High Side switching and common ground

Hello :wink:

I'm quite confused after a few search, so I'm trying to explain my little problem here.

Context : A circuit in two parts and a cable in between :

  • a micro-controller, for instance a wemos d1(other devices attached to MCU, that's why the 2 parts are separated)
  • a remote PIR sensor, powered by 3v
  • a TIP120 to remotely control a led strip with 12v pwm

Screenshot below for a better understanding :


Currently the cable needed is 5 cores. I wonder if I could use a 4 cores instead.
(I've bought 4 cores thinking I would be sufficient :()

By replacing the TIP120 by a P-Channel MOSFET, high side switching, can I share a common ground in the cable ?

I've seen such high side switching designs, but I'm really not sure about the common ground between 12v led circuit and MCU power)

Screenshot below is a just to understand my idea, it is not correct :


Thanks a lot for reading

That works only if the battery voltage (12V) is not higher than Vcc. Else the transistor never goes off and you risk to kill the output pin connected to the transistor base.

You have to spend an npn transistor as a level shifter from Vcc to 12V or the 4th pin on the connector.

Thanks for your answer DrDiettrich

The +12v source is a plug adapter. The MCU is powered by a USB plug adapter.
Different source, and VCC is not the same (MCU VCC +5v is obtained by a USB power adapter)
So I guess that VCC is not the same as you suggest :frowning:

To use a common ground, ff I understand correctly, I can use a NPN transistor to drive the P-mosfet, right ?

Is this similar to this kind of circuit below ?


MTP2P50E has a Vgs maximum of ± 20V.

Your MOSFET will be damaged at 82V.

That was just an example I found with a transistor driving a MOSFET

I'm quite open to components suggestions :slight_smile:

I'm driving a small led strip powered by 12v, max current will be under 1A

Then it's okay.

Based on the different signals, I don't see you reducing the wire count from 5 to 4 with the current circuit components.

You could reduce the wire count if you added a +12 to 3.3 volt regulator at the PIR sensor, eliminating the V3.3 volt wire.

A regulator is not suitable since I'm driving the Led Strip using PWM signals.

Seems to me that if I'm using a common ground, the signals in the cable would be :

  • ground (for both sensor and led)
  • +5v (sensor power)
  • sensor output
  • PWM 12v (led driving)

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