Voltage regulator and battery status output

Discussion in 'The Projects Forum' started by cyrand, Oct 31, 2012.

  1. cyrand

    Thread Starter New Member

    Oct 31, 2012
    (Sorry if this gets posted more than once, I get a server error when I hit the submit button)


    I recently built a circuit to monitor a pressure signal. The circuit is a pressure sensor powered by a 9V battery through a voltage regulator. Since that power regulator already had the comparator circuitry built-in and a power good status output, I figured I would use that output to turn on a red LED when the battery charge dropped.

    The circuit drawing is attached. The regulator datasheet mentions that the power good (PG) output is open-drain and can be used for the purpose of low-battery indicator. It is supposed to be pulled low when the output starts to drop. I connected the PG signal to a 74HCT00 NAND gate with the output of the gate driving a red 5V LED as shown. No other reason for the choice of gate and logic family than the fact that I had the part at hand.
    I built the circuit adding one part at the time and checking at every stage that the outputs in inputs were what I expected them to be and finishing with the red LED. Up to that point everything was fine.The gate output was at 0.0V and both inputs were at 5V.Then when I added the LED, I noticed it was drawing current as it was dimly lit, and the voltage at the input of the gate had dropped to 1.3V.

    Because I was not sure what the problem was and the power indicator would have been nice to have but not essential, I just removed the LED from the circuit. Since I closed the box and started to use it, I apologize for not being able to provide feedback about whether proposed solutions work or not (at least not immediately), but if anyone can spare the time and maybe spot glaring mistakes, I would appreciate some pointers about how to handle this correctly for future use.

    Thanks for your time,

    Voltage regulator:http://www.digikey.ca/product-detail/en/TPS7250QP/296-8059-5-ND/373269


    Pressure sensor:http://www.digikey.ca/product-detail/en/MPXV5004GC7U/MPXV5004GC7U-ND/951844