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I am planning to set up Home Automation System to my reading room using DTMF Technology. For that, I have searched different websites and found a post - DTMF Based Home Automation System Circuit.

That circuit seems to be simple and interesting. But, they mentioned few limitations.

1) It is limited to 16 tones only. Is it not possible to extend it by adding any other component?

2) It is mentioned that it has no security. Is it not possible to solve this problem by setting up a security password?

If anybody have an idea about it, let me know the solution. I would like to implement this same circuit to my room on next week.

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    \$\begingroup\$ No additional hardware is required (except for more relays). The features you want can simply be added to the firmware running on the MCU. \$\endgroup\$ – Dave Tweed Jun 14 '14 at 13:25
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DTMF is limited to 16 pairs of tones because it was developed for telephone keypads and the largest foreseeable matrix size I guess was 4 x 4 - this means 16 and there is no chance of extending this because the chipsets used are not going to be revamped.

No security - this is just a limitation in the foresight of the guy writing the article you linked - he envisaged 16 appliances that could be controlled remotely corresponding to the 16 keys. Why not use two key-presses to activate an appliance - this technique is only limited by the ability of the user to remember the codes AND why not have it password protected so, when you "connect" thru to the telephone decoder the microcontroller won't allow any activation or deactivation without a 4 digit code being entered.

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