Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

361 leds from 1 PIC18F4550 demo board.

Status
Not open for further replies.

spook

New Member
What I'm trying to do is pretty much the same as the monome project.
Except that I want a bigger grid size (19x19) , and that I need to use it for something totally different (not as a musical instrument).
Picture below shows the monome project:

**broken link removed**

I already had a PIC18F4550 development board (manufactured by Olimex) and I have written some software so that I can steer 8 of the outputs of the microcontroller and send them to a connected breadboard where I'm now doing some experiments with the individual parts that I was planning to use.

Because the grid size is so big I do not have enough outputs to steer the 361 leds from the microcontroller at the same time. For this reason I had the idea to use a couple of counter IC's to loop through the columns of the grid. While looping through the grid I will set the value of the leds in a flip-flop IC. Each flip-flop will remember 2 values and will keep the led shining (blue or red) untill the next cycle.

I used a counter-IC (CD4017) to alternate through 10 columns with only 2 outputs (clock and reset). I also ordered flipflop IC's, to store the value of each LED after it is set.

However I started to realize that powering 361 leds at the same time can be too much for a simple USB-connection. Making them shine individually is no problem, but as soon as I put 3 or 4 leds in series already the light is almost invisible.

I hope my problem-description is a bit understandable.

How does one usually solve this problem?
Should I use a relay-kind of circuit? This seems silly because it's only some leds. But maybe because there are so many leds I should use one after all.

The flip-flops have 5V-pins, perhaps I just need to connect this to a second power source? Bah ... I just don't understand why the USB doesn't give enough current to do it entirely by itself. Or is it my microcontroller that eats most of the current?

(Note: You may have noticed that my experience in electronics is limitted :) this is basicly one of the first projects I'm undergoing.)
 
Last edited:
Check out a 74hc595..it's a serial in parallel out chip that you can chain several together.
 
Check out a 74hc595..it's a serial in parallel out chip that you can chain several together.

The led's will be steered in parallel. Let's say that each led uses 20mA. I have 361 leds, then I need a total of 7,22 amperes.
Is this correct ?

My power adapter says "3-12V 300mA" so, my guess is that's not enough. So how does one usually steer such high number of leds in a device?
(I don't think the 74hc595 has anything to do with that or perhaps I don't understand.)
 
20 mA is .02 Amp so your math is a bit off.

Mike it's 7.22 amps the OP wins the button LOL

If he blinks them at 50% duty and want's it to look like there all on he needs to plan on having 4 amps of power

This is at 12 volts
each 560 ohm resistor dissipates 224 mW
the wizard thinks 1/2W resistors are needed for your application
together, all resistors dissipate 80864 mW
together, the diodes dissipate 14440 mW
total power dissipated by the array is 95304 mW
the array draws current of 7220 mA from the source.
 
Last edited:
Thank you for some help with these calculations.

So I guess I could integrate a lithium battery in my product:
Power Cell - LiPo Charger/Booster - SparkFun Electronics

Then I can charge (for example) a 7500mAh lithium polymer battery.
That way I can keep everything powered for more than an hour.

However, this brings me to the next issue.
I took the datasheet of the LEDs.

And it says 20mA current, 40mA maximum(!).
80mW power dissipation, 100mW maximum.

So, if I go with this solution, and only 1 led is in use at a certain time, then the thing will blow?
( So... let my beginners' mind give this problem a try ... )
If the leds are not used I need to make sure that a resistor takes over.
The ones I marked in RED will fix this, correct ?

**broken link removed**
 
Last edited:
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top