This fuel sender unit (latest K bikes)
can accommodate the installation of a fuel level gauge
The early models are equipped with an electronic sender
There are two thermistors housed in the fuel sender unit which is
located in the fuel tank. The thermistors are positioned at two
different levels corresponding to 7 and 4 litres. When the thermistor is
exposed to the air in the tank the appropriate light is illuminated.
Each thermistor is supplied with 12 volts that is current limited to
95mA, this current limit is controlled by the BD240A's and there
accompanying circuitry. With the applied voltage the thermistors self
heat, when they are immersed in fuel the heat is dissipated and the
resistance is approximately 600 ohms. While the thermistor is immersed
in fuel the inverting input of the op-amp has 12 volts applied to it.
This voltage level is above the 4.7 volt reference applied to the
non-inverting input of the op-amp and thus the output of the op-amp is
below the 0.7 volts needed to turn on the NPN transistor (BD877) which
controls the lamp. When the thermistor is above the fuel level and in
air, the self heating causes its resistance to drop below 53 ohms, the
voltage at the inverting input of the op-amp drops with it to below the
4.7 volt reference level applied to the non-inverting input of the
op-amp. This causes the output of the op-amp to swing to its maximum
positive level (+12V).