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I'm trying to design a custom PCB that is powered through USB (5V DC), has a touch sensor, powers a DC motor and can optionally have a Raspberry Pi Zero plugged into it through a GPIO header.

If the Raspberry Pi isn't plugged in, I'd like to forward the touch sensor signal so that it starts the DC motor directly.

enter image description here

If a Raspberry Pi is plugged in, I'd like the touch signal to first enter the Raspberry Pi. The Pi would decide whether the motor should be triggered or not using a GPIO output pin.

enter image description here

What would be the best way to solve such a problem with an electrical circuit? I've tried to figure this out for a while now and can't seem to find the right resources.

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  • 2
    \$\begingroup\$ You're confusing everyone here by drawing the "DC motor" part of the circuit upside-down. You should follow the conventions we all try to use to draw clear schematics, see: electronics.stackexchange.com/questions/28251/… \$\endgroup\$ – Bimpelrekkie Jan 13 at 10:53
  • \$\begingroup\$ @Bimpelrekkie thank you for the feedback. I'll fix this. \$\endgroup\$ – davidknezic Jan 13 at 11:30
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The way I see this, you need several signals to determine whether the motor should be on:

  • Whether a RPi is connected.
  • Whether a touch has been registered on the sensor.
  • Whether the RPi thinks the motor should be switched on.

The first can be achieved by repurposing a GND pin on the pinout and add a pull-up resistor to 5V (leave all the others tied to GND). This pin will represent the signal of whether an RPi is connected. If it's not connected, the pin is floating and pulled to 5V. If the RPi is connected, it'll be pulled low to GND via the RPi internal ground plane.

You already have the second and third signal (labeled TOUCH and MOTOR in your schematic). Remember to tie MOTOR to logic low so the pin doesn't float if no RPi is connected.

After that, you can feed the various signals into logic gates to determine whether the motor should switch on.

schematic

simulate this circuit – Schematic created using CircuitLab

In this case, the RPi MOTOR signal will always set the motor state if the RPi is present. If it's not present, the RPi MOTOR signal should be tied low and TOUCH should set the motor state.


An alternative solution could be to inline the MOTOR pin with the TOUCH pin, separated by a resistor.

schematic

simulate this circuit

When the RPi is disconnected, MOTOR will be left floating and will be completely driven by TOUCH. When the RPi is connected, the signal will be 'overridden' by MOTOR.

The exact resistor value will depend on output impedance of your touch sensor output and how much current it can sink/source. You may also need more signal buffering.

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