delay compensation. In that case, all your measurements
will be off by a fixed amount. You can subtract this
amount manually to find the speed of light, but it’s not
Let’s step through the circuit. First, you must use a
Schottky 74S86 (not the 7486 or the 74LS86). The
74S86 has a symmetrical, 10-ns, low-to-high and high-to-low delay. Of course, the other families have different
propagation delays. Remember, we want to resolve one
ns. The first thing we do is buffer the reference signal. The
XOR with one input connected to ground does this. Then
NUTS & VOLTS
10M 1/4 W
1.5K 1/4 W
1K 1/4 W
Part of laser pointer, see text
0.1 µF 25 V
33 pF 25 V
Key chain laser pointer (see below)
4 MHz crystal
74HC04 hex inverter
Note: There are many sources for these inexpensive lasers.
I’m quite sure I got mine from All Electronics a few years ago. They
currently have them listed as LP-506 on their website (www.
allelectronics.com) for $4.75. Hosfelt Electronics has a similar
item, 75-374, for $5.95 at www.hosfelt.com Officemax item
11134783 appears to be similar at $10.99.
1K 1/4 W
5-20 pF variable cap
0.1 µF 25V
74S86 Schottky Exclusive OR Do not substitute.
GP1FA551RZ Sharp fiber-optic receiver IC
( 13.2 MHz speed)
Available from Digi-Key, 425-1109-5-ND $2.38 each
4” x 4” x 1/8” clear plastic (see text)
Credit card/wallet Fresnel lens magnifier 2” x 4
Edmund Scientifics ( www.scientificsonline.com) part 3038456
@ $1.95 each looks identical to the one I used. Officemax part
11035435 ($9.99) might be useable, but it has a handle. A web
search brings up many listings.
Misc. 12” x 12” wood base, small wood strips to
mount parts, screws, etc.
we delay the signal with R1 and C1. This provides about
five to 20 ns of delay. Then, the signal is inverted by connecting an input to a logic high. Next, we delay this by five
to 20 ns with R2 and C2. Finally, the signal is buffered
again with the last section of the XOR chip. The delay is
30 ns for the three XOR sections and 10 to 40 ns for the
delay circuits for a total delay of 40 to 70 ns.
There are some subtle points to note. Two resistor-capacitor (RC) delay circuits are used because the XOR
output has asymmetrical drive. It sinks much more current than it can source. This means that the capacitors
will discharge faster than they charge. If only a single RC
delay was used, the rising edge would be delayed more
than the falling edge. By using two RC delays — one on an
inverted signal and the other on a non-inverted signal —
we can control the delay of each edge and can make
the delays symmetrical. (Note, if your receiver has a
significantly different delay, you may have to adjust the
values of the RC circuits. For more delay, increase the
resistor and/or the capacitor values, and for less delay,
The other point is a very useful property of the XOR
gate. A logic low at one input means the output follows
the other input. A logic high at one input will invert the
signal at the other input. This means that you can control
the inversion (or phase) of a signal with a logical value
without changing the propagation delay. If you do a lot of
digital design, you will find this little trick valuable.
Unfortunately, there is a problem with the receiver.
The actual IC is a small three-pin device buried in a large
plastic housing, which is normally mated with a fiber
optic cable. It’s very difficult to align the laser light to
shine deep into the small hole used for the cable. While
that is possible, it’s much easier to remove the IC from the
housing. I know because I tried it both ways. Look closely at Photo 2. At the front is a large, black component.
That’s the optical receiver. If you look to its immediate
left, you will see a small, clear, plastic object. That’s the
actual receiver IC that has been removed from the housing. A better picture of the receiver and housing is shown
at the bottom right in Photo 1.
I was successful in removing the receiver IC with two
different methods. The first, shown in Photo 1, was to very
carefully saw the sides of the housing until I reached the
cavity that held the actual IC. However, once I did that and
was able to examine the inside of the housing, I noticed
that two small retention barbs held the IC in place. These
barbs are accessible where the IC leads exit the cavity.
The next time, I used an X-acto razor knife to cut these
barbs away. With a little inward pressure on the face of the
IC through the hole (using a toothpick) and gentle pull on
the leads, the IC was removed.
Setup and Operation
Physically adjust the transmitter, beam splitter, and