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I'm fairly new to electronics & arduino.

When experimenting with the arduino wemos & the distance sensor HC SR04 I noticed that connecting the GND isn't necessary. It looks like the gnd is channeled through the Trig digital output instead. I conclude this as I measured 2 mA on trig where the 2mA is otherwise measured on the ground. If GND is connected Trig remains 0mA.

As 2mA is lower than the max of 10mA it will not hurt the Arduino wemos but will it hurt the distance sensor? I'd prefer not to use the GND as not using the GND automatically turns off the distance sensor when it's not used(saving half the power in deep sleep). So no need to use a PNP transistor setup then (I can't use the digital output as VCC as the arduino wemos only supplies 3V on it's digital outputs)

I'm not sure if it has any influence but the TRIG output is on a 10k Pull-up pin.

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    \$\begingroup\$ You need to diagram EXACTLY what you were doing, including scope grounds. \$\endgroup\$ – Scott Seidman Apr 24 '16 at 11:41
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I strongly suggest that you use the components as they were designed to be used. If that means to add the PNP transistor to shut off power to a part of the circuit then do it. The transistor and the possible bias resistors needed are not expensive.

Establish your design techniques to follow data sheets, use conventional practices and try not to cheap out at every turn. Do that and you will have success in your endeavors and gain the respect of your peers as one who can make reliable working circuits.

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