When placed between the speed sensor and the motor, this device receives the impulses from the speed sensor and only sends every second impulse to the motor. The motor thinks the bike is going half speed, and the speed limiter kicks in at 50 km/h instead of 25 km/h.

I've used it for over a year now, and don't want to go back.

Compatibility

It works with most bikes that have the motor driving the crank, it does not work with in-wheel motors. I'm using it with a Bosch motor.

Warnings

This product can be dangerous if used irresponsibly. Higher speeds mean higher stresses on your bike. Make sure your brakes, front fork, and other safety-related parts can handle it. Use at your own risk.

Only use on private grounds!!! Biking with this device on public roads is not allowed in most countries. When you configure it as a dongle, the device can be completely removed, and the bike can be used on public roads again.

Installation

1. Cut the wire going from the motor to the speed sensor.
2. Switch the bike on, and check the voltage on the motor side. It should be between 3 and 5 Volt. If higher or lower, bad luck, it won't work - solder the wires back together.
3. Did you get 3 to 5 Volt? Great, solder the 4 wires to the device and enjoy!

Dongle

Turn the device into a removable dongle with a switched 3.5mm stereo jack and plug, see drawing.

Technical Description

The speed sensor is a simple reed switch. The switch SW1 closes when the magnet on the wheel passes it. On the motor side the switch is powered by pull-up resistor R1, and the motor receives a low signal when the switch closes.

The device mimics the same configuration with SW2 and R2. It senses the closure of SW1 and closes SW2 for every second closure.

The device is powered by the motor via R1, and uses diode D and capacitor C to keep it powered while SW2 is closed.