Museum Hours:

Saturday 10-6

Sunday 12-5


Mechanical Television

LED Replacement for the Crater Lamp

Our Western Empire State receiver uses a crater lamp. These lamps are impossible to find, so every time we demonstrate the set the life of the lamp is reduced. At some point it will no longer work, so we have come up with a LED replacement.

The crater lamp

The problem is that we need a small point source - about 10 mils. Standard LEDs are that small if you remove the plastic diffuser, but they aren't bright enough. Super bright LEDs are bright enough, but they are made up of a number of standard LEDs, so their emitting surface area is too large - in the range of 1/8 inch for 1 watt models.

Karen Orton, a member of the NBTVA in the U.K., suggested that we use a door peephole lens in front of the LED to allow it to focus it as a small spot on the screen. We experimented with this approach, and it works nicely.

1 watt red LED

The LED glued to a plastic tube



The door peephole lens assembly mounted behind the lens disk

The lens disk, motor, and LED assembly, with the LED in place

The spot projected on a paper screen. The ringing is much less visible than the picture shows - the spot is saturating the camera. The spot is about 1/8 inch in diameter.

The raster on a paper screen

The raster on the ground glass screen in the Empire State

The next step is to build a driver for the LED. We will use a high power op amp, copying a circuit that we used in our color mirror screw project. Here is the schematic.

Today we experimented with using a 160 degree door viewer. It produced a spot somewhat larger than the 200 degree viewer, so we have decided that the 200 degree door viewer is the best choice.

The LED we are using is a red-orange ultrabright one with a light output of 21,000 mcd.

We then used a gray scale pattern generator and looked at the image. We noticed that there was significant time displacemnt of the 2nd and 3rd fields (this is a triple interlace 45 line disk)

We spent hours looking for the problem in the LED driver, then finally realized that it was a mechanical issue. The mounting arrangement for the LED assembly consisted of a 1/4 inch bolt about 6 inches long

This allowed the assembly to vibrate, moving the LED in and out relative to the disk. The vibration became synchronized with the disk rotation and resulted in the displacement of the fields. We built a more solid frame to hold the LED assembly

This solved the problem

We removed the glass cover on one of our LEDs and it appears to reduce the ringing (see photo above). We have ordered some amber LEDs of the same type to see if the color is closer to that of the crater lamp.

The color using the crater lamp. Notice there is displacement of the fields. The crater lamp was mounted on the same 1/4 inch 6 inch long post.

Images using the red-orange LED

Images using the amber LED. The spot is smaller using this LED, resulting in better resolution. It is closer in color than the red-orange to the crater lamp