Here
is the receiver circuit. It uses the OPT210 detector followed by an
op-amp. This gives a low level output which is further
amplified by an external LM386 audio amp that can drive headphones or a speaker.
The detector uses a 0.25 watt leaded 2.2M resistor as feedback. The components
around the first 071 op-amp are for a bandpass filter centered on 500Hz. This is usable
from 400 to 700 and offers significant attenuation from 50/60Hz sources of
interference. Design equations for this circuit are available in the ARRL
handbook.
As the circuit needed to be small, the
op-amps are TL071CD surface mount devices and the 4.7uF capacitors are all
tantalums. The supply is 13.8V and the 22nF capacitors used in the
bandpass filter should be 10% or better.
The pin out of the Burr Brown detector will depend on the case style. For the 8
pin DIL version the pins are:
Supply
1
Output
5
Feedback (2M2)
2
Ground 3
Zener
8
No connection 4 6 and
7
Printed Circuit Board Layout
This PCB is viewed from the
trackside. All the components are mounted on this side of the board except
the OPT210. The board has a ground plane on the reverse for the zero Volt
connection.
In this view the TL071 on the left is the bandpass filter. Both TL071s have pin
1 in the upper left while the OPT210 has pin 1 on the upper right. This layout
is slightly different from of the one used in the prototype pictured earlier.
Notes: 1) A link is
needed from pin 3 to pin 3 between the TL071 op-amps
2) In this layout the buffer feedback resistor (100k) has a capacitor in
parallel. This should act as a low pass filter, but in practice caused
instability. It was removed during testing.
3) Pin 3 of the detector is soldered on both sides of the PCB. This provides an
earth for the TL071 buffer at pin 4
4) Pin 6 of the detector is soldered to the groundplane for added strength
Comments:
October 2002
After using the circuit above successfully, I have now changed the feedback
resistor to 10M Ohms. This gives reduced bandwidth but better sensitivity. - See
NEP (noise equivalent power graphs on data sheet)
I've also looked at the noise generated in the TL071. It is quoted at
18nV/Hz. A better device would be the 5534 op-amp. This is used in
professional audio applications and it's noise is given in the data sheet at
just 3.5nV/Hz.
The OPT210 is no longer produced. I'm now using the OPT301 circuit which has only 4kHz
bandwidth but is perfectly OK for my modulated CW application.
Comments: January 2003
On the 28/December/02 I managed my first 'long distance' two
way QSO with Allan G8LSD over a 22.4km path. The detector above worked
very well. It was even possible to hear a signal when the incoming laser was
invisible to the naked eye! It may have noisy op-amps and the magnifying glass
lens in a piece of drainage tube was certainly cheap..........but it all works
really well.
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