System Architecture As previously mentioned, the design will consist of three main components: the sensor, the base unit and the remote receiver (See Figure 3). This section will describe the detailed design and operation of these components. 
Figure 3. Layout of Design Components
The sensor will consist of both the aforementioned variable inductor strap and transmitting circuitry. The transmitting circuitry consists of both an oscillator circuit, amplifier output stage, battery power source and an antenna. Figure 4 shows the arrangement of these sensor components.
Figure 4. Sensor Components (not to scale)
The most recent revision of the variable inductor strap utilizes flexible pantyhose material to connect two inflexible strips of nylon. 32 AWG wire is sewn into the flexible material in loops to form the inductor. The final sensor design will be based on this concept of using two materials with different flexibility coefficients. The infant sleepwear shirt will be less flexible than an elastic inductor section inserted into the front of the shirt near the diaphram of the infant. This will eliminate the notion of a "strap" altogether which should increase overall marketability.
The oscillator circuit will be responsible for generating a base frequency (2 to 5 MHz) which will then be varied according to the changing inductance. Operation in this frequency range will eliminate interference from AM, FM, television broadcast and short wave radio signals. The Colpitts oscillator model was used for this design as it requires a single inductor to be included in the frequency determinant components. This model can easily be modified for the purpose of this design by connecting the variable inductance section to the circuit as the tank inductor. Figure 5 shows the schematic diagram for the current modified Colpitts oscillator fitted with the new variable inductor.
Figure 5. Modified Colpitts Oscillator
Utilizing the most recent variable inductor strap, the current oscillator prototype produces a 10.77% change in frequency (1.22 - 1.35)MHz when subjected to a full stretch of the strap. Although the average frequency is slightly less than desired, more efficient wiring techniques for the variable inductor will reduce the overall inductance thereby increasing the average frequency. At Vcc of one volt, the circuit was observed to draw three milliamps from the source. Installing two 3V, 144 milliamp-hour watch batteries in series will thusly provide this circuit with 96 hours of operation time before battery replacement is required.
The second component of the transmitting circuitry will be a Class B output stage amplifier. This amplifier will drive an antenna which will act as a RF (Radio Frequency) transmitter. The transmitter will broadcast at whatever frequency is being generated by the oscillator.
The signal will then be received by an antenna in the base unit. The base unit will then amplify this signal and deliver it to a comparator, which will be built into the micro controller. The comparator will be designed such that it will generate a '1' signal when the voltage input is over the halfway point of the incoming sine wave and a '0' when the value is below the halfway point.
This new signal will be a 50% duty cycle clock line, which will clock a counter. The counter will start at zero and will count peaks on the sine wave from the '1' signal generated by the comparator. The counter will be allowed to take oscillation measurements over one millisecond intervals. This can be used to determine the frequency at which the oscillator is running.
Given the desired range of oscillation (2 to 5 MHz), the expected range of oscillations per millisecond will then be (1.2562 to 3.1447) x 10^4 A clock line at this frequency will be produced which implies the use of a bit counter at a value of 14 or 15.
Using a calibration table which is written in the memory of the micro controller, an algorithm will be created to determine when the incoming frequency has been in a steady state for ten seconds. Ten seconds of stopped breathing during sleep is the standard which defines an apnea event. After ten seconds, the micro controller will cause the monitor to generate a local alarm to hopefully wake the infant. It will also activate the transmitter on the base unit to send an alarm signal to the remote receiver. Upon receiving the signal, the remote receiver will sound an alarm to wake the parents.
As a precaution, the remote receiver will sound a distinctly different alarm when the sensor batteries on the infant have died. This process will be done similarly to that which checks for an apnea event. In the case of dead batteries, a frequency will no longer be generated or transmitted. The micro controller will signal the activation of this alarm when no incoming frequency is detected.
The remote receiver will have similar analog circuitry as the base unit for receiving signals from the base unit. A power supply, power switch, speaker and volume control will be included in this component. Detailed design of the remote receiver is not yet developed but is expected to ultimately resemble the basic design of any given baby monitor. A microphone could also easily be integrated into the transmitting circuitry of the base unit which would also allow this SIDS monitor to operate as a normal baby monitor. | 