I've been looking for an illuminated reticle eyepiece for quite some time, to assist in drift aligning my scope for polar alignment. The cost has always put me off, however, since they are generally more than $150 to purchase. However, when I saw a Meade 12mm on eBay from Telescope Warehouse in :as Vegas for about $80 Canadian, I couldn't resist - the only problem was that it was meant to plug into an LX200 control panel and thus simply had a cable with a phone jack on the end. However (as a recent ad on TV says) I am a handyman! How hard could it be? Pretty easy actually, so for those who might be interested, here's what to do.
First of all, the Light Emitting Diode (LED) in the eyepiece is pretty generic but a note in the box for the eyepiece noted that voltages above 1.5V would burn it out. What this really means is you can't just hook it up to any old battery because the current would burn the LED out. So, you need to put a resister (R1) in series with the LED (D1) to reduce the voltage to something that the LED can handle. With a resister I could use a 9v battery to keep things compact. A On/Off switch is of course required.
Of course, you want to be able to adjust the brightness of the LED as well, so you need a variable resistance (potentiometer, or pot - R2) in the circuit so you can brighten and dim the LED. It's a simple enough circuit, as can be seen in the schematic below. I chose a 270 ohm resister. An LED can handle between 15 mA and 30mA of current and will drop the voltage 2.6V so if you have a 9v battery, that means (9v-2.6v)/0.015A=426.67ohms (good old Ohm's Law V=IR). Since the pot added some resistance and I wanted as bright a light as possible, I chose a 270 ohm resister.
To construct the circuit, cut the end off the LED wire to the eyepiece (or find a female jack, which I didn't bother with - although it would be useful to get one that allowed you to dispense with the on/off switch), cut the appropriate holes in a small project case and solder all the components together. The white/black wire to the LED is positive. My project came out looking like below. Now I have a reticle eyepiece for roughly half of what it is to buy with a controller, and now so can you!
