The simplistic electrical circuit is as shown,
the battery through a resistor (about 50 k ohms for sigma types)
charges the capacitor (basically made by the leads) to the battery voltage.
The reed switch discharges the capacitor when the wheel magnet passes.
After the wheel magnet passes the capacitor recharges through the resistor.
The speedo senses the voltage on the capacitor and uses this to calculate the speed.
There are some other things going on but this will do as a simple
explanatory model.
The sensor does not look for either full or no voltage but rather for voltages about halfway (the red line in the diagram).
This helps remove noise and problems with contacts, drifting components, etc.
At high speed the capacitor does not have enough time to charge up through the resistor to the full voltage.
Eventually the speedo stops working.
Lengthening the lead, increases the capacitance , reducing the maximum operating speed.
The reed switch operates when the magnet gets close.
If you make the distance over which the switch is closed long compared to
the total distance the wheel travels then the speedo will stop working at a lesser speed.
The effect is small, but if your after the last bit, set the magnet so it does operate the switch,
but not over a great arc on the wheel. You can do this by positioning the
sensor and magnet as far from the wheel centre as possible.
As speed increases so mechanical flexing can be expected... just to complicate things.
I not going to get in to this... Too complicated, too much like hard work.
The mechanical inertial of the reed switch also comes into this...
producing a time lag, and a required minimum time for the mag field
to be above a certain level... way too much thinking for free.
If you quote me I would like an acknowledgment.
back to my home page & start again
You can e-mail me at Yahoo. Dated 10 March 2001