Since the sensor outputs voltage, which is by definition an analog signal, we must convert it into digital data suitable for transmitting to ground station. We decided to use two methods of pulse code modulation or analog-to-digital conversion.
1. Voltage Controlled Oscillator (VCO) to transform voltage into frequency.
2. IC that converts voltage to an 8bit scaling factor.
We initially tried to create a VCO with a NE555 timer IC. We found later, however, after testing the 555 timer VCO circuit that the input voltage range was extremely limited. Despite our attempts to expand the input voltage range by switching resistor values and capacitor values in the circuit, we failed to change the 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 initially used PBASICs "pulsin" command to measure frequency, however found that the command was limited by a maximum frequency value. Therefore, we set out to write our own frequency counter in PBASIC.
For the IC converting voltage to an 8bit scaling factor, we used a very popular ADC0831 chip by National Semiconductor. This chip is easy to use with the BASIC Stamp and is well documented in the "Basic Analog and Digital" kit manual (FREE download) by Parallax. For PBASIC coding, one would use the “shiftout “ command.
