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If your using an external server, you might be better of just using the hosted web app available here: http://rayshobby.net/apps/sprinklers
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Absolutely, you would need to forward the HTTP port on the OpenSprinkler so that it is accessible by the external server.
For example, if your OpenSprinkler is on your home LAN at IP 192.168.1.30 and using port 80. Then you would goto your router’s configuration and forward port 80 for 192.168.1.30. After that you would use your home’s WAN IP for the OpenSprinkler on the external server. The WAN IP can be found by visiting http://wimi.com inside your home.
A few recommendations for this setup:
1) Ensure your OpenSprinkler has a static IP using the menu settings on the device OR by setting a static DHCP reservation through your router against the MAC address
2) You should use a secure password on your OpenSprinkler since it is now accessible outside of your home
3) You should double check the ‘Ignore Password’ option is unchecked
4) If it is a shared server, ensure permissions are properly set to prevent unwanted viewing of the config.php.
If you need any assistance feel free to ask, hope this helps!