user3382: I want to make two or more boards to blink an LED at the same time (within 1 ms difference) The boards cannot be connected with wires to each other and will be located in different rooms. How can I do this? What components do I need?

user7859: Have both sync to a master clock using GSM shields. These shields are getting very cheap (as low as $14 on ebay).
The shields receive the time wirelessly from the cellular network and this time base is extremely accurate. Here is some example code to read the current time from the shield...
http://forum.arduino.cc/index.php?topic=163155.msg2102141#msg2102141
Note that you will need a SIM card to get the time, but you should not need an active cellular account. 

user8687: You can also use real time clock (RTC) modules to syncronize arduinos. These are cheaper than other solutions like GPS or WiFi.

3382: <p>I want to make two or more boards to blink an LED at the same time (within 1 ms difference) The boards cannot be connected with wires to each other and will be located in different rooms. How can I do this? What components do I need?</p>
 7859: <p>Have both sync to a master clock using GSM shields. These shields are getting very cheap (<a href="http://www.ebay.com/itm/like/181722612700?lpid=82&amp;chn=ps" rel="nofollow">as low as $14 on ebay</a>).</p>

<p>The shields receive the time wirelessly from the cellular network and this time base is extremely accurate. Here is some example code to read the current time from the shield...</p>

<p><a href="http://forum.arduino.cc/index.php?topic=163155.msg2102141#msg2102141" rel="nofollow">http://forum.arduino.cc/index.php?topic=163155.msg2102141#msg2102141</a></p>

<p>Note that you will need a SIM card to get the time, but you should not need an active cellular account. </p>
 8687: <p>You can also use real time clock (RTC) modules to syncronize arduinos. These are cheaper than other solutions like GPS or WiFi.</p>
