These pictures depict one way to mount the photodiode and FET.  The board has been laid out so that you could use either a through-hole or surface-mount FET and you may put the FET and photodiode on either the top or the bottom of the board.
Click on an image for a larger version.

• The PC board is mostly thru hole with a couple surface mount exceptions.
• Some day I may do a complete surface mount board, but don't hold your breath. If you do one, let me know!!
• From this site you may download a printable (and readable) schematic.
• From this site, you may download a printable 1:1 layout of each layer.
• From this site you may download a PC board file which may be sent to PC house to order boards.
• The board was made using ExpressPCB.com, an online line PC house.
• I used their free software to draw the schematic and to layout the PC board.
• If you want to edit the schematic or the PC layout, you will have to download the free ExpressPCB software, which is very easy to use. ExpressPCB free software site.
• The PCB layout is such that exactly 2 complete circuits fit on the ExpressPCB "proto" option boards, which are their least expensive boards. For about $60 you get 3 boards, each will have 2 circuits, therefore you will get 6 complete optical detector circuit boards. (about $10 each). Don't hold me to these prices, this is approximately what I paid in 2008.  (Note:  In 2015, the price is approximately the same for a non-silkscreen board.  For an extra charge you can get the "pro" version that has both a silkscreen and a mask on the board that makes parts location and soldering easier.)
• This site also has links to a few pictures of the final stuffed board and a couple of tips on building the circuit.

Lets get going

The following 5 files are in .PDF format. They will let you view the schematic and various PCB layers without needing the expressPCB software. It should print the layout files at 1:1 scale so if you have your favoriate way to make PC boards at home, you may be able to use these files directly.

• Schematic in .pdf format You wont be able to edit this file. If you want to edit the schematic, you will need to download the Express PCB software and download the Schematic in PCBexpress format. Use the links below)

The following are PCB layers in printable, 1;1 scale format:

• TOP i .pdf format (sometimes called part side)
• BOTTOM in .pdf format (sometimes called solder side)
• SCREEN in .pdf format (silk screen showing component placement)
• TOP and SCREEN in .pdf format Hand for figuring out where the parts go.

I do not have Gerber files for this PC board as I did not use a program that generates Gerber files. PCBexpress is quck and easy to learn. If you want a gerber file, maybe you could use my basic layout to start from. If you do make a Gerber file, let me know. That would be cool.

The following takes you to the ExpressPCB home page:  expressPCB free software site

The following 2 file are in ExpressPCB format. If you want to edit the schematic, edit the layout or order boards you should run the PCBexpress program then enter these files as required.

• Schematic in PCexpress format (you will need the expressPCB software to view this)
• PCB layout in PCexpress format (you will need the expressPCB software to view this)

• BILL OF MATERIAL in word .pdf format list of all of the components on the PCB. I have included Digikey part numbers for the less common parts that you may not have in your junk box.

Here are a few random notes on constructing the PCB

1. On the schematic: D4 is a protection diode, it in concert with F1, a thermally resettable fuse, provides reverse polarity protection to the circuit if you briefly connect the battery backwards, all too easy to do in the dark. 
2. The thermally resettable fuse is a surface mount device. If you leave it out, be sure to put a jumper in its place.
3. The following is important but a little confusing and it relates to mounting the detector diode.  See the following pictures - or look at the pictures to the right to get the idea:
• Prep the diode
• Mount the transistor
• Attach the diode
• Final assembly
• Stuffed board - with detector diode optionally on the part side

Important Note:  I artworked the board so the diode and FET may be mounted on either side of the PC board. This gives you a little more flexibility on how you mount the board relative to the optics.

Here is how to mount the diode, no matter which side:

• Mount Q1, the little surface mount transistor, with its gate sticking up in the air. Connect only the source and drain leads to the circuit board and have the gate. Be sure to get the D and S on the right pads!
• Straighten the anode lead of the diode so it sticks straight out.
• Solder the cathode lead to the circuit board in the proper pad.
• Solder the anode lead directly to the gate of Q1. Adjust things so the diode is parallel with the board and at the same height as the gate.

Have fun! Our record for over the air light communications using this circuit as the detector, is over 170 miles!

Let us know how you are doing with your optical communications experiments.

Good luck and have fun.

73 de Ron Jones K7RJ

K7RJ@ARRL.ORG

Go back to the "Optical communications" web page - or back to the main Modulated Light page.

This page last updated 20150819