transistor array

Thread Starter


Joined Jul 26, 2010
Hello, I am thinking about putting this picaxe circuit together, however, with me being a newbie, I dont understand the purpose of the Darlington Transistor Array.
It seems to me that you could just connect the Picaxe outputs directly to the LED's, instead of going through the array. Then I thought perhaps the array boosts the current, but there does not appear to be any power going into the array ?

Can someone please tell me what the transistor array does ?

thanks kindly



Joined Apr 30, 2011
The µC outputs can't source or sink enough current by themselves. It's a controller, not a driver. Each of the channels of the darlington array act like a switch controlled by the µC to connect the load (LED) to ground. This type of array is sometimes referred to as a low side (sinking) driver.


Joined Dec 23, 2010
What about checking the ULN2803 datasheet? Normally a Darlington transistor is used to drive a significant amount of current, where a normal transistor could not. However, maybe you can understand why that IC is used in the circuit taking a further reading at the link I reported.


Joined Jul 17, 2007
Each I/O pin on the PICAXE can only source or sink up to around 20mA. The way the LEDs are connected, they need a total of 80mA-100mA per channel.

Now if the problem is that you don't HAVE a ULN2003, the current requirement is low enough and the parts few enough so that you could substitute individual transistors with base resistors.

For the transistors, you could use 2N2222/PN2222, or 2N4401/PN4401's. For the base resistors you could use anywhere from 390 Ohms to 470 Ohms.