I need to build a circuit where light is converted to sound. The light falls on a CDS cell and the current is passed through the NPN transistor to amplify it.
I know this is a very simple schematic, but what would be the real life model of this? Why is the resistor going to the ground?
That is a really crappy circuit since it depends on the gain of the transistor being just right. Real transistor gains vary widely, even within the same production lot. Competently designed circuits work from the minimum guaranteed gain of the transistor to at least 10 time that, preferably to infinite gain.
I would start with the transistor stage setting its own DC bias so that the output voltage is near the middle of its range. The signal from the LDR would be AC coupled into that stage. The LDR would have a pulldown at least so that it and the pulldown produce a voltage signal by themselves. This would be the signal fed into the amplifier stage thru a coupling capacitor.
For extra credit, come up with a auto-biasing scheme that keeps the output near the middle of its range regardless of average ambient light level.
It is a simple non-inverting transistor amplifier. The resistor to ground is what allows it to be non-inverting. When the transistor is off (the CDS cell not conducting), the Headphone output at the emitter of the Transistor is pulled to ground level through the 47Ω resistor. When the transistor is on, the headphone output gets pulled up to 3V, with the resistor limiting how much current goes through the transistor, and preventing a direct short.
This is known as an emitter follower. There is no voltage gain in the circuit, this acts as a impedance buffer i.e. it can drive heavier loads that the CDS cell could on it's own.
Light on the CDS cell generates current which goes into the base of the transistor, this current is multiplied by the transistor (this gain is known as Hfe). This current when forced through the resistor generates a voltage which is your output signal. Your Hfe (from some old data sheets) varies but is at least 40 (and can be as high as 100).
The real world application varies upon the application and the type of the signal. If linearity is not a concern, then the above circuit diagram can be improved to remove the 47(ohm) resistor and the circuit will act as a digital circuit, with almost no sound when there is no light.
On the other hand if linearity is a concern for analog applications, then the above is suitable to provide analog output specially if the amplitude of the light is to be varied on both sides of the normal value (i.e. if the value changes both above and below the dc value). Just check up non-inverting transistor amplifiers for more information for the circuit.
A lot depends upon resistor values of the CDS cell and the resistance of the headphones. Moreover, you might want to connect a capacitor between the output and the headphone to dc couple the output.
