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I am designing a simple controller for an automotive application. As you can in the schematic below, they system is consisted of a sensor, a gauge and a controller. This particular sensor shorts its own power line (SIGNAL in the schematic) to ground, creating a PWM signal which represents the measured quantity. The gauge works by balancing a needle between the magnetic fields produced by two coils.

When the gauge is present, the controller's only job is to sense SIGNAL (which switches between 0 and ~6.5V). However, I also need to support the configuration where the gauge is removed from the circuit and the sensor is directly connected to the controller. In this case, I need to provide power to the SIGNAL wire (perhaps using a current limited regulator) AND sense it at the same time.

I am looking for a decent way to support both of these configurations (gauge present and not present). One solution I can think of is to use the controller MCU to monitor SIGNAL at power up and if a PWM voltage is not detected (say after 1 second), turn on a MOSFET which will provide power to SIGNAL. But I feel like there should be an easier way. Any ideas?

enter image description here

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  • \$\begingroup\$ Sounds like a job for a 555. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 29 '14 at 9:46
  • \$\begingroup\$ could you elaborate please? Is using a 555 instead of the micro worth the cost and PCB real estate? \$\endgroup\$ – lyxicon Dec 30 '14 at 0:38
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You could very well likely add a pullup resistor to the +12V in the controller so that the controller can "see" the PWM signal from the sensor when the gauge is disconnected. You would have to do some experimentation to see if various size pullup resistors that you add will cause much change to the PWM signal when the gauge gets connected. If it does cause a problem then add a PNP transistor between the pullup resistor and the +12V so that you can disconnect the pullup via an output from the controller when you discover that the guage is connected.

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