Introduction: Bontrager Duotrap S Cracked Case and Magnetic Reed Switch Repair

Hi, what follows is my story on saving a broken Bontrager duotrap S digital sensor from the trash.

Its easy to damage the sensor, a portion of it protrudes out of the chainstay to be in close proximity with the wheel spokes. It's a fragile design. And I managed to break it.

Hopefully, this example may come in handy for others that run into the same issue. You can pocket $70 of a replacement for beer money or a romantic dinner out. Back to the story.

The chainstay on my bike scraped hard along the side of an indoor flywheel trainer, breaking the protruding finger of the duotrap off the bike. The circuit board also came out, luckily in one piece.

But one of the components attached to the board was torn off, stuck inside the other 1/2 of the duotrap still inside the chainstay.


utility knife


solder iron

wire cutter

replacement mini reed switch

Step 1: Separate the Case

Separate the halves of the duotrap case using a utility knife blade. Do this carefully, and slowly. Push the blade into the seam and twist. The seam will come apart, eventually the halves will come apart as you work along the whole seam.

Once separated, the broken component was revealed. Inside the remaining part of the sensor, stuck inside the chainstay was a tiny broken glass cylinder, with metal conductor in the middle. The circuit board had the other side of this conductor, but the glass was gone, so it looked like a plain wire. Turns out these are pieces of a broken reed switch. There are 2 of them on the board, one for cadence the other for wheel rotation. The cadence reed switch was broken on mine.

The pictures shows the two halves separated. The finger is also re-attached with superglue.

Also shown is the circuit board, notice on the left, the broken reed switch.

Step 2: Order Replacement Reed Switch

A quick search for mini reed switches brought up an exact match, ~14mm in length and ~2mm diameter. I bought pack of 15 on amazon for ~$7.

Step 3: Trim and Solder Source Lead of Reed Switch to Circuit Board

You'll need to trim the leads on the switch, i layed the replacement inside the case to find trim dimensions.

I noted the orientation of the working reed switch (wheel rotation), blades inside the switch flat are oriented horizontally. And soldered the replacement in the same orientation (not sure if this is critical).

Step 4: Solder Return Lead to Circuit Board

The return lead is a thin wire, enameled green.

Solder it to the bottom pad on the flip side of the circuit board. See picture above where start of return lead is wired to circuit board pad. Both pad and lead start are noted with boxes. I touched up the lead with some

Step 5: Paint Enameled Return Lead With Nail Polish

The enamel on this end is most likely burned off when soldering the lead back to the circuit board. Coat it with nail polish to prevent a short with the other pads on the board.

Step 6: Test the Circuit

Powerup the circuit board with a 2032 button cell battery. Pinching the battery and circuit board between my index finger and thumb did the trick.

Swipe a magnet next to both read switches.

You should see the led indicator lights light up. The wheel rotation is red, cadence is green.

At this point, you can also pair the board with your bike computer. Swiping the magnet should generate velocity and cadence on your bike computer. I was able to get it up to 22mph, ;).

Step 7: Repackage Circuit Board Into the Case

I used some black silicone to seal the case. Hopefully, i'll be able to re-open it again if something breaks inside in the future