Navigation Light Switch Panel with warning lights

[Blain Albin]

I am attempting to build a navigation light panel that will turn on a warning light or buzzer if one of the nav light burn out. The system is 12 VDC. I have a circuit diagram I found in the Transport Canada Electrical Standards book but can't seem to get my head around a certain part of it. I have included a picture of the diagram and circled the area that doesn't make sense to me. I wondered if someone could explain in words what happens here and the components that make it work. I just don't see how it would work the way it is displayed here.

If anyone has any better ideas i would love to hear from you.

http://picasaweb.google.com/lh/photo/CCzj5YDRwten7pzzeBjJxQ?authkey=Gv1sRgCMLVicjyrMjQfg&feat=directlink

[CDK]

I also don't understand the circled part and I spent the last 60 years learning, building and designing electrical/electronic circuits.

The simplest way to check the filament of a light bulb is use a LED and a 1 K-ohm resistor connected in series to the contacts of the light switch. If the contacts of the master switch are closed and those of the light switch are open, the LED will emit light if the bulb is OK.
The same principle is used in all cars to show that several circuits are OK.

To get a signal when the lamp is powered is a bit more difficult. You must sense the current in the circuit. For that you need a few more parts, like a transistor and a wire wound resistor of 0,5 - 1,5 ohm, depending on the type of lamp you will be using.

[Blain Albin]

CDK
I know, that diagram just doesn't make sense. I thought maybe that circled part was some kind of relay but I gave up on that after playing with a couple relays all afternoon.

The current sensing circuit intrigues me. Would you be able to provide a circuit diagram for this. The navigation light is 25W so that means about 2A.

Thanks for the response.

[CDK]

This circuit measures current in the negative lead and draws no current itself when the lamp is off. For the transistor, any small NPN type can be used.

The same can be done in the positive lead if you mirror the circuit and use a PNP transistor.