# Switch that turns one lamp on while keeping the other one off - Mystery

We tried to connect outside light bulp (AC) to a switch where an another light bulb is already connected. We did a parallel connection. Roughly like in the schematic. (I'm not an engineer so the schematic can be wrong).

simulate this circuit – Schematic created using CircuitLab

If I understand it right it should work like this: When the inside light is ON the outside light should be also ON. Because both lights are connected to the same switch. Or If I understand it right the outside light should light all the time.

But the lights work like so: When outside light is ON the inside light is OFF and vice versa.

• Welcome to EE.SE! A correct schematic is crucial here. Also, what is your question? – winny Aug 31 '18 at 7:27
• If your schematic is correct (which I hope it isn't) then SW1 will turn the outside light ON when closed, and OFF when open. When the switch is OPEN both the outside and inside light are connected in series, so (if they are identical) they will see half the voltage, and so will be dim. When switch is closed, the outside lamp will be off (as it is shorted out) and the inside light will be brighter. – Puffafish Aug 31 '18 at 7:34
• Note that in your schematic, the outside bulb is connected in parallel to the switch, when what you want is connect it in parallel to the inside bulb. – jcaron Aug 31 '18 at 8:59
• But as Dampmaskin wrote, if you don’t know what you are doing, don’t! – jcaron Aug 31 '18 at 9:00

If the schematic is correctly drawn, the circuit works like this:

When SW1 is closed, all the current passes through Inside Lamp, and through the switch which can be considered 0 Ohm. Outside Lamp is shunted out, no current passes through it. (But it should still be considered live.)

When SW1 is open, the current has to go through both lamps. The lamps are in series. If the resistance of each lamp is the same (this resistance is temperature dependent), each lamp will have half the voltage it should. Probably enough to light them up, but not at full brightness.

If one lamp is emitting light and not the other, it's probably because of a temperature difference, or minute differences in the filaments during production. Both lamps are effectively passing current, but one of them is cool enough that you don't see the glow.

It is highly unlikely that this is how the lamps are supposed to be wired.

I suggest that you hire a professional for this job. Doing electrical wiring when you don't know exactly what you're doing is dangerous. It can lead to the house burning down. Or someone getting electrocuted. It's not somehing to be taken lightly.