Design

The Classroom Noise Buster (CNB) monitors the noise level generated within the classroom and turns on either a red or yellow light once the noise level exceeds acceptable thresholds as shown in the figure below:

The CNB consists of four main subsystems:

  1. Signal Transformation
  2. Signal Processing
  3. Noise Threshold Modification
  4. Visual Indicators

system block diagram is shown below:

1. Signal Transformation

Signal transformation employs the use a sound transducer (microphone) to convert sound (variations in air pressure) into an electrical signal. This is necessary since the device used to determine the sound level of the noise generated inside the classroom can only process electrical signals. This electrical signal is then amplified, DC shifted and band limited in order to obtain a suitable signal for digital processing by the PIC’s analog to digital converter. The three stages pertaining to signal transformation are illustrated in the figure below:

2. Signal Processing

A PIC16F876A device is used for:

The PIC16F876A and its external connections are shown below:

A suitable measure of the classroom noise is obtained by sampling the input signal at a sufficient sampling rate and calculating the average over a small interval. This average can then be used to describe the noise level at some instance of time along a discrete time axis. Performing this over several intervals and averaging these intervals avoids sporadic turn-on / turn-off of the lights (visual indicators) and thus provide a good measure of the classroom noise generated over time.

3. Noise Threshold Modification

The initiation, transmission, and reception of real-time noise threshold modification are discussed in this section.

Initiation / Transmission

A wireless keyfob transmitter (shown below) operating at 315 MHz is used to initiate a modification of the threshold noise levels pertaining to the turn on of the visual indicators. This is accomplished by pressing the one of the five buttons on the transmitter corresponding to the desired change of the noise thresholds. Pressing S3/S1 will increment/decrement the noise threshold for the red light whereas pressing S4/S2 will increment/decrement the noise thresholds of the yellow light. Pressing S5 causes the noise thresholds to return to the default noise threshold values. This scheme, therefore, allows for a dynamic range between the thresholds of the red and yellow lights.

The keyfob operates as follows: when a button is pressed the power is applied to the internal circuitry and the encoder is enabled. The encoder then detects the “hardwired” address and logic states of the five buttons. The address and five button logic states are formatted into a 3-word message that is continuously transmitted until the button is released.

Reception

The receiver is “hardwired” to have an identity address identical to that of the transmitter. The matched receiver is used to decode the transmitted 3-word message. The first 10 digits of the received message correspond to the transmitter’s address. If this address matches that of the receiver the Valid Transmission (VT) line, which idles at low logic, is set high to indicate the reception of a valid signal. Upon valid transmission, the decoder outputs data onto data lines corresponding to the logic states of the 5 buttons pertaining to the last transmission and then resets all data lines to a low logic level.

The receiver and its external connections are shown below:

4. Visual Indicators

A yellow light and a red light are used to indicate to the students in the classroom that they are too noisy and need to quiet down. If the noise generated inside the classroom exceeds the threshold corresponding to the yellow light but remains below the threshold corresponding to the red light then the yellow light will turn on. Once the noise level exceeds the threshold for the red light the yellow light will turn off and the red light will turn on. If the noise is below either threshold both lights will remain off. Note that if the threshold level for the red light is updated while the red light is on and the new threshold exceeds the current classroom noise level then the red light will turn off and the yellow light will turn on.

Two digital output pins on the PIC are used to control the red and yellow lights. The lights consist of super bright light emitting diodes made by Luxeon that require a driving current of 350 mA. A device for driving these LEDs is required since the I/O pin on the PIC can sink/source a maximum of 25 mA and one port (8 I/O pins) can source a maximum of 200 mA collectively. The driver circuit shown below incorporates feedback to maintain a constant LED drive current of 350 mA.