This doesn't really differ that much from option 2, but it's a litte more complicated, but cheaper. You can easily use a transistor or mosfet to switch another (higher) current. The only problem, however, is that you will need a 5V on the input of the board. This little board will change output 5V, when given 3.3V at the signal in. For connecting a low current data-connection, it can actually work since the 3.3V might be seen as a logic HIGH. This might work in some scenario's, but not this scenario. You can fix this by adding a capacitor to 'buffer' the power, so it can be releases at the very moment the coil needs to be energized, and it won't take the power away from the rest of the board. Often, the power supply isn't 'responsive enough'. The coil may 'suddenly' take a lot of current, you may see that the voltage on your board drops, or that it resets. Your source should be able to supply enough current for the relay. And that your input voltage shouldn't be higher/lower than what the coil is rated for. Mind that some relay breakouts already have circuitry for this. (TODO add diagram, I'm currently unable to, give me 12h) You will have to make the control signal of your board and switch the 'input power' over the coil of your relay.

These sources can often provide the power required to energize the coil of your relay. Usually, you're ging to power the 3.3V device from something like 5V USB or 9V plug. Check the specs to find one that fits your needs. Your best bet would be a 'solid state relay', since these do not require a coil to be energized and may work on low voltage/current.

You can choose a relay that can work with low voltage/low current. It is somewhat accepted to drive a LED directly from an output pin (if using a high enough current limiting resistor), but it may lead to problems, since the total current over multiple pins can also be limited by the hardware.įor communications, it may sometimes work to use 3.3V since it could be registered as a logical high, but you better not rely on it, for any serious applications. Output signals are low current control signals. Output pins shouldn't be used to drive anything. Mind that this answer is for driving relays, if you're going to switch 20VDC at ~3A, a MOSFET will be a better/cheaper solution, as you don't need the relay. This edited part should be an improved version of the original one. The original/accepted answer can be found below. There are an awfull lot of ways to connect a 5V device to a 3.3V output pin.

Arduino relay