Measure actual Voltage of super cap when voltage source and s.cap in series

What is "bulbe"? What is its resistance and purpose? That "voltage sensor" must have 3 or 4 wires, 2 to 24V + and ground, and 2 to Arduino GND and analog in pin.
Do you have a voltmeter? Do not connect that sensor to the Arduino until you KNOW the voltage at the sensor pins!