In automation projects it is often needed to drive multiple outputs. Combine this with the reduced number of pins of a microcontroller, such as ESP8266, and you got a problem. In this post we'll explore the methods of converting the parallel inputs of a relay module to some kind of serial protocol, which allows connecting even more relays, without the need of additional control pins. I will use for exemplification an 8-channel relay module, however the methods I will show will allow you to connect more than 8 relays to the same bus.
To achieve this purpose, I have to use some kind of bus expander IC. There are a few available options here. However, as we will see, both communication protocol and output port capability are different. And even the common relay modules use a rather unusual method of turning on the relay driver transistor. I already discussed the difference between current sink and current source in the previous post. Let's use that knowledge.
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8 Relay module with PCF8574 bus expander |