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I am working on a project for a watering controller where I would need to turn on or off an solenoid valve (electric valve), my first thought would be to use a relay, but I know relays are good for things like 110V/220V AC, but I feel like it would be too overkill for 9V/12V DC. Would a transistor work as a switch for 9 to 12 volts? Or is a relay still a better option?

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  • \$\begingroup\$ May be related, if not duplicate. This. \$\endgroup\$
    – Unknown123
    Apr 6 '19 at 19:39
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Relays are still used and with good reason. In your case:

  • The relay separates your logic circuit from your high-current and higher voltage pump circuit.
  • Any faults on your pump circuit will not affect your logic circuit.
  • A faulty relay can often be a plug-in replacement.

A transistor is cheaper but requires that the common line must be shared between both circuits.

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    \$\begingroup\$ And the OP will still need a transistor to turn on the relay as the I/O pin almost certainly can't handle the relay current required. So the transistor is still in the picture. Just not the same picture. \$\endgroup\$
    – jonk
    Apr 6 '19 at 19:23

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