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I'm trying to remote control the doors in my stand-alone garage over two wires. There's three operate switches in the house, and three SPDT position switches on the doors. I think I want to end up with a relay board at each side to reflect the state on the other, and some kind of multiplexer in the middle. Off-the-shelf is somewhat preferable to having an Arduino on each side and having to program them.

There's a chance I can abandon the phone in the garage to pick up a second pair, so each direction could have its own.'

I think it's possible an answer would be, "use a 74LS" chip on each end, and connect the output pins a, b, c to the relay controller, the inputs to pins x, y and z and connect the twisted pair to the drivers on pins P and Q.

That would be perfect, having no software, and I'd be able to build that on a proto board, or know what to look for COTS.

The question is what to put in the ? box enter image description here

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    \$\begingroup\$ This is a Q&A site, you need a question to be answered \$\endgroup\$ – Voltage Spike Jul 18 '19 at 16:08
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    \$\begingroup\$ Asking for something "off-the-shelf" makes this question off-topic for this site. If you change your mind and want to design your own solution, an Arduino on each end is not a bad way to start. \$\endgroup\$ – Elliot Alderson Jul 18 '19 at 16:25
  • \$\begingroup\$ What's the question, exactly? Can you narrow it down to a particular obstacle you've encountered? \$\endgroup\$ – JYelton Jul 18 '19 at 16:29
  • \$\begingroup\$ Seems like a job for wireless remotes and a normal garage door system (which includes limit switches and safety interlocks - the latter required by code in most places.) Just a thought. \$\endgroup\$ – hacktastical Jul 18 '19 at 16:34
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    \$\begingroup\$ You need at least a block diagram to show the automation required. If you have only a single pair of wires (sounds like you have one pair from a two pair phone line) available then you really only have one signal wire and one ground for DC signaling. By far the easiest is to implement a half duplex serial link, and that is easier using a small MCU. You can do this by hardware (non-MCU) but it actually becomes much more complex. You could also use a 'tone' based solution which allows multiple signals on the wire simultaneously, but again this just becomes so more complex than an MCU solution. \$\endgroup\$ – Jack Creasey Jul 18 '19 at 17:13
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Have a look at DTFM (dual-tone, multi-frequency) chips. These are used in telephones. You'd need a transmitter and receiver at each end and some way of of mixing the signals so that they can be injected at both ends.

Table 1. DTMF tone pairs including the seldom used A, B, C and D tone pairs.

         1336 Hz  1633 Hz  1209 Hz  1477 Hz 
697 Hz      1        2        3        A 
770 Hz      4        5        6        B 
852 Hz      7        8        9        C 
941 Hz      *        0        #        D

Choose, say, row 1 (1, 2, 3 and A) for signals to the garage and row 2 (4, 5, 6 and B) for signals from the garage.

Links:

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It looks like a plausible solution is to use a CAN-bus module. such as the $5 each 1

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