🚀 go-pugleaf

aw2t+@andrew.cmu.edu (Alex R.N. Wetmore) writes:

>We are attempting to use a phototransistor to recieve a signal from a 
>modulated laser light.  We have a 741 chip, a lm386 chip, and various
>resitors, caps, etc...  Can anyone suggest a good circut to use?  

>If not, can anyone explain how a phototransistor works?

>thanks,
>alex

For the phototransistor, if it is a true transistor (i.e. three legs,
not two), the way to use it is this.  It works by using the light input
to generate carriers (i.e. make current flow), instead of the more
normal method of making a transistor conduct by injecting a base
current.  Thus one uses the transistor in a standard configuration, but
with base tied (with a high value, say 1MegOhm, resistor) to ground.  

Eg.
              _____  +V (say 5V, whatever)
		T
		|
		 >
		<  Say 4.7K
		 >
		<
		|
		|------------------->  Output to amplifier.
		|
		|  Coll.
		\ |
		 \|
		  |  Base
		  |---------------*
		  |		  |
		 /|		  >
		V |		 <
		|		  >  About 1Meg.
		| Emitter	 <
		|		  |
	      --*-- Ground  ------*----

That should cope with the transistor.  If it is only a two pin device,
the missing pin is the base.  If that is the case, the same circuit
(minus the base resistor) still works.  The reason for the base
resistor, is to improve circuit stability at high frequencies.

This input circuit operates as follows.  Under dark conditions, no
carriers are injected into the base, and no current flows in the
collector-emitter path.  Thus there is no voltage dropped across the
collector resistor, and the output voltage is (very nearly) 5V.  When
light is allowed to hit the transistor, carriers are injected into the
base, and current begins to flow.  This causes a voltage drop across the
collector resistor, and the output voltage falls.

To amplify this voltage, a circuit similar to the following could be
used.  

					R3
				-----/\/\/\------
				|		|
				|  |\		|
Input from		R2	|  | \		|
above	-----| |-----/\/\/\-----*--|- \		|	
circuit.			   |   \________|___  Amplified output
			R1	   |   /
		-----/\/\/\--------|+ /
		|		   | /
		|		   |/
		|
		|
	     ---*---  Ground

This circuit gives a reasonable amplification for minimum parts.  There
are problems though:  1.  Most of the op-amps need dual power supplies.
In this case, say +5V, and -5V.  This is usually 
Ok in a lab, but sad for commercial equipment.  2.  The input capacitor
which is necessary to de-couple from the input stage, limits the
frequency respose.  The reason for the cap. is that the input circuit
"idles" at 5V (for dark conditions), and the amplifier idles at 0V.

However if this isn't a problem, the way to choose the values for the
resistors is this.  Make R1 and R2 the same value, and the same as the
value of the collector resistor in the input circuit (i.e. 4K7).  Then
figure out how much gain you need (make it reasonable) and then use the
formula R3 = R2 * Gain.  (this is an approx. for gain >10 it's pretty
accurate).  So, if you wanted 10x gain, make R3 47K, etc. etc.  suggest
keeping the gain down below 1000 for stability (also, it might not need
that much).  If you need more, build a second stage, and split the gain
across both stages.   An interesting feature is that the op amp circuit
is inverting.  This means that in increasing light, the input circuit
voltage decreases (towards 0), and the amplifier output increases.  So,
the output voltage from the amplifier, should be in phase with the light
fluctuations.  Choose the value of the capacitor appropriate to your
frequency response.  Start at about 1uF, (if electrolytic, put positive
towards input circuit, negative towards op-amp).  If you need lower
frequencies, make the cap larger value.

One final note, since you are using lasers, you have to be careful not
to saturate your transistor, since it will swamp the signal.  

Hope this gives you something to start with.

[dav]  aka
David Emrich
gude@uniwa.uwa.edu.au