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I'm trying to use a Raspberry Pi to switch on and off another circuit. This other circuit draws a LOT of current, so I'm using a power supply dedicated to that load.

I tried to do a simple op-amp comparator (1.65V coming from a resistor divider off the Pi into the V-, 0 or 3.3V coming from the GPIO pin on the Pi to the V+, and then 0 and 9V from external battery to the VS- and VS+ pins), with the output then connected to my load and to the external battery ground.

However, I then realized that op-amps can apparently only have up to 25mA passing through them when there's no feedback. (Just a figure I found, it might not be exact.) My load can take up to 1A or higher.

I was thinking of doing a CMOS switch, but as far as I can tell I would need to have some sort of negative voltage source as well as positive to be able to make the switch. I'm not sure how to do that with just batteries, though.

Is there any way to do the CMOS switch, or just have another switch mechanic in general?

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  • \$\begingroup\$ "as far as I can tell I would need to have some sort of negative voltage source" Where did you read this? \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 21 '17 at 2:26
  • \$\begingroup\$ Relays are very easy to use and many can handle the current you need. \$\endgroup\$ – Dave Jan 21 '17 at 2:26
  • \$\begingroup\$ @IgnacioVazquez-Abrams My not-very-good understanding of CMOS. I was under the impression that if one of the PMOS or NMOS has a positive voltage threshold, then the other one has a negative threshold. \$\endgroup\$ – Jashaszun Jan 21 '17 at 2:27
  • \$\begingroup\$ @Dave Thanks for the suggestion, I hadn't heard of those before! I'm looking at relays now and they seem very useful for this. \$\endgroup\$ – Jashaszun Jan 21 '17 at 2:28
  • \$\begingroup\$ The threshold is relative. If you have +9V and ground, then ground is -9V from +9V. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 21 '17 at 2:31

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