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.
Best Answer
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.
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.