Override One Lamp in a Four-Lamp Circuit Using Transistor?

Question

^{simulate this circuit – Schematic created using CircuitLab}

I have a circuit of 4 light bulbs, with single switch on the positive terminal that will turn all the 4 bulbs at once. EDIT: Light bulbs circuit is parallel design. source is 12V DC and all bulbs are running 12V DC.

I want to be able to still have the original switch to turn on all 4 bulbs, with a new switch that overrides bulb #3 to keep it turning on. Essentially, when bulb #3 is switched on, the original switch ignores bulb #3.

To solve this I go with installing a diode at the positive terminal of the bulb #3. When original switch is on, electricity will go thru that diode and then turns the bulb on; when new switch is turned on, electricity will go to bulb #3 thru new line but will not go to the original circuit because of the new diode.

A friend said this might be able to be done with a transistor but he couldn't figure out the solution. Is that true we can use transistor for this? it's for automotive brake lights.

Answer 1

Given your control signal is just another switch, your diode approach in the question should work fine.

If the control signals came from two different supplies you would need a second diode in series with the override switch. However as both are fed from the same supply, this is not required.

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An alternative solution would be an SPDT relay:

^{simulate this circuit – Schematic created using CircuitLab}

When you turn the relay on, it will turn on lamp 4 regardless of the state of the switch. When the relay is off the lamp goes back to being controlled by the switch.