Random flickering on LEDs connected to ULN2803s

I’m using the pwm outputs of a series of PICs to dim ten strings of LEDs up and down individually.

Each PICs’ pwm signal is fed to two paralleled inputs of ULN2803s. The corresponding darlington outputs are also paralleled and fed to the LED strings which incorporate current limiting resistors. So, ten pics, ten doubled-up darlington pairs, ten strings of bright white LEDs (the ones grouped in threes, mounted on flexible tape with a self-adhesive backing).

The power supply is a 12v switch mode supply rated at 2.5A which powers the darlington chips. A small 5v voltage regulator (a 7805 regulator and a few capacitors) drops the 12v to 5v to provide volts to the PICs. The PICs and ULN2803s are all mounted on the same PCB.

Problem: When I have more than about ten LEDs connected to any of the paralleled darlington outputs, regardless of whether all ten or more LEDs are on one output or spread around the ten outputs, I get random brief flashes on most outputs. This seems most pronounced when the pwm duty cycles are around 85% of full brightness. Even when just single LEDs (with resistors) are connected to each of 8 or more outputs and those outputs are a bit above zero brightness, it provokes the random flickering - which is especially evident in any channels which are at minimum (off) level. More LEDs = more flickering.

An LED probed across any PIC pwm output/darlington input while the flickering is evident at the darlington output of that channel shows no sign of flickering - so it seems the ULN2803s are somehow at the core of the problem. I don’t have access to an oscilloscope or any test gear fancier than a multimeter.

Does anyone have any ideas on how to clean this up? I’m really new to all this so would appreciate all the help/hand holding I can get.

Jay

You better make a drawing or schematic of this. Please provide as much detail as possible about all the wires that go wherever. Also do not neglect to show how the ground/minus/negative wires are going. My first hunch is a voltage drop along the ground wires causing the Pic-pins or ULN2803 inputs to be shifting.

How much current each LED draws? The ULN has a limit on how much current it can source overall (and is not the max current of each driver multiplied by the number of drivers).

Thanks for the thoughts guys, I’ll get back with drawing and more info in a couple of days’ time when our house-full of NYE guests depart.

J

Am waiting for an administrator to tell me how to post a circuit drawing, will get it on here once I know how!

J

You attach it as an image… under Full editor, it’s the “Upload Attachment” tab under the text box.

Thanks Ross Robotics, don’t know why I hadn’t noticed it there! Duh!

Here’s the circuit diagram as a pdf.

All of the components (except the darlingtons used to power the channel 1 incandescents - actually the toy theatre footlights) are on two smallish adjacent PCBs which are mounted on spacers about 8mm below (and back to back of) a larger pcb on which all the slider pots are mounted.

Initially the master fader was simply hung between 0v and +5v rails and its output fed to the ‘top’ of the channel faders. There didn’t seem to be any flicker problem then as I recall. The folks I am building it up for asked for more sophistication in the future such as automatic ‘one touch’ dimming up and down so the concept of a serial digital signal from the master fader as opposed to a basic changing maximum voltage range was included. Provision has also been made for an on/off switch for each channel plus a dim up and down rate pot… these have been physically installed but not yet connected… hence each channel 08m2 chip currently has two unused inputs pulled to Ov, awaiting connection to those as yet unused controls.

J

Langeur, the LEDs are bright white ones, with 560 ohm series resistors, drawing as far as I can find out abt 12ma.