Hardware
Microcontroller
The microcontroller for this project was selected from the Atmel AVR range because of their low power and low cost factors. Since the Atmel AVR microcontrollers have an internal oscillator, no external crystal is needed, thus reducing the number of components required and PCB size. Programming of the Atmel chip can be done by using the Atmel ISP (In System Programmable) cable, which can be purchased at a very low cost.
I have selected the ATMEGA8515 to be the main controller. The ATMEGA8515 has 5 input/output ports, which makes it suitable for this application where the controller needs to be interfaced with many devices. (LCD, external SRAM, sensor inputs, etc)
Port A: 8-bit data bus for LCD and Memory Module.
Port B: Address lines for memory and Cadence sensor input
Port C: Address lines for SRAM.
Port D: Enable lines for SRAM, IrDA, GPS and LCD
Port E: Push Buttons and Heart Rate Sensor Input.
Liquid Crystal Display
The display unit for this project is the alphanumeric Trident TRIMODS1535 Liquid Crystal Display module. The LCD has 16 x 2 lines and comes without backlight.
The LCD can operate in 4 bits or 8 bits of data lines depending on pin availability on the host controller. Noting the complexities that will be introduced by using 4 instead of 8 bits, I decided to implement the display unit with the standard 8 data lines setup.
Heart Rate Sensor Circuit
Transducer
The Heart Rate transducer consists of a matched Infrared Light Emitting Diode (QEE113) and an Infrared Phototransistor (QSE113), both produced by Fairchild Semiconductor. The QSE113 is a NPN Phototransistor with daylight filter to produce a more controlled testing environment and minimize interference from other unwanted light sources.
In this project, unlike most plethysmograph designs where the Infrared emitter and receiver are placed opposite each other, the pair of Infrared LED and phototransistor here are placed beside each other and the phototransistor picks up the reflected infrared rather than the infrared absorbed by the blood. This arrangement method allows for greater comfort when wearing the device over the finger. The “side looker” package (flat package) type of component was also a design factor to reduce the size of the heart rate transducer.
Signal Conditioning Circuit
The Signal Conditioning Circuit is basically working with amplifiers and configuring them to amplify, filter or produce waveforms. I have selected the LM124 Quad Amplifier package for this circuit because it is low powered and runs on a single low-level supply (minimum 3 Volts). Since the design uses quite a number of amplifiers, a Quad package also helps to reduce cost and component count.
Global Positioning System Module
The GPS module which I selected was the LS40-EB (5Volts) OEM module by RF solutions. This module uses a 10 way 2mm pitch header which is quite hard to find and therefore a custom made connector was used to instead.
The module has 10 pins, of which, only 3 are important: VCC, Ground and Data Output. My first task was to connect the module to a computer using a MAX 232 chip to monitor the type of data coming out from the GPS module.
Following the datasheets closely, I configured the GPS module to output only the required data for this project to reduce the amount of processing required by the microcontroller. The module produces NMEA-0183 sentences at 4800 baud rate with 8bits of data, no parity and 1 stop bit. Since the logic levels are the same as those accepted by the microcontroller, the Data Out pin is directly connected to the USART Receive Pin of the microcontroller.
Memory Module
One aspect of this project is data logging therefore a memory module is needed to store information as it is gathered and to be uploaded onto a computer later. The possibilities for the types of memory modules to be used are the Static RAM or Flash.
Flash Memory
Flash Memory would be ideal for this project because it costs significantly less compared to SRAM and the memory size is huge. However, a setback to using Flash would be its implementation. Flash is written in blocks and depending on the block size, a similar buffer storage would be needed to accumulate and hold the information till it is ready for transfer.
Static RAM
SRAM is easy to implement but is quite costly and the memory size is quite small. However, I decided to use a 256K Static RAM chip since this chip runs on 5 Volts, and interfacing with the main controller would be easy. The memory module I selected was the BSI Very Low Power CMOS SRAM BS62LV256.
Cadence Sensor
Transmitter
The cadence sensor is a simple device consisting of a Radio Frequency transmitter (RTFQ1 433 by RF Solutions), a magnet, a reed switch and a battery. The magnet is mounted on a pedal and activates a reed switch that is fixed to the frame of the bicycle every time it makes a revolution. When the reed switch activates, it sends a signal to the transmitter, which will in turn send out a “logic 1” signal at 433 MHz.
Instead of buying an antenna, to reduce cost, I made a 433MHz antenna from a strip of wire cut to the length of 17.3 centimeters.
Receiver
To enable the Sports Monitor to receive information from the cadence transmitter, a RF receiver tuned to 433 MHz (RRFQ1433) by RF Solutions is attached to the main controller. The output of the RF receiver is used to drive a transistor to provide a clean square wave to the microcontroller for cadence processing.
Again, a 17.3 centimeters strip of wire was used as an antenna for the RF receiver.
Wireless Communication (Infrared)
A wireless link to transfer data between PC and microcontroller is needed. A very common and low cost method would be to use an IrDA (Infrared Data Association) port. IrDA is a standard for a two-way wireless infrared light transmission of data.
The IrDA controller that I am using is the MCP2150 chip by Microchip. The clock source is an 11.0592Mhz crystal that is divided by 1152 to run the data transfer at a baud rate of 9600. The MCP2150 chip requires an Infrared Transceiver to communicate with a host computer. To cut development time, I bought an integrated package infrared transceiver by Vishay (TFDU4300) that was easily available in the market.
