ULN2803 driving LEDs with common cathode

[Andres Rodriguez]

The photo shows an RC airplane propeller with a number of surface mounted LEDs. The purpose is to create a POV sign as the propeller rotates.
The problem is that I soldered the LEDs with a common cathode. I did this before I realised that the total current would exceed the max tolerated by the 28X1. To handle the current I added the ULN2803 (see attached schematic) and now I don't think that the common cathode set-up will work.
If there is a way to avoid reworking the propeller I would appreciate your input.
I thank you in advance

 

[alphamike27]

Place the series resistor from +DC to the anode of the LED, and connect the cathode to -DC.

Connect the point between the resistor and the anode to the output of the ULN2803.

When the input to the ULN2803 is high, it will pull this point to ground and turn the LED off.

When the input to the ULN2803 is low, the LEDwill be illuminate.

.................................

 

[Andres Rodriguez]

Place the series resistor from +DC to the anode of the LED, and connect the cathode to -DC.

Connect the point between the resistor and the anode to the output of the ULN2803.

When the input to the ULN2803 is high, it will pull this point to ground and turn the LED off.

When the input to the ULN2803 is low, the LEDwill be illuminate.

Thank you for that advise, I modified the schematic.
Is this right?

 

[Technical]

A better way may be to just use a UDN2981A instead, which is the 'opposite polarity' of a ULN2803 (sources rather than sinks)

http://uk.farnell.com/allegro-microsystems/udn2981a-t/driver-led-relay-2981-dip18/dp/1193495

 

[MartinM57]

Thank you for that advise, I modified the schematic.
Is this right?

YMMV, but the 2803A doesn't pull the output directly to ground - there is something like 1-1.2v Vce(sat) which means, depending on your LEDs, they may still be dimly lit.

I'd recommend a breadboard prototype before you commit to your circuit/any PCB layout....

 

[westaust55]

UDN2981A = opposite of ULN2803 (sources rather than sinks)

Futurlec also have the UDN2981A in US (US$2.20ea) and Australia (AUD$2.97) at US$2.20ea.
All dispatched from a warehouse in Thailand for just a couple of dollars. So if not looking for overnight delivery worth a visit

http://www.futurlec.com/cgi-bin/search/search.cgi

By comparison, Farnell in Australia price AUD$5.27ea

 

[Andres Rodriguez]

Westaust,
Thank you for the info.
In this application I need to use a SOIC package and I found the same IC from Allegro in that package. Digikey carries it (US$ 2.03) and I just ordered it.