This is an interior shot of a very famous non-trek prop which also used the rotating cam timers. It is indicative of the technique used for the trek displays, and is used here for reference purposes. This is a double bank, 8 switch timer made by the Cramer Co. They were mostly used to sequence industrial machines. A similar timer is used in many washing machines today to step the machine through the various wash cycles.
Each switch is a SPDT, which means “Single Pole Double Throw”, it feeds power into a common wire, and out 2 wires, each to one side of a “flip-flop” group of lights, then back to ground. Only one side gets power at a time, and if you move the switch lever back and forth, the lights will flash like a RR crossing. In this case, rollers on the switch lever ride on the cams as they rotate, following the peaks and valleys on the cam, which cause the lights to flash on and off. The cams can also be adjusted to give various flash rates.
This prop is the head display unit from a Burroughs 205 computer from the 1950’s. All the tiny wires go to neon bulbs on the front of the console, and originally displayed contents of data and address registers for the computer operator. Now the wires are simply grouped together and are wired to the timer switches to flash in different patterns.
These consoles were seen in various incarnations in Sci-Fi of the 50’s and 60’s, from the flight controls on the Jupiter II from Lost In Space, to the “Bat Computer”.
I'm using a very simple circuit to flash the lights and mimic the workings of the original cam timers. When grouped together, they provide an endlessly changing flash sequence, whereas the cam timers eventually repeat the sequence. Six of these circuits in parallel are powering the display linked to at right. At this point the timings are not correct, but shows that the circuits are working properly. A little experimentation will eventually get the right look. For R1 & R2, any values from say 22k to 82k in different combinations should get close to the correct timings.
A fancy power supply is not critical with this circuit. Anything from 5 to 12 volts should be fine. I'm using an unregulated 5V 500ma wall-wart charger from an old cell phone, and have had 6 stages powered with no problems. You can usually get them from surplus sites for 2-3 dollars apiece. The output voltage will affect the timing, so a circuit working the way you want with a 5V supply will be different with a 12V and so on.
Despite the cost, I choose to use solid state relays because they are silent when operating. If you don't mind the clicking of mechanical relays though, they can be used instead, and one SPDT can be used for each stage reducing costs. ( Since they are SPST, two solid states are needed for each stage at about 8 bucks each!) In this case, one of the LED's in the circuit is replaced with the relay coil, and the other is eliminated altogether.
I'm using C7 clear lights that I got half price after Christmas from Lowes. I got 25 bulbs, 25 sockets, and 25 feet of cord for under 4 bucks! Considering they sell a 4 pack of replacement bulbs for $2.50, it's a steal. They do put off some heat though, so a cooling fan can be used. The string can be dissassembled easily by rocking the base of the socket back-and-forth with a pair of pliers while pulling up. Considering what these exact sockets cost online, it's well worth the effort!
--more to come