How do I connect 20 outputs and 20 inputs to an Arduino board?

Question

I am a mechanical engineer starting my journey with Arduino, so please excuse me if I put some very generalized message.

• I need to sequentially operate 20 output relays, and then verify if the correct relay has been triggered. If the correct relay was triggered, the next relay will be triggered after a delay of 1-2 seconds.
• I plan to do this with an Arduino Uno so that I can generate error messages if any output has not been triggered.

However, the Uno doesn't have sufficient inputs and outputs to do this. So what are my options?

I want to ask you the below:-

1. If there are I/O expansion boards available for the Uno? If yes, sharing the details will be helpful;
2. A friend recommended to use a multiplexer. Can I use a multiplexer to convert a 4 pin output from the Arduino to 16 pin output, and probably use the saved pins to monitor the inputs? If yes, how?
3. Is there something available like a reverse multiplexer wherein the chip can convert the 16 output signals to 4 pin inputs to the Arduino?

Please let me know the best solution wrt the below parameters - reliability of operation, cost, compactness.

Thanks for your time and support.

Answer 1

Look for port expansion boards for the arduino, there are a lot of them many under a dollar. Typically they are in 8 or 16 channel designs. They probably will have PCF8574 or PCF8575 chips. These are low voltage (typically 5V) for the Arduino. You will have to use an interface if it is connected to another voltage. Each of these modules the channels can be individually programmed as an input or output. You can turn on or any of the outputs in any order and pattern you want. These devices operate with the I2C interface so you need only two lines from the Arduino to drive up to 8 of these modules. The pins should be labeled A4 (SDA), A5 (SCL) depending on the model of Arduino you have. Good luck!

Answer 2

Instead of using expansion boards, you could use an Arduino Mega or an Arduino Due instead. These have more than enough GPIO pins for 20 inputs and 20 outputs. The price isn't much higher compared to the extra expansion board and the extra wiring required.