Since the sensor outputs voltage, which is by definition, an analog signal, we must convert it into digital format suitable for transmitting to ground station. We cam up with two methods of implementing the pulse code modulation or analog-to-digital conversion.
1. We use a Voltage Controlled Oscillator to transform voltage into frequency.
2. We use an IC that converts voltage to an 8bit scaling factor.
For the VCO, we initially tried to create one with a NE555 timer IC. We found, however, after testing the 555 timer VCO circuit that the input voltage range was limited and could not be expanded to our desired range. We then moved to an off-the-shelf VCO chip, the 4046 by Motorola. I will discuss the challenges of using this chip in the next slide. We started out using the pulsin command to measure frequency, however found that it was limited by a maximum frequency. Therefore, we set out to write our own frequency counter in PBASIC.
For the 8bit scaling factor, we used a very popular ADC0831 chip by Dallas Semiconductor. This chip is fairly easy to use with the Basic Stamp. One would use the “shiftout “ command in the PBASIC code.
