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I would like to use Arduino to control locks around a building which is 5 storeys high and on every floor there is 2 door.

I'm concerned whether the wires is able to travel all the way from the ground floor to the 5th floor to provide enough power to the relay located at the door.

The lock will be a 12V magnetic lock and it will be unlocked thru a SSR relay which will be controlled by the arduino

  • Even without the problem of long wires, if you are using a typical Arduino, like UNO, you will have to add something in between the pin and the lock (btw, adding a reference to the specific lock would help understanding better the question). – Igor Stoppa Apr 21 '16 at 11:06
  • Pulling wires across a whole building seems a bit of a hassle. Have you considered using a radio connection? There are many radios available, over which you could send signed messages. – Igor Stoppa Apr 21 '16 at 11:07
  • @IgorStoppa What would be the something be? – Exceptions Apr 21 '16 at 11:08
  • Something like this relay shield: seeedstudio.com/wiki/Relay_Shield_V2.0, but maybe your lock already has something similar built in, therefore I was asking you to be more specific about what type of lock you want to drive. Even if you have a relay, that might still require an additional transistor, depending on the specs of your relay. – Igor Stoppa Apr 21 '16 at 11:09
  • @IgorStoppa I'm afraid that the walls of the building will affect the radio signal. The lock will be unlocked by touching 2 wires together which I will be using a relay to do that – Exceptions Apr 21 '16 at 11:12
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Yes, losses in wires are proportional to the length, so an excessively long wire may not be able to deliver enough power. Many factors come into play here:

  • higher current consumed by the relay increases the losses
  • smaller wire diameter increses the losses
  • the voltage required by your relay defines how much you can afford to lose

For example, a pair of AWG23 wires 30m long will have a total resistance of 4 Ohm. If your relay consumes 100 mA, you will lose 0.4V in the wires. If Arduino delivers 5V, and the relay requires a mininum of 4.5V to work, you're just good enough: you're delivering 4.6V to the relay after the losses.

  • Possible to add a amplifier between each level? – Exceptions Apr 21 '16 at 11:14
  • Technically, yes. Do you have a power supply on each level? – Dmitry Grigoryev Apr 21 '16 at 11:15
  • Yes that would be possible. What kind of amplifier should I be looking for? – Exceptions Apr 21 '16 at 11:16
  • The simplest one, something like this. But like I said, maybe you will get away without one, if the wires you have are thick enough and the relays don't consume much current. – Dmitry Grigoryev Apr 21 '16 at 11:19
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You are addressing a device control problem that auto, home and building automation has concerned itself with for years.

As such there are many solutions. Most require a transmitter and one or more receiver(s). C-BUS is one such solution. Another is CAN-BUS.

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Use wireless nodes which are self-powered. It is easier (no need for lot of cables) and also solves this issue completely.

You can use ZigBee, which is mesh network. Every node re-transmit signal for nodes which are far. Because you have one on every floor, you would not have issues with reception.

EDIT: If you want to use wires, I would still keep self-powered nodes and you can communicate with them using i2c or similar. You would need only 3 wires (SDA, SCL and GND) and they could be very thin, because they transfer only signals (<1mA range)). These 3 wires would connect all nodes. All nodes should have distinct address. I think this could be very cheap because you can create nodes with arduino micro which can be bought for ~1.5$ each.

  • Wireless would need a lot of additional work to maintain security – ps95 Apr 22 '16 at 11:42
  • ZigBee already have security mechanism (128-bit key) implemented. – Darko Apr 22 '16 at 12:38
  • He mentions Arduino + basic lines. This will drive costs through the roof compare to that – ps95 Apr 22 '16 at 12:52
  • I have added second (wired) option which is very cheap. It needs only 3 thin wire for whole building because all nodes would have distinct addresses. If you take into account number of thick cables he needs this can be even cheaper. – Darko Apr 22 '16 at 13:30

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