Polar scope illuminator


One thing the original HEQ5 didn't have was a polar scope illuminator. Fortunately Rajiva has built into his MCU update board the capability to add one. There is no on/off or brightness control but these are minor issues if you choose your components carefully. Here's what I did.

Components and supplies
  1. 2 9-volt battery pigtails
  2. 5100 ohm resistor
  3. 12.5mcd 2.0v 20mA rectangular red LED
  4. Heat shrink
  5. Hot glue gun

I wanted to have the LED permanently mounted, but still be able to take the MCU update board out if I ever had to work on it. This is the reason for the 9v battery pigtails. They form a nice connector; a bit large, but there is room in the HEQ5 for them. One of the pigtails will have the LED attached and the other will be soldered into the MCU update board. You will have to add a resistor to the MCU update board in position R2. The first pigtail will be soldered into the MCU update board at position D3 (both highlighted below). Then you want to mount the LED somewhere where it will illuminate the polar scope, but not get in the way.

The MCU Update board
    The current limiting resistor for your LED goes in position R2 (highlighted in pink). For my LED I selected a 5100 ohm 1/4W carbon resistor. This will knock the brightness of the LED down to a usable level.

    One of the 9v battery pigtails is connected to the position D3 next to the 15-pin ribbon cable. Take care you don't un-solder any of the connector leads, it's pretty tight.


The LED pigtail
    Slide suitable heat-shrink over the first 9v pigtail and then solder the LED on. Now connect everything and test your work. If you have the polarity wrong on the LED it won't illuminate. Once you have light shrink-down the heat-shrink to cover the connections and enough of the LED for support.


Positioning the LED
    I chose to mount my LED between the motors with the LED just overhanging, and resting inside the RA shaft. I used hot glue to mount the LED and pigtail to the 'post' between the motors. With the fairly weak LED that I used this setup works fine, if you are using a bright dome style LED then you'll have to use more resistance. You only want about 1.5-2mA of current through the LED.