Max no of LED outputs

moxhamj

New Member
Max number of leds all on together - chain them up in series and parallel and run them off 12V or more via transistor drivers. But I don't think that is what you are asking (crystal ball still cloudy).

If you want to drive lots of leds and you want them all turning on and off at different times, then daisy chain any number of HC595 chips. You will get 8 leds per HC595 chip. This is fine for speeds up to tens of times per second. If you want to go faster then use a bigger picaxe chip (28 or 40 pin).

How many leds do you want to drive and how fast?
 

Rod McCrea

New Member
Sorry for lack of clarity, what I'm after is the max no of led's on at once from one output?

Thanks
 
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Rickharris

Senior Member
Sorry for lack of clarity, what I'm after is the max no of led's on at once from one output?

Thanks
Search for charleyplexing either in the forum or on Google.

For a single output the answer would be 2 LEDs - one sourcing current and One sinking i.e. one connected point to ground tail to picaxe, the other point to picaxe and tail to + volts - with suitable resistor.

The picaxe output will source 20ma so you may need a driver transistor depending on your LED current.
 

Michael 2727

Senior Member
1,000 Red LEDs @ 20ma +/- a few hundred if
you use an IRF540 on the output.
Or 5 Red Leds @ 4ma each off a plain output
(you need limiting resistors for each LED )

Still a little unclear what your requirements are,
bit like saying how fast can I go in a car ?
 

hippy

Technical Support
Staff member
If you mean how many LED's can be controlled in unison by a single PICAXE output pin with no buffering and only passive components, you are constrained by the PICAXE the current which can be drawn from the pin ( 20-25mA ).

As Michael 2727 says, 5 LED's in parallel drawing 4mA each will work. You can also connect them in series as well as in parallel then you are limited by the voltage they are driven from, the PICAXE supply voltage if connected between the PICAXE and 0V but you can have more if the LED's connect between a higher voltage and the PICAXE.

You could probably drive 15 LED's from a PICAXE running in series and parallel with a 5V supply, but there's a maximum current for all eight outputs combined so you could not do that for all pins.

The number becomes almost unlimited if you use transistors or FET's controlled by the PICAXE and have a suitable power supply.
 

BeanieBots

Moderator
Which PICAXE?
From one output, you could pull 20mA at about 5v if you crank the supply up to about 5.5v.
That would would give you about 100mW of power.
Up to you how to distribute it.
One or two quite bright or a thousand very dim, the choice is yours.
 

inglewoodpete

Senior Member
To take Michael 2727's 4-LED suggestion a little further:

4mA through a standard LED is quite bright, nearly as bright as 20mA. Since a standard red, green or yellow LED drops about 2V (give or take a bit), you can run 5 parallel chains of 2 LEDs + current limiting resistor in each chain. The calculation of each resistor will be a little tricky, depending on the supply voltage. It would be safer to have the resistor value a little on the high side to keep the current under 4mA for all operating voltages.
 
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