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I am planning to for an Arduino to read status of a switch, and accordingly switch on and off AC appliance, for a demo purpose. I want to add a feature to it of easy recovery to manual switch control.

schematic

simulate this circuit – Schematic created using CircuitLab

First circuit to the left is when Arduino reads status of switch, and controls a bulb by digital pin P2 which drives SSR/relay. Circuit on the right is normal switch board controlled light bulb.

So basically what I am trying is: when I remove Arduino board, my previous control (light bulb controlled only through switch) should be regained. Really stuck on this, help will be highly appreciated :)

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I solved it by using a SPDT relay instead of SSR. The N/C contact is attached to the manual switch control. The uC then puts the relay in on state to turn the output off. That way even if uC failed, I could still operate it manually. There are corner cases, of course.

BTW, you might need a DPST or DPDT switch for mains control. One pole directly to relay, the other to uC.

It's kind of relay logic, actually. Think of it as staircase wiring (e.g. http://electricaltechnology.org/2012/11/stair-case-wiring-circuit-diagram-or.html).

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  • \$\begingroup\$ @Vasu- Thanks for quick response, but I am having space constraints on my PCB, and want to control multiple such appliances. Can't really add so many relays. any way out for that? \$\endgroup\$ – Pudi Sep 30 '14 at 16:20
  • \$\begingroup\$ @Pudi: I usually use sugar-cube relays, off board. Having said that, the challenge with SSRs is they are unipole, unithrow. The control logic tends to be complex, if not impossible. One alternative is to try working out a diode-resistor logic or transistor-resistor logic that has two parts: One is controlled by Arduino & other by switch directly. The uC overrides (e.g. inhibits) the switch control, but in the absence of uC override, switch drives SSR directly. Similar to high impedance state controls found in CD4503. Let me know if you need an example schematic. \$\endgroup\$ – Vasu Oct 1 '14 at 4:54
  • \$\begingroup\$ -Example schematic would be great !:) \$\endgroup\$ – Pudi Oct 1 '14 at 8:48
  • \$\begingroup\$ @Pudi Were you able to find a solution for this? \$\endgroup\$ – SE_User Feb 2 '17 at 12:25
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Use a two way switch and a two pole relay to act as an two way switch. Normally closed relay with switch in series or switch in parallel but must turned off to give control to the automation.

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