Plug n' Play usb switch - arduino

Question

I'm currently developing a hardware electronics application and I want to be able to easily toggle my circuit with a USB cable. All I want to do is have an arduino or other device that I can just plug into any computer, run a small application and toggle one wire to 5v. This would be extremely practical for lots of small applications.
Question:
-I want a small device that I can plug into any computer and toggle a 5v wire with a small application to control electronics. How do I do this?

Answer 1

If your MCU doesn't have USB frontend you can use assembler, eg. http://www.cesko.host.sk/IgorPlugUSB/IgorPlug-USB%20%28AVR%29_eng.htm This allows to implement simple USB1.1 low-speed device. If you need high-speed your MCU should have USB engine built-in and then you probably find proper application note for USB from MCU's manufacturer.
Let's notice that depends on your device class you need or not to write your own driver. You can use HID, serial or CDCACM class. Finally you can just use FTDI's bridge chip http://www.ftdichip.com/

Answer 2

I've got some code for that over here. I use a simple transistor IRFU5305 where i attach one lead to a pin of the arduino and the other 2 bridge the power line of the usb.
Depending if you're using n or p channel transistor you may need to adjust the code.
ArduinoCode:
String inputString = ""; // a string to hold incoming data
boolean stringComplete = false; // whether the string is complete
void setup()
{
pinMode(8, OUTPUT);
Serial.begin(9600);
// reserve 20 bytes for the inputString (input is max 20 chars long)
inputString.reserve(20);
Serial.print("waitingforinput");
digitalWrite(8,true);
}
void loop()
{
if (stringComplete)
{
Serial.println(inputString);
inputString.trim();
if(inputString=="F8")
{
digitalWrite(8,true);
}
else if(inputString=="T8")
{
digitalWrite(8,false);
}
inputString = "";
stringComplete = false;
}
delay(500);
}
void serialEvent()
{
while (Serial.available())
{
Serial.println("inputreceived");
char inChar = (char)Serial.read();
inputString += inChar;
// if the incoming character is a newline, set a flag
// so the main loop can do something about it:
if (inChar == '\n')
{
stringComplete = true;
Serial.println(inputString);
}
}
}
Pythoncode:
import serial
import time
import os
ARDUINOPORT = os.getenv('ARDUINOPORT', None)
def send_string_to_arduino(cmd):
"""sends a certain command string to the arduino over Serial"""
print ARDUINOPORT
ser = serial.Serial()
ser.port = ARDUINOPORT
ser.baudrate = 9600
ser.parity = serial.PARITY_NONE
ser.bytesize = serial.EIGHTBITS
ser.stopbits = serial.STOPBITS_ONE
ser.timeout = 10
ser.xonxoff = False
ser.rtscts = False
ser.dsrdtr = False
ser.open()
time.sleep(2)
ser.readline()
ser.write(cmd+"\n")
ser.close()
def switch_pin_on(pinnumber):
"""switches the pin with pinnumber(int) of the connected Arduino on"""
cmd = "T"+str(pinnumber)
print cmd
send_string_to_arduino(cmd)
return
def switch_pin_off(pinnumber):
"""switches the pin with pinnumber(int) of the connected Arduino off"""
cmd = "F"+str(pinnumber)
print cmd
send_string_to_arduino(cmd)
return
def set_arduino_port(comport):
"""sets to which comport the arduino is connected to, comport should be a string ("COM2")"""
global ARDUINOPORT
ARDUINOPORT = comport
return
if __name__ == '__main__':
set_arduino_port("COM17")
switch_pin_off(8)
switch_pin_on(8)