Nuts & Volts - April 2005

Measure the Speed of Light

The important thing is to make sure that the parts don’t interfere with each other.

The easiest way to find the precise location for the holes in the beam splitter and lens is to fix the components on the bench and turn on the transmitter. It is easy to see where the light passes through the plastic and lens. Mark that spot and drill an 1/8-inch hole. You may have to enlarge it slightly to allow the whole beam to pass through, but don’t make it any larger than necessary. You lose this area when the reflected beam comes back. Since the beam splitter is mounted at a 45-degree angle to the laser beam, the hole should also be at 45 degrees. Otherwise, the beam will hit the edges of the hole. This should be done after the initial hole has been drilled. If you used a hand drill, secure the plastic in a vise with the drill through the hole. Then carefully start the drill and slowly move the drill to the proper angle. The edge of the bit will cut/melt the plastic. If you used a drill press, start it up before you manually twist the plastic. Use gloves or a drill press vise to hold the plastic for safety.

The Receiver

After a couple of attempts to build a simple, low-cost, high-speed optical receiver that resulted in marginal success, I decided to use the Sharp GP1FA551RZ fiber-optic receiver. It’s very cheap — under $2.50. It’s simple to use — three pins. Also, it’s fast, with a response of 13.2 MHz. Figure 3 shows the circuit, but the actual receiver circuit is trivial. Connect the Sharp IC to power and ground and run the output to the XOR. Bypass capacitor C3 is required (placed at the receiver IC), as the circuit will oscillate without it.

What’s with the other three gates, though? What are they there for? They’re not in the block diagram. These gates are needed to invert and delay the reference signal

Photo 3. This is a typical output pulse with a 300-MHz ‘scope.

APRIL 2005

Photo 2. The basic layout. The base is a 12 x 12-inch piece of particle board.

to compensate for the delays in the laser and receiver. We want to measure only the time it takes light to exit the laser and return. Unfortunately, all electronic components require some time to operate. There are a few nanoseconds (ns) of delay from when power is applied to the laser and when light is emitted. There is also a delay from when light strikes the photo diode in the receiver to when a signal is available at the output pin. Surprisingly, the receiver has a worst case delay of 180 ns. This is very large. The good news is that the jitter — or delay variability — is typically only one ns. Since 13.2 MHz corresponds to a wavelength of about 57 ns, I figured that a realistic “typical delay” estimate should be no longer than that (for a strong signal). Therefore, I chose to design the delay circuit for about 60 ns. (Note, you can choose not to use