user4843: I'm looking to connect an 8-relay board to an Arduino Uno. A board like this one. However, I don't really want to use up 8 digital outputs if I don't have to. Is there some intermediate circuitry I can either buy or build that would do this? 
For example, can I connect something to the serial so that sending '3' would result in binary 00000011 and thus turn on relay's 1 and 2?
Alternately, and probably more lazy, is there an extension board that would just give me a pile more digital pins?

user20245: A likely solution is to use the Arduino SPI interface and the Arduino SPI library to control a SPI relay board.  Here is a 6 year old thread talking about such a board. And another more recent relay thread where SPI is brought up.

4843: <p>I'm looking to connect an 8-relay board to an Arduino Uno. A board <a href="http://www.ebay.com/itm/221389425632" rel="nofollow">like this one</a>. However, I don't really want to use up 8 digital outputs if I don't have to. Is there some intermediate circuitry I can either buy or build that would do this? </p>

<p>For example, can I connect something to the serial so that sending '3' would result in binary 00000011 and thus turn on relay's 1 and 2?</p>

<p>Alternately, and probably more lazy, is there an extension board that would just give me a pile more digital pins?</p>
 20245: <p>A likely solution is to use the Arduino SPI interface and the Arduino SPI <a href="https://www.arduino.cc/en/Reference/SPI" rel="nofollow">library</a> to control a SPI relay board.  <a href="http://forum.arduino.cc/index.php?topic=8209.0" rel="nofollow">Here</a> is a 6 year old thread talking about such a board. And <a href="http://forum.arduino.cc/index.php?topic=293858.0" rel="nofollow">another</a> more recent relay thread where SPI is brought up.</p>
