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I am working on a home automation project using:

And basically I want to know how to supply power to (future) PCB (with all listed components); via

  • rPi 5V GPIO pin, or
  • rPi USB port, or
  • should I add an external power supply just for MCP23017 and relay boards?

So I did some thinking and calculations, but in the end I would still like to hear your opinion.

Regarding MCP23017: I will power it with 5V with level shifter as seen on the following picture: enter image description here

For this I'll be using 2 x BSS138 transistor and 4 x 1kOhm resistor.

Relays are off when reading HIGH and are turned on when reading LOW. Therefore, I will use pull up resistors with switches - here I will have 10 kOhm resistor + 100 kOhm internal resistor in MCP23017. So 5 V/110 kOhm = 0.045 mA/per switch is flowing when reading HIGH and 0.5 mA/per switch when reading LOW.

In the worst case, when all 20 switches are on I should be using 20 x 0.5 mA + 3 x 160 mA (for all relays ON as written in added link to sainsmart). This means 490 mA. Did I forget to calculate something? And in the final version if I add a few components and reserve for them additional 100 mA. This would mean approx 600 mA of current.

Now what would be the best solution for power source? Can rPi supply 600 mA and/or sink it? Would be best to use external power supply? Can I use rPi USB port for it - does that even make a difference compared to 5 V GPIO?

Thanks for answers!

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TheoryX
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2 Answers2

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You mention a "2.5 A power source" - provided it actually does supply 2.5A at the rated voltage (most don't) you should have adequate power for the components listed (although I don't know what you mean by "20 x physical wall switch"). See Raspberry Pi Power Limitations for further information.

By the way you don't need level shifters for I²C interfaces, provided you don't have devices with pull-up to 5V.

Most people who use MCP23017 with the Pi run them off 3.3V - but this may depend on what you are connecting to them. The relay boards do not need 5V to drive inputs.

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Milliways
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You should use N-channel MOSFETs for anything which consumes >5mA. IIRC my SS 5V relays use 5mA on the signal pins, and 30-50mA to VIN/+. Keep your load on the DRAIN side, wire SOURCE to GND, and use >=1k ohm in series to GATE to switch on.

The 2N7000 MOSFET for example can drive up to 60V/200mA, and 50 of these cost you at most a few euro. There is only a negligible voltage drop on these.

I often use the 2N4033 (because I have many, and they're in a metal case, so easily heatsinked) PNP transistors - good up to 80V/1A. For these I use a 4K7 ohm pull-up on the BASE, a 1k ohm resistor in series from GPIO pin to BASE , and keep load on the COLLECTOR, then wire EMITTER to GND.

You should consider giving your circuit its own power rail (do not forget common ground) at 5V and just driving your various gadgets using 2N7000 N-channel MOSFETs. The pi pins really are only meant to do this - in my opinion.

Edit: I read that some of you use a rpi 'official charger'. IKEA sells a 3-port USB charger that can supply 2.4A per port, and supply 3-4A total. It's ~7 euro.

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