RS-232 Laser Transceiver

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Laser based projects used
to be expensive, until the development of solid state lasers. This
project is designed for the entry level laser experimenter. The circuit
allows any two computers with serial (RS-232) communication capability
to communicate over 200 meters using a laser beam. A low cost
transmitter only circuit is also presented here for use in one way
communication and other laser based projects.

Introduction

If you are like me and
always wanted to buy a laser pointer to play with, but could never find
practical uses for one, here are a couple of circuits to convince you to
finally make that purchase. Before we begin, however, it is necessary to
give a word of warning: Never look directly into the laser beam as eye
damage may occur. I will present the project in 2 sections: the first is
a full-duplex transceiver, and the second is a transmitter only. The
main reason for separating the design is to offer a cheaper solution if
only half-duplex communication is required. For full-duplex
communication 2 transceivers and 2 lasers will be required, and for
half-duplex communication a single laser, a transmitter and a
transceiver is needed. The transmitter can be also used as a stand-alone
circuit if you only want to control the laser in other laser
experiments.

RS 232 Laser Transceiver
This view shows the assembled transceiver. In this photograph you can
see the conductive dummy battery used to reach the negative
contact inside the case of the laser.

The laser source for this project is an
inexpensive laser pointer pen. As well as being readily available, the
circuit is designed in such a way so that the laser pointer is not
damaged, and can be used for other experiments. Because this is an entry
level circuit, costs have been kept to a minimum around $20 for the
transceiver and approximately $10 for the transmitter (excluding the
laser pointer).

Why use a laser? A laser as
a communications medium has some unique properties compared to other
forms of media. A line-of-sight laser beam is useful where wires cannot
be physically connected to a remote location. A laser beam, unlike
wires, also does not require special shielding over longer distances.
Lasers offer at least an order of magnitude longer distances compared to
infrared LEDs. Although RF transmitters may offer longer distances than
line-of-sight lasers, they are subject to interference from other
transmitters. Since the laser medium is line-of-sight and the beam being
only several millimeters in diameter it is very difficult for the data
stream to be tapped. This offers secure communication since any attempts
to intercept the laser beam would be detected at the receiver as a loss
in data. A laser medium also allows for the sender and receiver to be
galvanically isolated from each other.

The Transceiver

The transceiver is based on
the MAX232A IC for generating and receiving RS-232 compatible voltage
signals. The receiving sensor is an NPN infrared photo-transistor
(OP505A). I chose an infrared photo-transistor to minimise ambient light
interference. Although the laser wavelength is in the visible spectrum
(~670nm) the photo-transistor’s broad response band (550nm to 1050nm) is
wide enough to sense the intense laser beam. The signal from the
photo-transistor is buffered via a pair of Schmitt trigger buffers to
clean up and square the signal. The output of the second buffer is then
directly converted to a RS-232 standard signal via the MAX232A.

The MAX232A generates +10V
and -10V voltage swings using a dual charge-pump voltage converter from
a single +5VDC rail (see RS-232 standards below). Several different
versions of the MAX232 chip exist. The A version requires only 0.1 uF
capacitors for the charge-pump and inverter, whereas the MAX232 requires
1uF capacitors. The advantage of the A version is that it has faster
response times, and allows for faster data rates.

RS 232 Laser Transceiver #2
Figure 1. The schematic of the transceiver. The MAX232A IC provides
the interface to the PC, and the 74LS05 is used to drive the laser diode
inside the laser pointer.

The laser diode driver
consists of a 7405 open-collector hex inverter IC. All the outputs of
the inverters are coupled together to provide enough drive current for
the laser diode which draws around 35mA @ 3V. A 7805 voltage regulator
is used to provide the IC and laser diode with a stable 5V voltage
source. The two 1N4001 diodes, in series with the laser diode, step down
the voltage from +5VDC to around 3.6VDC which is close to the nominal
voltage for the laser diode.

The transceiver is designed
in such a way that when no signal is present the laser is on. This helps
you see where the laser is pointing during the laser-detector alignment.
The transceiver is powered by a 9V battery and draws approximately 80mA
(laser on) and 40mA (laser off).

The Transmitter

The transmitter differs from
the transceiver in the fact that it can only transmit data. The
transmitter consists of an opto-isolator and an open-collector hex
inverter and a handful of other components. The transmitter is also
powered by a 9V battery and draws approximately 70mA (laser on) and 30mA
(laser off).

RS 232 Laser Transceiver #3
Figure 6. Here is a close up of the assembled transmitter. The
transmitter accepts both RS- 232 and TTL compatible signals as inputs.

The circuit uses an
opto-isolator (4N33) to couple a standard RS-232 signal from a computer
to the driver section of the circuit. The resistor/diode configuration
at the input to the opto-isolator converts the +12/-12 voltage swings of
a RS-232 signal into a signal suitable for the LED in the opto-isolator.
A second input on the board is also provided for external TTL compatible
signals. This can be wired to the parallel port of the computer or other
microcontrollers. Note: Never use the TTL input signal at the same time
as the RS- 232 input signal as these are shorted together only via a
resistor.

