Friday, June 22, 2007

Working up top

tomsk lid
Originally uploaded by Phil_Parker.
The reason the inside of the wheelhouse has been detailed is because the roof is off. And the reason the roof is off is because I needed to detail it.

Now this wouldn’t be necessary if I hadn’t planned on putting working lights up there. In a boat this size it seems a shame not to do this though. I like sailing Pigeon Pie at dusk with her lights on and so I want the same option for Tomsk. Besides if the boat sinks again, at least I stand a chance of seeing it in the water !

The navigation lights are brass and the mast light plastic. I think they are both from Robbie but have thrown the packaging away. If I’m wrong it doesn’t matter too much as there are plenty of people who do these things.

Putting the wires up the mast took some serious fiddling – they are very thin and prefer to roll themselves up in the tube rather than be pushed through. I reckon that little job took half an hour and nearly resulted in the roof being flung across the room !

The “grain of wheat” bulbs in the lights are 3V, and I didn’t spot this until I had installed everything. Since the battery gives 8V I’ll need a dropper resistor to stop them frying. My physics was never good enough for this so I’ll use the same ones I use for dropping 12v down to power LED’s and hope that there is enough juice to light the bulbs.

The radar done is scratchbuilt and on the small side. I used the largest plastic tube from the local model shop and it’s still only ¾’s the size it should be. I’ll have to keep an eye out for a commercial replacement. It’s not worth buying big enough pipe as I only want about 10mm and will need to buy 6 feet !

1 comment:

tomdg said...

Hi there.

You've probably already done it and proved me wrong, but my guess is that using the same resistors you use for LEDs to reduce the voltage for a grain-of-wheat light bulb will probably make the lights rather dim. From memory LEDs use less current - 20mA from memory, compared to 65 for the bulb (according to this google link - I've never used one myself).

From my attempts to do the maths, I'd reckon something like 76 ohms should work for the light - compared to 500 or more for the LED (I use 1K). It depends on the resistance of your bulb - do you have a voltmeter? If so, 5/3 of the resistence of the bulb ...

You might need a chunkier resistor than normal too - if my maths is right it will be taking 0.3w, and I think the little resistors tend to be rated at 1/8 or something. Although what you'd have to do to actually burn one out, I have no idea - I've never managed it.

Alternatively, if your model conveniently has three lights on it, then wiring them in series would work fairly well; or you could build a little circuit with two resistors and a transistor ...