Relay with mechanical fallback

Question

In my circuit I have a relay that will turn on some light bulbs this is my circuit:

The switch over the 5V is in fact my Arduino. A HIGH will turn on the relay and a LOW will turn it off.

This is working fine, but because of security measures I need to add a mechanical switch to be able to turn on and off whenever needed. This is the circuit I'm using for this configuration:

This circuit also works fine, the lamp is turned on and off by changing either switches but the Arduino loses track of the status of the lamp. The only thing it knows is if the relay is on or off.

Is there a simple way to tell when the lamp is on?

Answer 1

If you add an optical sensor appropriate for the lamp (UV, IR, visible light?) and add a test line to your code, the Arduino would "know" if the light is on. Your second circuit could remain as presented, while the sensor would provide a status of the light.

If the light failed for an unknown reason, the code for your Arduino would not know the difference between that and a manually opened override switch.

Answer 2

I got the terminology wrong in my comment. My suggestion is not to use what is commonly called a “three way switch”, but instead a single pole triple throw switch. This is not available in the schematic editor, so I had to improvise. Imagine that the switch labeled “SP3T” below has three contacts on its left side:

^{simulate this circuit – Schematic created using CircuitLab}

• The top contact connects to nothing: this is the OFF position
• the middle contact connects the controlled circuit with the relay: this is the AUTO position where the Arduino is in charge
• the bottom contact closes the controlled circuit: this is the ON position