Application Examples - Driving LEDs

This area of the Mirrorbow website provides applications information and answers some of the more frequently asked questions from customers 

Driving LEDs

The pins of the USBIO25 and EthernetIO board are both capable of driving small LEDs, the logic output from the board should be connected through a resistor (minimum 330R) to the anode of the LED, and the cathode (normally denoted by a flat or dot on the LED) connected to 0V. The small test LEDs found on the USB25IO board and Ethernet IO board are connected through a 1kR resistor. The lower the resistor value the higher the current and the brighter the LED becomes. The 330R resistor limits the current to around 15mA which is within the IO pins capability. Remember that 100mA is the total current allowed to be drawn from all the IO pins, so you cannot have more than 6 LEDs connected this way with 330R resistors.

Increasing the current capability: You can increase the driving current capability by using a buffer transistor or IC. You can use a number of logic ICs for the purpose, such as the 74HC04 (this has 6 NOT gates so will invert the signal) or 74HC244 which is an 8 bit buffer, its up to you. For driving an LED with a larger current you can use a simple transistor (see diagram below), dont forget you still need a resistor in series with the LED otherwise it will be very bright for a short time then stop.

Driving LEDs from a higher voltage: You can use a transistor as shown below to power an LED from a higher voltage. Alternatively you can use a driver chip such as the UDN2981A. This chip has a separate power supply input and is also used for driving relays.