An Isolated MOSFET Serial Port Relay Controller
Using DTR (data terminal ready) and RTS (request to send) pins of a PC serial port (RS-232) without actually using the serial data pins, we can interface at least two relay devices with a computer. Windmeadow Labs has an excellent article on how to achieve this using a bipolar transistor (BJT). Here I will show you a similar relay control circuit built using an opto-isolator and a MOSFET.
Here is the circuit diagram:
An Isolated MOSFET Serial Port Relay Controller
The benefit of using an opto-isolator is that the rest of the controller circuit is totally isolated from the serial port and thus prevents the port from accidentally being damaged by wrong wiring or current surge due to a failed component. And by replacing the drive circuitry from using a BJT to a MOSFET, the switching capability is improved dramatically, making switching higher current load more reliable and more efficient.
Here is a picture of the finished board:
Circuit Board
And here is a simple program that toggles the relay on and off at an interval of 0.5 second.
#include <sys/ioctl.h>
#include <fcntl.h>
#include <stdlib.h>
#define _SERIAL_PORT "/dev/ttyS0"
int main(int argc, char **argv)
{
int fd, status = 0, cmd = 0;
if ((fd = open(_SERIAL_PORT, O_RDWR | O_NDELAY)) < 0) exit(1);
while(1) {
if (cmd) cmd = 0; else cmd = TIOCM_DTR;
ioctl(fd, TIOCMSET, &cmd);
ioctl(fd, TIOCMGET, &status);
if (status & TIOCM_DTR) puts("ON"); else puts("OFF");
usleep(500000l);
}
close(fd);
}
hi kerry,
i m a student of electronic communication and i have a very low knowledge on computer programming.
i have an electronic project that required me to turn on and off an light and buzzer by using serial port from dos window.
from the above project it is the perfect circuit for me because of the cause and the functionality. but i could not get it to run using the script you give above. is the above script written in c++ or linux? can i run those script in dos? or what software do i have to use to complier it?
please help me out here, many thank in advance.
Thank,
Alan
The code in the article above is in C and it was compiled under Linux. Unfortunately, this particular code will not run in DOS or Windows, but if search on the internet you should be able to find the equivalent. For example, a quick google search returned this link (http://www.codeguru.com/cpp/i-n/network/serialcommunications/article.php/c2503) which should help getting you started.