NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Paul Dolkas
Date: 2013 Dec 3, 22:29 -0800
I second the observation about coiled wire filaments, and the small useful range of rotation on the pot. Alas, strait-wire filament bulbs have so far eluded me, so I just put up with using the coiled wire ones. Any idea of a source?
Paul Dolkas
From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Geoffrey Kolbe
Sent: Monday, December 02, 2013 11:20 PM
To: paul@dolkas.net
Subject: [NavList] Re: Bubble sextant parts and calibration
On 03/12/2013 05:14, Paul Dolkas wrote:
As for the bulb that illuminates the bubble cell, this should be incandescent. LEDs can’t be dimmed by using a potentiometer, so unless you want to add a bunch of circuitry, just stick with a 1.5 V bulb. You can get them at RadioShack, if nowhere else.
Well, you can use an LED, which can be dimmed. The way you do it is to built a little multivibrator and connect the LED on one side of it, preferably powered by a driver transistor. So, three transistors in all and easily fitted - with a battery - into the existing battery space. The potentiometer is used to vary the mark-space ratio of the square wave the multivibrator generates and this is how the LED is 'dimmed'. This is not just theory, I have built one to provide the background illumination to a theodolite's cross hairs for night observations on stars, and it works very well indeed. It is a good idea to use a red LED, as mentioned, which is what I did.
It should also be mentioned (again) that the best incandescent lightbulbs to use to ensure that the dimming is a smooth function of potentiometer rotation, is the bulbs with 'straight' filaments rather than the usual 'coiled' filaments. Coiled filaments retain the heat in a way which a straight filament does not and this leads to the fact that the useful range of rotation on the potentiometer is much smaller for coiled filament bulbs.
Geoffrey Kolbe
--