RS 232 Laser Transceiver #4
Figure 2. The circuit diagram of the transmitter. The laser diode
driver is identical to the driver in the transceiver. The bi-polar
RS-232 signal is converted to a TTL compatible signal via D3, R1 and the
opto-coupler.

The laser diode driver section is
identical to the one used in the transceiver. The driver section of the
transmitter is also designed so that the laser is on when no data is
present to help point the laser.

Construction

Construction of both the
transmitter and transceiver is fairly straight forward so I will
describe them together, pointing out the differences when needed. First
start by checking the PCB to make sure it is clean and free from dirt.
Next mount all the passive components, this includes the resistors and
capacitors. You can now mount the diodes taking note of their polarity.
Next fit the active components, this includes all the ICs and the
voltage regulator. The voltage regulator does not require a heat sink,
so it can be placed flush against the PCB. The ICs may be mounted in
sockets or soldered directly to the PCB. Now fit the pin-headers to the
appropriate holes on the PCB. If you prefer not to use pin- headers and
connectors, you may solder the wires of the external components directly
to the PCB.

Now you are ready to start
attaching the external components. These include the laser pointer,
photo-transistor, battery connector, switch and the DB-9 connector. I
will leave it up to you as to how you want to house the project, but I
suggest that you use a zippy box so that you can mount the
photo-transistor and switch. To mount the photo-transistor, you can
simply drill a snug hole in the side of the zippy box and use that to
secure the photo- transistor. Alternatively a block of plastic or wood,
with a hole drilled in it for the photo- transistor, can be attached to
the top of the zippy box. The advantage of the block is that it shields
the photo-transistor from ambient light. Take particular care with the
orientation of the photo-transistor when clipping the pins and soldering
wires to it.

RS 232 Laser Transceiver #5

RS 232 Laser Transceiver #6
Figure 3a & 3b. Here are the PCB patterns for the transceiver and
transmitter for those who want to make their own. The layouts are
copyright to GKDesign.

Now you need to
prepare the serial connector. You may use a standard
(female) DB-9 or DB-25 connector depending on your needs. I
will describe the connections for the DB-9 connector as
this is found on most IBM-PCs. (See Fig. 8 for the
connections to both a DB- 9 and DB-25 connector) The IBM PC
serial port contains several data and handshake lines. We
will only be using the Transmit Data (TD), Receive Data
(RD) and common ground (GND) lines. Handshaking will be
done in software. In order to make the serial port happy we
need to connect the Data Terminal Ready (DTR) line to the
Data Set Ready (DSR) and Data Carrier Detect (DCD) lines.
We also need to connect the Request To Send (RTS) line to
the Clear To Send (CTS) line. This has the effect of
tricking the serial port into thinking that it is always
ready to receive and send data. These links should be
soldered inside the connector itself. Only 3 wires are
required for the connection to the transceiver. Connect the
three wires to the RD (pin 2), TD (pin 3) and GND (pin 5)
lines of the connector. For the transmitter, wire only the
TD (pin 3) and GND (pin 5) lines. A length of 4 core
flexible telephone cable works great. Make sure no one is
using the phone at the time! Again ensure that the wires
are correctly wired to the PCB.

RS 232 Laser Transceiver #7
Figure 8. This solder side view shows how to wire the
DB-9 (female) or DB-25 (female) connector for an IBM PC
compatible computer.

Next connect the black wire
of the 9V battery clip to the PCB and the red wire to one
contact on the switch. The other switch contact should then
be wired to the PCB. You can use light duty hook-up wire to
achieve this.

The last, and most
interesting, component needs to be wired to the circuit –
The Laser. We must first prepare the laser pointer since
almost none have wires already hanging from them. The
preparation will vary on the laser pointer you have, but
most should have access to the battery compartment. The
most suitable laser pointers are the ones that require 2
AAA or 2 AA batteries. The laser pointer used in this
circuit is the LX1000 purchased at Radio Shack. First
remove the batteries carefully noting the polarity of the
contacts. You now need to connect a wire to each battery
contact. Depending on the laser pointer case you may need
to create a conductive dummy battery in order to
reach the contacts. For the laser pointer used in this
design, in order to reach the negative contact deep inside
the case, a metal rod was cut to the length of both
batteries and a wire was soldered to the end of it. The rod
can be made from a bolt or large diameter nail with the
head cut off. The rod was wrapped in electrical tape to
insulate it from the aluminium case which formed the other
contact. An exposed wire was taped to the insulation of the
rod so that it made contact with the case when the rod was
inserted. This made the positive contact.

RS 232 Laser Transceiver #8

RS 232 Laser Transceiver #9
Figure 4a & 4b. Connections to the external components
are shown here for both the transceiver and the
transmitter.