Click to Enlarge Red Led Dimmer

Following on from an article which had an illuminated finder, here's some more ideas on having an illuminated finder and dimmer.
 
The diagram below shows my set-up, which, as most of your DIY people will tell you is over powered. The LED’s are 1.7 V so a total of 7.3 V has to be dropped over the 100 k pot and the 5.6 k bridging resistor. My logic behind this is the components on my build were what I had on hand and as such I used them.

This circuit could be simplified and optimised by using a two 1.5 V batteries, a 1k Pot and the 5.6 k bridging resistor. The resistance range is then 57 to 0 Ohms. This would give much better range of glow than mine does. 21 Ohms on the Pot is a balanced circuit so be careful not to overdrive the LED’s for too long. If in doubt put a 15 Ohm resistor between the pot output and the wire to the LED’s this will limit the V across the LED’s to about 2 V.

Total current draw is about 60 mA using the high output LED’s, so best to use it momentarily as needed. I could have put them in series to drop the current but then there is no redundancy, although I don’t think I have ever seen a LED burn out of its own accord.

Click to Enlarge Circuit Diagram

The basic design is as shown below using a film canister as the holder. The 9 V battery is held to the canister with an elastic band and the whole assembly strapped to one of the fork arms with a plastic tie. The LED’s for the setting circles are held in place sandwiched between two Velcro pads. Prior to the Velcro I used a small piece of black card as a cover and some cello tape.

The finder shown in my setup is currently just a trial unit, a cheap 6 x 30, which I did not mind experimenting on. I simply drilled a hole the size of the LED head in the plastic just in front of the cross hairs and pushed the LED in. I painted one side of the LED black so that it could not be seen in the filed of view. This will not be necessary on my final guider eyepiece as I will not be poking it so far in.

In the photo it is shown uncovered but in practice it is hidden under a piece of Velcro strap which I took off my Laptop power supply cable. I could have glued a bezel on to make it prettier but this was only a trial. I am now modifying an eyepiece in the same manner. I have purchased an etched plain lens for this purpose, however the diameter that arrived is 0.2 mm too big so I am having to grind it down, hence no photo of the final item.

I hope someone finds this useful.

Click to Enlarge Dimmer switch		Click to Enlarge Cheap Piggyback Mount