The objective of this lab was to take a weak IR light signal (whose amplitude correlates with an audio signal), amplify the signal, and use it to drive a speaker in order to hear a secret audio message. To accomplish this, I created a two stage circuit that first senses light signals through phototransistors, then uses a differential amplifier to filter the weak light signal from the ambient light and amplify the resulting signal, then uses AC coupling to offset the signal properly, and finally buffers the signal in order to provide enough current to drive the speaker to hear the audio. I was able to detect and translate the weak light signal from a distance of over one foot from the source.
I selected the LM6142 Dual R-to-R Op-Amp for the differential amplifier because it had the highest Common Mode Rejection Ratio, or CMRR, of the eight op-amps available to us at . This is important because the high CMRR indicates the op-amp is well suited to reject common-mode signals, or noise. Prior to the differential amplifier, the signal through one phototransistor is the ambient light. The signal through the other phototransistor is the weak light signal superimposed on the signal. The differential amplifier subtracts the signal from the superimposed signal leaving just the weak light signal. The gain on the differential amplifier stage amplifies this signal. The signal then passes through a capacitor for the purpose of AC coupling to offset the signal properly. For the buffer portion of the circuit, I opted for the TCA0372 Dual Power Op-Amp since it had the highest output current () of the eight op-amps available to us. This allowed the amplified signal to be output to the speaker while providing enough current to drive the speaker. This op-amp has a gain of 1 since the signal is already amplified from the previous stage.