Here's one my son brought home from school and so far he's got me stumped:
Create a circuit in which initially, four lamps are ON while the fifth lamp is OFF. Also, you must be able to unscrew one of the four burning lamps in order to turn on the fifth lamp. You are not allowed to change any of the wiring to get this to work.
Darn College-boy!!
What kind of parts can I use?
A battery, the lamps, (& holders) & any # of wires you need.
That's it, no switches, etc. A simple circuit connected in any combination of series or parallel.
Thyristor, zenor diode and SCR to control a relay? Multiples of each could do it arranged correctly, oh a few resistors too. Also what is the voltage and current?
You posted while I was pondering...
[This message has been edited by e57 (edited 07-11-2006).]
Hmmm, not legal to install? But operational?
[This message has been edited by e57 (edited 07-11-2006).]
Are all of the light bulbs the same wattage?
If the 5th one was a small fraction of the others and in parallel with the one you unscrew it would work.
It sounds like one of those "all american five" radio filiment deals.
Voltage and lamp wattage?
You guys got me again in the two minute delay of my slow typing...
[This message has been edited by e57 (edited 07-11-2006).]
Thats what I was thinking too.
Guys,
This is a college physics lab question.
No NEC code or inspectors to worry about.
Wattage of lamps was not given. The info in my original post is all the info the prof gave them.
I'll send a little sketch showing my solution to the moderators. IFF the 4 outside lamps are equal, then the center lamp will not light, that is until one of the outside lamps is unscrewed.
Radar
![[Linked Image]](https://www.electrical-contractor.net/PC/LightBridge.jpg)
Edited to add image.[This message has been edited by pauluk (edited 07-13-2006).]
"Light" is certainly going to be a figure of speech. One of these will probably burn out and 2 will be very dim if they were sized for normal brightness when it was a balanced bridge.
http://members.aol.com/gfretwell/lite.jpg
The above suggestion is clearly not intended to be code compliant or to pass any sort of inspection. Whether a light is lit is certainly a relative term. As I read the original posters remarks, this is a physics (basic circuit continuity theory) question of series / parallel combinations and no more than that. This is something one could wire up using some flashlight lamps and a couple of small batteries on your desk. I make no claim that this is any kind of an operational lighting circuit.
That said, the voltage across the center lamp will be zero so long as the other 4 lamps are both equal and operating, and the 4 lamps will glow equally bright. Take any one of the 4 out of the circuit, and the center lamp will light. Also - the operating lamps will not all be equally bright.
You can see this for yourself using Kirchoff's current laws if you pick a resistance value for the lamps and a voltage for the circuit and apply Ohm's law. The exact values you pick don't mater much, just pick reasonable values to make the math easier.
Radar
You know... the next time I have a customer who's a PITA, I just might wire his lights this way! (EVIL!)
For another example of how differing filament resistances in bulbs can be made to work in "unusual" ways in a bridge arrangement, just look at how the front side marker lights are wired on General Motors cars.
Instead of just connecting betweeen the parking/tail light circuit and ground as with most Ford and Chrysler vehicles, they connect the marker between the parking light and the appropriate turn signal feed.
When a turn is signaled, the 5W or less side marker bulb is in series with the parallel combination of all the parking/tail lights, so it flashes with the turn signal while the other running lights don't get enough power to light.
With parking/running lights turned on, the side marker is in series with the front turn signal lamp. Again, the small side marker lamp has a much higher filament resistance compared to the much larger turn signal lamp, so the marker lights. (*)
When both running lights and turn signal are turned on, the marker lights via the front turn signal lamp during the off periods of the flasher, then goes out during the on periods as both sides are at 12V, hence the way that the marker flashes alternately with the main signals when the running lights are on.
(*) For a demo of this, try removing a front parking/turn signal bulb and then switching on your parking/tail lights. The green warning light on the dash is connected in parallel with the front turn signal, so with the main bulb removed the side marker is then in series with just the dashboard warning lamp, which is of similar rating to the side marker. You should see the arrow on the instrument panel glow dimly, and the side marker itself will be dimmer than usual too. (If you have one of those cars with independent amber turn signals on the rear, you might need to remove that bulb too, depending upon how it's wired.)
[This message has been edited by pauluk (edited 07-16-2006).]