AN203 issue driving ULN2803

[twesthoff]

Yes, you can get them bright enough using 5V.
Just make the 390 ohm resistor smaller to make the LED brighter. Not knowing the color of the LED makes it hard to tell you what resistor value to use, but if you use a resistor value that is 1/2 the value you have, the LED will be roughly twice as bright as it is now.  Making the resistor value 1/4 what it is now will make the LED 4 times brighter.  So in this case 390/2=195 so you can use the closest standard value of 180 ohms.

Of course if you make the resistor too small you can burn out the LED...  If you want to quickly test this idea, just put another 390 ohm resistor in parallel with the one you are using.  Two equal value resistors in parallel have 1/2 the resistance.

On 4/20/2010 9:42 PM, General wrote:

[G Herzog]

Success..... just a follow up report

I am now running four 12volt Finder relays (type 44.62) rated at 55ma each on a cycle of 1 second ON in an endless loop each driven by a separate pin on the ULN2803.

The four unused inputs are grounded. No pull ups or pull downs on the active UNL2803 inputs.

I intend to leave this running for a week to determine stability. Also running is a NetMedia Serial LCD display with real time display of time and date including backlighting.

Conclusion - all of my problems were due to not fully understanding the configuration of various hardware components. The ULN2803 is quite straight forward for 12volt relays being fed 13.5 volt supply. I suspect that 5volts is marginally low for the Darlington pairs.
These particular 12 volt relays will function between 8 and 18 volts DC. The goal is to have this eventually powered by an automotive supply ranging from 14.5 to 10 volts depending of what the alternator is providing.