Nuts & Volts - April 2005

Measure the Speed of Light

reflection is slightly scattered and is not a tight beam. For that reason, a lens is needed to focus the laser light onto the receiver. This lens is an inexpensive plastic Fresnel lens.

The beam splitter (just a piece of clear plastic) reflects only the returning light from the lens toward the receiver. Otherwise, the receiver would have to be in the same position as the laser. (We’ll discuss the beam splitter and lens in more detail later.) The receiver creates an electrical pulse that is a duplicate of the transmitted pulse, but slightly delayed. These two signals are combined with an exclu-sive-OR (XOR) gate that “subtracts” one pulse from the other, leaving only those parts that are different. These differences are the results of the distance/delay and are displayed on the oscilloscope.

The Transmitter

Figure 2 shows the schematic diagram of the laser pulse generator or transmitter. As you can see, it’s very simple. The first section of a complementary metal-oxide semiconductor (CMOS) inverter is used as a crystal oscillator. A second section is used to buffer the oscillator, and the last four sections are used in parallel for increased power to drive the laser diode. The rectifier diode, D2, does double duty. It prevents problems if the power is reversed, and it drops the six volts to about 5. 3 volts so the inverter won’t get upset. Resister R3 and LED D1 are optional and are used as a power-on indicator. (It’s just a habit of mine to include them.)

There are a few notes. The inverter can be almost any CMOS family (74H04, 74HC04, 74HCT04, etc.), but bipolar inverters won’t work (7404, 74LS04, 74S04). You can certainly use a commercial oscillator instead of the crystal oscillator circuit. (In that case, you can eliminate R1, R2, C2, C3, and Y1. Connect your oscillator output to U1 Pin 3, and ground U1 Pin 1.) While the oscillator circuit is very robust and will work with nearly any crystal, keep the value around 4 MHz to match the receiver delay circuit (to be discussed later). A 3.57-MHz color-burst crystal should be fine.

Photo 1 shows the transmitter and some parts that require “field modification.” I put the transmitter in a box because I have a feeling that I will find a number of uses for a laser transmitter that can be modulated at up to 20 MHz. As you can see, the actual electronics fits on a small, perforated circuit board, and point-to-point wiring was used.

The laser I used was a key-chain type that used three 1.5-volt button batteries (right center of Photo 1). I chose to remove the laser head from the body for ease of use. (You don’t have to, but you will have to make power connections somehow.) I carefully used a hacksaw to remove the head assembly. You can see the spiral cut in the body (left center). The laser head consists of a lens, laser, and a tiny PC board (center). I attached power wires that

APRIL 2005

Figure 1. It is useful to have crystal stability because jitter on the oscilloscope trace will make it hard to read. This clock drives a semiconductor laser (which is just a cheap laser pointer).

bypassed the switch so I wouldn’t have to press the button for the laser to work. The long leads were only for testing. (The two parts in the lower right of Photo 1 are the optical receiver and will be discussed later.)