Introduction: Fit a Digital Speedo and LED Lighting Into a Vintage Bicycle Speedometer Lamp

About: Swiss expat in Germany, husband, father, teacher, cyclist, tinkerer, former theatre propsmaster 🇨🇭 🇩🇪 👨‍👩‍👦‍👦 👨‍🏫 🚲 🛠️ Heimwehschweizer in D, Ehemann, Vater, Lehrer, Radfahrer, Bastler, Ex-Requisiteur

Since a long while I had this old "Radsonne Ratar" bicycle lamp taking up space in my parts bin. It had a built in speedometer which was broken. When I was looking for a lamp for my last bike, it suddenly came to me that a modern bike computer should fit underneath that little window. And why not put a LED light in at the same time.
And here we go with another project....

Step 1: Materials and Tools


• vintage Radsonne Ratar bicycle lamp with (broken) speedometer
• Bikemate bicycle computer
• Osram Crosser LED Lenser / Varta LED Headlamp (I fried the first lamp with the soldering iron..)
• sugru
• wires, switches, battery holder

Main tools:

• Dremel
• soldering iron
• drill

Step 2: Void All Warranties

First, I took this brand new bike computer, and attacked it with my dremel. I strongly recommend wearing safety goggles on this step. Lots of hot almost molten plastic flying around here!

Be very careful here for not damaging the circuit board. When taking apart I quickly marked "+" and "-" on the board for later reference.

Then I slowly started to grind the top of the bike computer into a shape, that would fit into my lamp (which I had already taken apart before I started documenting for this instructable). Again, be very careful here, when it's gone, it's gone!

As there is some space between the actual LCD Screen and the top layer, I recommend to clean said space from grinding dust with a few short blasts of compressed air.

Step 3: Desolder and Solder

Now it's time to fire up your soldering station.

As the first step, I soldered on a battery holder and put in some fresh batteries. Then I tried to figure out, where to solder on my switches. The setup in the picture worked very well for me: a wire with some small pins or needles on each end and a small mirror to check if I get any kind of response while touching the various soldering spots.

Like this I quickly figured out, where I had to solder on the external switches. But before doing so, I obviously had to desolder the original switches...

After a quick function check the computer is ready to be built into the lamp.

Step 4: Fitting It In

First I widened the two holes that were already on the sides of the lamp (the original speedo was fixed there) to accommodate the two switches.

Then I applied some sugru on the inside edge of the window and pushed in the computer. Then I removed any excess sugru on the outside, an used it to give some extra fixation on the inside.

Like this I let it sit for 24 hours to cure and went on to the next step: light!

Step 5: Void Warranty Again

As I already mentioned in step 1, my first source for the light was a Osram Crosser. I disassembled it, de- and resoldered switch, battery case and lamp and thought all clear, but then realized, that somewhere on the way I must have fried some components on the circuit board. The light didn't work anymore and the board did get that hot, that I almost burnt my finger on it....

So back to square on and go shopping again (there goes low budget). The Crosser was not available anymore, so I decided to give the Varta headlamp a chance...

It was fairly easy to disassemble and the circuit board was soldered directly to the battery casing. This time I wanted to solder as little as possible. So instead of desoldering the original switch, I soldered the new switch directly onto the old one. Then I cut the cables to the lamp an soldered a longer one inbetween.

And hey, it worked this time!

Step 6: Fitting the Light In

As the light was working now, it was again time to get out the sugru. I used it to fix the new reflector kind of on top/inside the old one.

Just to be save, I used some leftover sugru to insulate the contacts of the switches and the battery case.

Step 7: Putting It All Together

Now, after all the sugru had enough time to harden out, all I had to do, was to fit everything into the housing of the lamp. It was a thight fit, but it worked and there is almost no rattling.
The light is very bright, but there is no real focus point. But it works fine for me. At night I use it mainly for "self defense" as in beeing visible. While cycling in town, there is more then enough light to see where you go.

Although everything is inside the lamp as planned there are two major drawbacks:

  1. The speedo seems unable to make contact to the sensor. The most probable reason is, that the metal casing of the lamp shields the radio signal. So I have to identify the antenna and wire it to outside of the lamp.
  2. Somehow I must have wired the switches wrong, as the buttons don't have the functions they should have. I'll update that step, once I have figured out what went wrong.

It's not working perfectly, but I will get back to the two issues later. The light works, the speedo is optional. So I call this done for now and update this intructable later...

Step 8: Debugging the Speedometer

As I mentioned in step 7, I had some troubles with the speedometer. So I urgently needed some debugging to make this thing really usable.

One bug that showed up after the first night ride was the fact, that I soldered the light switch directly onto the original switch. It happend a few times that even though there is almost no rattling, that the switch was activated and the light went off. To avoid that, I put a bit of glue onto the original switch, and I also fixed the batteries with cable binders.

So now on to the switch issue. For this I decided to take the speedo back out of the lamp. Then I had to search for the original switch. Unfortunatly I had to go through my waste bin for that... So lesson learned for further similar projects: don't throw away any parts until the project is really done!Once I took apart the original switch to get a better look onto the circuits, my mistake was obvious and easily corrected. Now (without switches, just the bare cable ends) it worked fine, so I soldered the switches back on, and.... bummer! Again, strange behaviour. So switches desoldered again. I went to a local electronics supplier and told them my problem and he pushed my nose onto the obvious: one of the switches was a opener instead of a closer! So two new switches and... tadaa!... it works!

So back to step 4 and reassembly...

Last the easiest part, the sensor problem. The thing to do here is to lower the distance between sensor and speedo. So the sensor has to get as high as possible and the lamp has to get low enough to receive a signal. I used a strip of metal with predrilled holes, some screws and epoxy to get the sensor right over the tire underneath the fork bridge and taped a flat but strong neodymium magnet inside the tire. Problem solved! Bonus: the ugly sensor is hidden. Malus: tire will probably pick up not only tools...

I had to adapt another fixture for the lamp in the lower position, because the original one wouldn't work here...

Bicycle Contest 2016

Participated in the
Bicycle Contest 2016