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low power ultrasonic and g2553

James Woodrow
Intellectual 360 points

Hi,

Ive got an MSP430 that goes into LPM3 and wakes up once every 5 seconds, I added the HC-SR04 ultrasonic sensor which I am powering from the MSP430s USB port 5V supply.

I had a 2n3904 NPN transistor setup which switched to ground, but i noticed that between +5v source and VCC of the sensor that it still draws current around 11mA. Its seems to be the trigger and echo pins, because after disconnecting them the device stops draining current. 

It seems to be a port pin issue, I only set the pins up in the routine as input for ECHO and output for trigger. I also have a 2.2K base resistor (calculated was around 2.3K ohm).

Any advice on pin setup etc?

I also have a BC557 and 2n3906 would it be better to source the VCC that way?

thanks guys

  • 0
    Guru 243080 points
    The ESD protection diodes can make any pin with a high voltage a Vcc connection, and with low voltage, a GND connection.

    Unless the device actually is designed to allow voltage at its pins even when powered off, you have to disable all connections (i.e., make them high impedance = inputs) when powering off.
  • 0
    Guru 46710 points
    >I had a 2n3904 NPN transistor setup which switched to ground
    You shall switch +5V, not ground of sensor. Use your npn transistor as driver for P-MOS, not as power switch.

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