This circuit is intended to show a general solution for implementation a low cost ADC and a 2-way multiplexed LCD using COP840C 8-bit microcontroller. The implementation is demonstrated by means of a digital personal scale. Details and function of the weight sensor itself are not covered in this article. Also the algorithms used to calculate the weight from the measured frequency are not included, as they are too specific and depend on the kind of sensor used. This circuit is use a V/F Techniques for driving the LCD. The diagram is shown in below;
Today's most popular digital scales all have the following characteristics:
They are battery powered and use a LCD to display the weight. Instead of using a discrete ADC, in many cases a VFC is used, which converts an output voltage change of the weight sensor to a frequency change. This frequency is measured by a microcontroller and is used to calculate the weight. The advantages of a V/F over an A/D converter are multifold. Only one line from the V/F to the microcontroller is needed, whereas a parallel A/D needs at least 8 lines or even more (It also offers A/Ds with serial output). A V/F can be constructed very simply using National Semiconductor's low cost, precision voltage to frequency converters LM331 or LM331A. Other possibilities are using Op-amps or a 555-timer in unstable mode.
The principle work of the circuit is a capacitive or resistive sensor's weight related capacitance or resistance change is transformed by a 555 IC (in unstable mode) to a change of frequency. This frequency is measured using the COP800 16-bit timer in the “input capture'' mode. After calculation, the weight is displayed on a 2-way multiplexed LCD. Using this configuration a complete scale can be built using only two ICs and a few external passive components.
Today's most popular digital scales all have the following characteristics:
They are battery powered and use a LCD to display the weight. Instead of using a discrete ADC, in many cases a VFC is used, which converts an output voltage change of the weight sensor to a frequency change. This frequency is measured by a microcontroller and is used to calculate the weight. The advantages of a V/F over an A/D converter are multifold. Only one line from the V/F to the microcontroller is needed, whereas a parallel A/D needs at least 8 lines or even more (It also offers A/Ds with serial output). A V/F can be constructed very simply using National Semiconductor's low cost, precision voltage to frequency converters LM331 or LM331A. Other possibilities are using Op-amps or a 555-timer in unstable mode.
The principle work of the circuit is a capacitive or resistive sensor's weight related capacitance or resistance change is transformed by a 555 IC (in unstable mode) to a change of frequency. This frequency is measured using the COP800 16-bit timer in the “input capture'' mode. After calculation, the weight is displayed on a 2-way multiplexed LCD. Using this configuration a complete scale can be built using only two ICs and a few external passive components.
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