This circuit was developed after the
original OPT201 detector was made obsolete by Texas
Instruments.
The OPT301 is in a TO-99 8 lead package
and has good sensitivity but a reduced bandwidth of
4kHz. The peak response is just into the Infra Red
region at 750nm but its sensitivity in the visible red
spectrum at 670nm is only a few percent down.

Detector (left) and a box housing an
LM386 audio amplifier with waterproof speaker /
headphone socket.
The detector is connected via a 2m
length of twin core screened cable which supplies the 12
DC feed to the detector and feeds the audio back to the
amp.
The circuit is almost identical to the
original receiver. Changes are the new detector / PCB
layout and improved op-amps. It uses a single supply at
12 - 13.8V. The detector has a a 10M Ohm feedback
resistor soldered directly between pins 2 and 5. The
detector output is followed by a NE5534 low noise op-amp
(3.4nV/Hz) which is configured as a bandpass filter. The
final NE5534 is an inverting buffer amplifier with a
gain of 20. The complete detector assembly is designed
to be mounted in a small metal box at the focal point of
a lens.

PCB Track
Layout

PCB
Overlay
The track is viewed from the top or
component side. The detector is fitted to the other side
of the board where a groundplane in unetched copper has
holes countersunk for the TO-99 package.
The 1N4002 diode protects against
reverse polarity. It is soldered to the undrilled pads
on the PCB.
Circuit notes:
A 10k resistor is needed between Pins 1
and 8 on the OPT301. This is shown on the PCB artwork
but I've not yet added it to the overlay.
The capacitor coupling the signal out of
the detector to the 39k was changed from 0.1 to 0.22uF
The capacitor across the zener diode was
also changed to 0.22uF
With a single supply line the body of
the detector is held at the zener voltage. Therefore it
should be isolated from the groundplane and from any
metal enclosure.
Four earth connections are required
through the board.
-
NE5534 pin 4 (on the left above)
-
Supply decoupling 4.7uF tantalum
(above and right of 5534 pin 8)
-
Detector Pin 3
-
Pad in lower right of PCB connected
to the 10k resistor and output ground.
Circuit Diagram

Performance of the bandpass and buffer
amplifier.
To evaluate the op-amp section I
connected a 600 Ohm sig gen to the input of the bandpass
filter and measured the response at the output of the
buffer. The Input was adjusted from 50Hz to 3kHz with a
constant level of 200mV Peak to Peak.
As you can see from the results below,
the 6dB bandwidth is just 200Hz. This makes the detector
ideal for transmitters using modulated CW on a fixed
frequency.
An OPT301, using 10M feedback with a
bandwidth of 200Hz gives a Noise Equivalent Power of 3 x
10-11 Watts.
My TX uses a 4MHz crystal and a CMOS
4060 oscillator / divider to generate 488Hz. This degree
of accuracy gives the option of using a lap top and
modern DSP software (e.g. ARGO or Spectran by IK2PHD) to
receive signals 20dB below normal noise level.
The 40dB rejection at 50Hz gives a high
tolerance to interference from street lighting.
Voltage gain at the design frequency is
x11 or 20.8dB
Bandpass filter / buffer amp
characteristics
50 |
20 |
40.8 |
100 |
80 |
28.8 |
200 |
215 |
20.2 |
300 |
430 |
14.2 |
350 |
630 |
10.86 |
400 |
980 |
7.02 |
450 |
1700 |
2.24 |
500 |
2200 |
0 |
550 |
1500 |
3.32 |
600 |
1050 |
6.42 |
700 |
620 |
11 |
800 |
445 |
13.9 |
900 |
360 |
15.7 |
1000 |
295 |
17.5 |
1500 |
170 |
22.2 |
2000 |
120 |
25.3 |
2500 |
95 |
27.3 |
3000 |
80 |
28.8 |
The bandpass filter center frequency is
selected by 2 capacitors and 2 resistors. It can be
changed to any audio frequency of your choice. Design
equations are published in the ARRL handbook. |