Gasoline tank control

Equivalent circuit for the Suzuki SV 650/1000 when the original speedometer is exchanged for an accessory speedometer.
here there is the wiring diagram for download [200 KB]

The function :

About the resistor 82 Ohm / 5 W , the NTC is permanently supplied with power in the tank . However, it is kept cold , so its resistance remains high through the gas. Decreases the gasoline and the NTC is therefore less or no longer cooled , it heats up and its resistance decreases.
At the connection point between resistor and NTC voltage is then measured . The resistors R9 and R10 provide a reference voltage. IC1a is a voltage follower. It provides , with its high input impedance for a non influence of the reference voltage . At the output of the reference voltage is low are available. IC1b is a comparator with a hysteresis . At its non-inverting input the reference voltage and the inverting input of the measuring voltage from the NTC is connected.
The measured voltage exceeds the reference voltage ( the tank is full ), the output of the comparator low level. If the measured value of the reference voltage, the comparator output flips to a high level . The measured value must now ready to rise again , that the tipping point of the comparator is reached. The hysteresis thus prevents constant switching on and off at sloshing gasoline.

Note: The operating voltage is not stabilized. Thus , the circuit may very well respond to changing power in your motorcycle. Since the components operate in a linear , all the measurement points to move simultaneously. The circuit is thus stable.

optional part : At the output of the comparator could turn an LED to ground. The functional fuel gauge was already finished
. In the example , the LED should flash but . This assumes a downstream astable multivibrator. On the basis of the timer NE555 can be something realized with little effort. The output of op-amp switches directly on the operating voltage of the timer on and off.

The whole circuit was with me on a sample board with the dimensions 4cm x 2cm square. And all this without SMD - use .

With a small expansion of the circuit to a second comparator , one can also use the second NTC Suzuki tank with newer sensors. The output of this comparator can then drive the transistor of the LED directly , which corresponds to a steady light. Thus, both functions of the OEM gauges are shown . Only then flash continuous light .