In EE230, Electronic Circuits & Systems, we were assigned two projects in the semester. Both projects were centered around active and passive filters.
The first project involved designing and building a three-channel crossover network for audio. The low-pass channel needed to have the ability for the user to be able to increase the amplitude of the signal, or drop the bass.
The second project was slightly more complicated, where we were asked to build an analog infrared transmitter and receiver that would be capable of sending two different signals. In demonstrate the correct behavior of the system, each signal would light up an individual LED.
Our design was fairly simple. For the transmitter we used two op-amps on an LM324 for two oscillators, with one oscillating at 1 kHz and the other at 20 kHz. Two switches would output the signal from the oscillator through an infrared LED. A circuit diagram of the transmitter can be seen below.
For the receiver, an infrared photo-diode was connected between an op-amp terminal and the negative voltage. Because the output from the photodiode was very weak, one op-amp in the circuit acted as a preamplifier for the high and low-pass filters that separated the input for each LED. To determine what LED should be lit from input, we used a high-pass filter to block the 1 kHz signal to one LED, while a low-pass filter blocked the 20 kHz signal from the other LED. The single-order active low-pass filter was designed with a corner frequency of roughly 2 kHz and a 0 Hz gain of 10 v/v. The output from this filter was sent to an AC rectifier, where the DC output was passed to a small resistor and LED. For the 20 kHz signal we decided to build a second-order filter with an infinite frequency gain of 10 v/v and a corner frequency of 10 kHz. The output from the high-pass filter was passed to a non-inverting amplifier of 10 v/v before being output to the other LED and small resistor.
This was the second circuit class I have taken at Iowa State in a planned sequence of three. Circuit design and analysis are two skills that I have really enjoyed learning. The skills have proven to be very useful on personal projects and also in a past co-op.
A video of the final solution for the project is shown below…