A stupidly simple way to connect 3.3V logic to a Sainsmart relay board

Jovian Outpost

A lot of hobbyists are using Sainsmart relay boards with their projects because the boards cost less than buying the relays, or come really close. The boards include circuits to drive the relay coils so that a typical logic output can be connected directly to the board. Each driver circuit uses an optocoupler so that the driver and the coils only share a ground potential. This is a great scheme to allow the driver to work at a different voltage than the relay coils, but the board was intended for use with 5 volt supplies for both. The drivers start to work reliably from around 3.8V up to 5.5V or so. The result doesn’t work well with a 3.3V supply for the drivers operated by 3.3V logic; some relays will function, and others won’t. The solution is to give the drivers a 5V supply, but then the floating driver inputs…

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