[Laser] low voltage optical transceiver - receiver

[Tim Toast]

Hi all,
 
I finally got around to building that little hearing-aid amplifier circuit i mentioned last year. 
http://www.aladal.net/toast/5mwkit1.jpg
 
It run's off 1.5 or 3 volts. As a hearing aid, i have to say it didn't seem that great at first. It would drive an
8 ohm headphone just fine - not real loud but the volume was ok. It turns out though, building it with a few 
random transistors will affect things negatively sometimes and you will need to adjust the bias on Q1 
depending on which transistors you use. Also the battery voltage affects it a little. So i just replaced the 
two biasing resistors for Q1 with a 100k pot and added a 1 k ohm current limit resistor where it connects
to B+. Now you can adjust the bias easily and can experiment with different transistors and other parts.
 
The 100k pot works a bit like a volume control and the higher the setting the more current used. But you 
reach a point where Q1 saturates if turned too high. On a current meter, the circuit draws between 0 and 
about 25 ma. depending on the bias setting. It seems to be loudest around 10 or 15 ma, depending on 
battery voltage. The circuit keeps on working as the battery voltage drops. It actually had more gain at a 
slightly lower voltage than 1.5, and the best bias point changed too. I think it operates all the way down 
to about 0.85 volts or so. I had an old "dead" 9 volt battery laying around that i tried and it worked! It will 
not with a fresh 9 volt though.
 
The first thing i tried was a phototransistor in place of the microphone and it worked well. I can hear the 
hum from the florescent lights in the shop. Buzz from the TV, and remote controls are really loud up 
close. Turning it toward the window i could hear the hiss from the sunlight. 
I tried a photodiode next and it was noticably weaker picking up as expected. I'll probably try to add 
a FET front end to perk up the sensitivity. Hopefully they will work at such low voltages. I also have a 
few of the old germanium transistors in my junk box to try. Those are known for working well at lower 
voltages. 
 
...more later
 
-toast