John LonerganThe DM9368driver is a constant current device and drives the LEDs at 20mA. Looking at the datasheet I'm thinking that's 20mA per segment which is 140mA per digit.
If I then include say 8 of these in my CPU I end up burning more than an Amp on LED's - a whole Amp!!
That's a bit rich for my needs (my SPAM-1 CPU), I don't want to burn that much juice.
This is a common cathode driver IC and in the PCB that I have on order I left a space for a resistor on the between the LED cathode and ground to limt the current.
I as a bit concerned at this approach because if one want's the segments to light up consistenty as the digits change then you really ought to have one resistor per segment. But that's a lot of work and space so I figured I'd live with a horrible flickering if needs be.
Well I just put it on breadboard and tried a driving the LEDs with the cathode directly attached to ground. Wow are they bright!
I then tried a 1K resistor on the cathode and sure enough they are a whole bunch dimmer.
The driver IC high is about 3.1v and the resistor is dropping about 1.4v with the LED dropping 1.7v forward. So with 1K at 1.4v I'm using 1.4mA for the whole device rather than 140mA. The display is defnitely not 100x dimmer.
I also tried changing how many segments were lit and was pleased to see that the brightness didn't seem to be that sensistive to the number lit. I am guessing this is because at this low forward current perhaps they are really efficient. With all 7 segments on I'll get about 0.2mA per segment and perhaps the difference between 0.2mA and 1mA at this current range is irrelevant.
I looked at a datasheet but it's graphs of intensity vs current didn't go below 1mA
Anyway looking forward to the board arriving, but a bit worried because I realised last night that I hadn't been careful enough when setting out the PCB and the pin headers to the breadboard aren't as perfectly alligned as I intended. Fingers crossed.
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I saw folk using a programmable logic device. Or I could use a I little microcontroller. Either way I'd need one with 12 gpio lines. For 4 in, 7 out plus an output line to select the digit . Plus a couple of transistors. Also an oscillator.
I'd end up being tempted to some how make it variable.
In my primitive solution the brightness is controlled by the resistor.
I do want to try GALS and microcontrollers as I've never used them. But I had this load of Drivers already.
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The other way you could get dimming is to feed a PWM signal into the display enable pin if there is one. That reduces the effective current through the LEDs by the duty cycle of the PWM signal. A 555 or a couple of CMOS gates can easily generate this signal.
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Yeah 20mA seemed crazy.
I remember as a teen doing projects where the LEDs needed like a 200ohm resistor or something like that , and when I recently started SPAM-1 I was looking at single LEDs with my boy and was really surprised that it was still glowing well at 2k and more.
But I looked at a few discrete LED data sheets and they still quote forward currents of 20mA . Presumably that's the max?
They are crazy bright at 20mA.
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The DM9368 is from an older era when LEDs were not so efficient. As you have discovered, it only takes a couple of mA for good brightness with a modern display.
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