12V output from 08M

[penovak]

Hello,
I'm a newbie with the PIC and have a hopefully simple question. I am trying to drive 4 strings of 5 LED's each, 1 from each output of the 08M. The problem I''m having is that the output is insufficient to power the LEDs. I have access to a 12V DC power source, but I'm not sure if or how I would connect that in to make this work. I am using the starters kit board.

Thank you in advance.
Paul.

 

[BeanieBots]

It's all covered in manual 3 (interfacing circuits).

 

[alexthefox]

connect the output of picaxe to transistor and drive the led with it.

 

[penovak]

Thanks for the responses. I've looked at the manual and it looks like I need a Darlington transistor to cover all 4 outputs. I don't quite understand where the 12V power supply is connected though. Will it go after the transistor, with the + being connected to the + of the LED strings, and the - to the - of the LED strings?

 

[moxhamj]

Basic circuit. A BC547 any small signal transistor will be fine - they are good up to 100mA and the leds will only be drawing 10-20mA.

The number of leds depends on the colour. Green leds drop around 2.3V so I don't think you will get more than 4. You will probably get 5 red leds though.

The dropping resistor is calculated using Ohms law V=IR where V is volts, I is amps and R is ohms. So if you used green leds, the drop is 2.3x4=9.2V. The resistor thus drops 12-9.2=2.8V. V=IR and I=20mA = 0.02. So 2.8=0.02xR. R=2.8/0.02=140 ohms. Just to play it safe, go for something a bit higher, eg 220 or 330 ohms. The brightness won't vary much.

 

[SD2100]

Here's another option using a ULN2803, the 2803 has 8 outputs so if you wanted you could add another 4 strings of leds and swap the 08m for a 18x etc.