Technical Tips


This project is for connecting analogue signal to serial port. Included a VB6 software, PIC assembly, C code and a circuit diagram. The PIC12F675 micro controller is programmed to convert the analogue input to 10 bit digital and transmit it in RS232 protocol to the serial port. The microcontroller is powered by the signals of the serial port, so no external power supply is needed.
You can read more about serial interface in

Software includes PIC code and VB6 code (20KB). You are free to use the circuit diagram and the software with no limitations.

The VB6 application is a combined digital and analogue voltmeter.

Circuit Description
See also Technical Tips

Pin 4 at the serial port is turned to +12V by the software enabling RTS. It is regulated to 5V by 78L05. The 5V is also the reference voltage.
Pin 3 is at -12V and pin 7 at +12V constantly and they are used to drive RX input between -9V (input HIGH) and +8v (input LOW) . These levels are sufficient to drive the RS232 input.
Pin 4 at PIC12F675 is reset and pin 2 is output. The other pins are not in use but must be left open circuit.