Need to used 74HC595 with 12vdc LEDs - ULN2803A to be used?

T&E Engineer

I have a project that I want to use an SX28 to send data to several 74HC595s. However, the LEDs are rated for 12 vdc so I can not connect them directly with current limiting resistors to the outputs of the 74HC595s. I was thinking of using ULN2803As between the 74HC595s and the 12vdc LEDs. Has anyone done this before? The ULN2803A data sheet is a bit confusing in that it states that:

"The ULN2803A has a 2.7K ohm input resistor for 5v TTL and CMOS".

However, on the Max ratings page it states: Output voltage is 50V and Input voltage is 30V.

Does this mean I can or can not use an external 12vdc power supply for the ULN2803A and LEDs

AND use a 5vdc power supply for the SX28 and 74HC595s (with grounds tied together from each power supply).

??

Here is a link to the 12vdc 80 mA LED clusters to be used: http://www.sureelectronics.net/goods.php?id=394


Post Edited (T&E Engineer) : 1/29/2009 4:09:16 PM GMT

kwinn

I have not used this module with the ULN2803, but I have used the ULN2803 often for similar circuits.
The ground pin of the ULN should be connected to the same ground as the '595, SX28, and 12V ground.
The output pins of the '595 can connect directly to the input pins of the ULN ( 2K7 resistors built in).
The output pins from the ULN go through a current limiting resistor** to the individual cathode connections of the display.
The +12V goes to the anode(s) of the display.

** Not sure if current limiting resistors are required. Check the data sheet for the display. Put them in if you are not sure.

The ULN2803 is basically 8 darlington transistors with the emitters connected together to pin 9(gnd), a separate pin connecting to each base through a 2K7 resistor, and the collectors connected to output pins. For your application (non inductive load) the diodes are not required so pin 10 can be left disconnected but there is no reason it should not be connected to +12V.

Post Edited (kwinn) : 1/29/2009 6:21:44 PM GMT