a forgotten treasure... the Belt Beacon strobe light

Somehow, a conversation with a friend led to the Belt Beacon, a bicycle strobe light from the mid 70's. I used one back then, and it was a big improvement over the other humble lights that were available for bikes. It used actual electronics!! It could run from a 9v battery for hours, and was much more visible than other lights (although that seems unlikely in retrospect). In any case, it was the best available for quite a few years.

Naturally, I had to go dig through my parts box and see if I ever got rid of it. Somewhat to my shame, I still had it. Here are a few shots of it....




so what was in this light??
Well, you had to pop off the amber lens and remove two screws to remove the plate that the bulb was mounted to...



amazingly, the screws that hold the light together are also parts of the circuit.. they connect two wires from the board to the contacts for the bulb. The wires from the board are fitted into the threaded sockets that the screws go into. This is certainly a case of thinking outside of the box!

The circuit itself was very simple. Two transistors, three resistors, and one capacitor.
The board itself was single-sided, with no jumpers on the top of the board. This is harder to do than you might think.





I reverse engineered the circuit, which is much easier to do than with modern circuits. The values and part numbers are all clearly marked, and the traces are all visible. Piece of cake! The only complication is that the transistors are no longer being made, and their manufacturers, Motorola and National Semiconductor, are no longer in the business of making transistors. I did have to probe the transistors with a meter to figure out if they were npn or pnp (answer: one of each).

Here's the schematic....



I even ran a simulation using LT Spice to see if my schematic made sense. It didn't work that well with generic transistors, but substituting common 2N3904 and 2N3906 transistors caused the simulation to work fine. The voltages shown below did help me get a better feel for how the circuit works, and just how clever it is.
The bulb flashes roughly once a second.



the blue trace is the voltage at Q2's base. The green trace is Q1's collector, the low side of the bulb.



Okay.. I'll admit.. this may be of interest to just a few folks, but I felt this was definitely the place to share.
enjoy!

Time for an LED upgrade?

Wow, a great piece of kit to find after all these years, and beautifully documented as well. Not sure why you said, "Somewhat to my shame, I still had it.", that's a proud find, and in excellent condition to boot.

It seems to be quite an elegant piece of engineering, with everything you need, and nothing you don't. Thanks for sharing that most interesting piece of bicycle lighting nostalgia, Steve!

+1
And Steve, don't even think of throwing a 21st century LED in there!

I kept the light because it is/was kinda cool and a great design for its time. My parts boxes are full of this sort of stuff, though... a nice Soubitez dynamo block light, an old Schwinn bottle dynamo and light that was gifted to me, a more recent dynamo block light, etc. You should see my collection of derailleurs! I've got a number of neat and quirky SunTour rear derailleurs. I still use the SunTour Cyclone GT on my commuting bike.

The Belt Beacon would be a fun candidate for an LED upgrade. I'd probably run it off of a couple of AA NiMH cells and try to use a modified version of the Belt Beacon circuit. The modification would need to be reversible, though. It seems very reasonable to do.

There are times when I like to have a strobe that is bright and visible over a wide viewing angle, and a LED version could do this well. I do have a couple Lightman xenon strobes already... powered by two AA's. It's fairly bright, but is also pretty inefficient. It would make a nice host for a LED mod too.

Time for a lithium 9v battery.
LA522 come on down.

Thanks Steve K for your report. I have not used mine for so long I had forgotten how it operated and did not want to break it. I popped it open and unfortunately there was an old 9-volt battery. Although the top was exploding outward and terminals coming off, all was actually well; just needed a pair of pliers to carefully remove one terminal. Did a very minor amount of cleaning.

First experiment no go, checked the 9-volt battery, it was dead. Got a fresh Energizer and was disappointed for a second or so and then it started blinking. Yay.

Any idea what bulb it takes or whether they are still available? I did not want to remove the bulb to see if there were any markings, better to leave it working. ;-) I probably won't use it enough to wear out the bulb anyway.

I probably got mine sometime in 1980 or thereafter. I did read some history here: http://www.cyclelicio.us/2011/belt-beacon-bicycle-light/

Donald said:

Thanks Steve K for your report. I have not used mine for so long I had forgotten how it operated and did not want to break it. I popped it open and unfortunately there was an old 9-volt battery. Although the top was exploding outward and terminals coming off, all was actually well; just needed a pair of pliers to carefully remove one terminal. Did a very minor amount of cleaning.

First experiment no go, checked the 9-volt battery, it was dead. Got a fresh Energizer and was disappointed for a second or so and then it started blinking. Yay.

Any idea what bulb it takes or whether they are still available? I did not want to remove the bulb to see if there were any markings, better to leave it working. ;-) I probably won't use it enough to wear out the bulb anyway.

I probably got mine sometime in 1980 or thereafter. I did read some history here: http://www.cyclelicio.us/2011/belt-beacon-bicycle-light/

Click to expand...

I saw that my flickr account was showing a bunch of views of my Belt Beacon stuff, so I figured I should see what's going on at CPF! Good heavens, it's been a while!

I checked my photos, and all I can see is that "Italy" is part of the marking on the bulb. While the battery is 9 volts, the pulse is so short and the bulb so bright that I suspect that it must be rated for a lower voltage. 6V? 4.5V? I could dig out my Belt Beacon, but that sounds like work.
I don't think it is possible to wear out the bulb in any reasonable amount of time. Mine still has the original, and I did use it for quite a while.

By coincidence, I was thinking that I might need a light to clip onto a saddle bag, and the Belt Beacon could work. I admit that I was contemplating swapping in a new board and a white LED, but I'm not sure I could fit 2 or 3 AA nimh cells in there. Might have to check again.