PWM multiple 7-segment LEDs

[Demon]

I would use what Maxim suggest for the displays and would not bother with mods to drive common anodes. ...

Yeah, I wasn't going to keep fighting against the current.


I'm looking at using PICs instead. Concatenating values and remembering which one is in which position is going to be a massive chore, with lots of place for error.




I'm looking at using PICs instead. My 7-segment displays will range between 2 and 6 digits. I just send each PIC a USART message with the value they have to display and forget about them until the value changes.

This is why I had gotten common anode in the first place. The power to the LEDs was going to be controlled by pot & PWM from a main PIC. I was planning on using transistors to control the 8 segments. It's cheaper to use MAX7219, but in my case I don't think it's worth it.

Another thing consider is that this is the "basic" console; the Cessna 152 is one of the simplest planes with a single engine in MSFS 2020. I plan on making consoles for much more complex planes, most having 2 - 4 engines. That's a truckload of displays; that's why USART-and-forget is my favorite technique so far.

I started looking at 16F PICs on JLCPCB. I forgot about those and had only looked at 18F PICs the first time. So far I'm looking at:

- 14-pin 16F1823 @ $1.16
- 28-pin 16F1933 @ $1.51

And I'm just starting, I'm pretty sure I saw some 44-pins in there. I can easily control 6 digits with that and a few transistors.

[tumbleweed]

This is why I had gotten common anode in the first place. The power to the LEDs was going to be controlled by pot & PWM from a main PIC

Wouldn't it be simpler with common cathode? That way you only have a single sink transistor/fet for each set of digits, making it easy to PWM.

[Demon]

Wouldn't it be simpler with common cathode? That way you only have a single sink transistor/fet for each set of digits, making it easy to PWM.

Hmm, is sink more efficient than source?


EDIT: Gotten from a quick google:

'While this may sound strange or non-ideal, it is generally easier for the microcontroller to shunt this current to ground and absorb a little power as heat rather than try and generate the sourcing current. Sourcing current can cause large voltage swings on the input which can introduce noise.'

Interesting, I never really put a lot of thought in this before.

I suppose that's because the PIC didn't have to source the power consumed by the device, only the leftovers.

Where-as if it has to source, it has to source everything. Never thought of that.

[Demon]

PWM is done at a higher level by a central PIC, so all LEDs across the console get the same VDD-LED.

My largest display has 6 digits with 1 decimal point. This was my most recent gameplan. I can use smaller PICs for smaller displays:




EDIT: Right now JLCPCB has 500 units of 16F946 in stock for $2.15, 2889 units of the 16F1947 for$2.21.

When you consider the MAX9217 costs me $20 for each DIP for my testing, I'll need 20 to test the entire console.

Using PICs in this case is not much more expensive and so much more affordable and convenient, especially when it comes to transmitting the values to the displays.

[richard]

i see a lot of smoke in your future

apart from the upside down mosfet

pic1f946 max chip current 95mA although each pin can sink or source 25mA equally well
@ 42 leds [or 48] if you use dp's thats 2.2mA per led 2 Full bright , the 270 ohm seg resistors are not going to do that for you

pic1f1947 max chip current 120mA [the industrial ver can sink more] although each pin can sink or source 50mA equally well
@ 42 leds [or 48] if you use dp's thats 2.8mA per led 2 Full bright , the 270 ohm seg resistors are not going to do that for you

[Ioannis]

The sink is always better choice than sourcing.

Then N-channel MosFets along with NPN transistors are much better than P equivalent ones and more choices available.

And finally, even if a PIC can source or sink 20, 30 or 50mA per pin, there is a maximum TOTAL dissipation for the whole chip, so that you cannot source at the same time on all pins a max of 50mA on each pin!

You will melt the chip!

So, yes. I am for the sink topology too.

Ioannis

[Demon]

(I'll move the PWM LED posts to an off-topic thread in a few minutes to keep this about Chuck's code example. I just want to put down some comments first.)

... apart from the upside down mosfet ...

I was hoping you wouldn't see that. It was late and I went to bed.

i see a lot of smoke in your future ...

Usually I'd agree with you, and yes I'm aware of the current limitations on PICs and pins.

But ... LEDs are special. You can get different performance out of them via PWM. This is identical LEDs:

- left, 5V rail with 270ohm resistor.
- right, PWM without resistor.




I can get identical brightness with 2.5mA less. If I add a 270 ohm resistor to PWM, I get 1/2 the current.




Yeah, I thought I could improve this even further down to 2mA but that isn't gonna happen. The LED won't be bright enough.

And yes, I've been playing with using transistors/MOSFETs and I'll be going back that way again.




I had dropped that idea cause HPWM has lower limits on Frequencies and I didn't like not have total control (64 MHz is 3905Hz).

But I just realized I could pulse the devices using a timer interrupt; switching between different ON-time and OFF-time.