycle between the headlight modes (blink-strobe-steady-off), and four clicks between the horn modes. That being said, one press of the headlight button turns the headlight on. When it’s on, pressing the button cycles through the three light modes, and after the third mode, next button press will turn the head light off. Likewise, you need to press (and hold) the horn button to sound the horn, and cycling the button cycles through the four alarm tone modes. Frankly, this is hard to do while riding, and it’s a bit like changing television channels one at a time instead of just jumping directly to the one you really need!

Before I started hacking my bicycle headlight, I was hoping that I could find some easy ways for that by examining its circuit board. So, I took it apart:

The single-sided printed circuit board uses a small 8-pin microcontroller to do the job which means that there isn’t too much data I could get by just staring at the circuit board. Sadly, the marking of the microcontroller chip was sanded so I couldn’t figure out the actual microcontroller used in the circuit easily either. By looking at the circuit tracks, the chip seemed like a cheap PIC microcontroller. Anyway, I decided not to dig any further!

I studied the circuit well and prepared a rough schematic diagram.

The key parts are:

  • uC: Unmarked SMD microcontroller
  • C1-C2: Unmarked SMD capacitors
  • Q1-Q2: Y1 (MMBT8050D) SMD Transistors
  • PZD: 25mm Piezo Disc
  • L1: 3-Pin Alarm Booster Coil
  • R1-R2: 102 (1KΩ) SMD resistors
  • R3: 1.5Ω ¼ w Resistor

The headlight section consists of just three parallel-connected 5mm super-bright white LEDs and the 1.5Ω current limiting resistor (R3 in the above schematic).

For some, the 3-pin Alarm Booster Coil is still a mysterious part. But note, it’s just a small autotransformer ( I’ve examined this in detail before

So, the bicycle headlight consists of a multimode headlight and a multitone horn in a common housing with plastic optic. There may be several reasons to open the bicycle headlight, here it’s for some small hacks. So, you now need to extend the button connections by soldering three thin wires to the button pads, but it’s not a big deal.

When doing this, make a small slit in the battery door and try to pull the wires out through it.

Disclaimer: Do this stuff at your own risk. If you mess up and break something, that’s on you. Also, doing any of this will void your warranty!

Frankly, there’s not much you could mod on this pretty cheap bicycle light, but you can control both buttons independently by means of external electronic switches or sensors (you can also use electro-mechanical switches).

When it comes to an external microcontroller interface, you only need one dual-channel optocoupler between them to ensure galvanically-isolation. The MCT6 optocoupler will be a clever pick here.

The MCT6 is a dual channel phototransistor output optocoupler. Each channel consists of an optically coupled pair with a gallium arsenide infrared LED and a silicon NPN phototransistor ( I chose this optocoupler because I had it in my hand, and as readily a available component.

This optocoupler, especially designed for driving medium-speed logic, can be used to replace relays and transformers in many digital interface applications, as well as analogue applications (take a look at the below wiring scheme).

Now you can start the hardware hacking of your bicycle headlight with the help of appropriate optocouplers and/or miniature reed relays (see below).

For example, you can rig up a nifty handlebar-mountable (or pocketable) wireless button pad with a couple of touch buttons or sensors so that you can use it to cycle between modes like you would use the headlight’s own light and sound buttons. I’ll move on to this idea in my own way later.

Thanks for reading my post. If you’ve any questions about this topic, I’d be more than happy to answer them. Happy Hacking!


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