I have the following relay board (pictured below), and to activate any of the relays, you simply connect one of the IN1-IN8 pins to ground. Easy. The current going between gnd and INx is negligible, because of the transistors already on the board, so the current going through the serial port of the PC is not an issue.
How can I electronically use the serial port to to activate a relay on this board? I'm not concerned about the software aspect of it. The standard serial port has DTR and RTS (pin 4 and 7) that can be set high or low through software. I believe that "high" on a serial port is -3 to -25 volts and "low" on a serial port is +3 to +25 volts.
Is there some easy way to use that to set IN1 to GND? Look at the schematic below, and see if there is an easy way to accomplish this:
By the way, VCC and JD-VCC are connected together with the jumper, I'm not doing high voltage or concerned about isolation.