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ULN2803 Darlington Array

I would hate to count all of the transistors that I have purchased. Let's say more than hundreds! If you don't like collecting (a little joke) transistors then maybe this isn't the backwater of the internet for you.

Of all of the transistors in my collection, the SS8050 continues to be my favourite - but sometimes you need more power (mosfet?), and sometimes you might need extra goodies that make life easier for some applications.

So one of my "other" favourite transistors is actually an IC chock full of transistors called the ULN2803 - and it has some great features that make it a perfect choice when dealing with some situations such as large inductive loads (e.g. a motor), or maybe you just want to draw heaps of current in an easy "monolithic" way.

Let's say you have a motor kicking along at 12V and drawing 500mA current. Your microcontroller is going to spit the dummy real early for a couple of reasons. For instance the typical AVR (I use the Attiny13a way too much) cannot output that much current (40mA upper limit) from it's pins. Then when the motor stops and you get a bit of inductive kickback - the AVR could easily fry.

You can increase the current available by using a transistor as a switch and also you could protect your μC through the use of a flyback diode and all the other blah blah bits to make the circuit work OR you could just chuck a ULN2803 into your circuit which includes all the goodies shown in this block diagram as follows:

So many components for your viewing pleasure

The Darlington Pair at the heart of the block diagram (and remember there are 8 inputs and 8 corresponding outputs just like the one above on this single lovely chip) allows a total output current of 500mA. Because there are multiple inputs and outputs you can add as many inputs and outputs together as you need to get more current! Two inputs, parallel linked, gives 1A via parallel outputs, 3 gives 1.5A and so on. Need more power? No problem!

So the task for this week is to output some bulk mA and drive two 3W LEDs from a simple square wave generated by our "other other" favourite IC the 555 timer. As the timer kicks out a high, the ULN2803 should open the gate on heaps of current to simultaneously light the 3W LEDs enough to give us all a nice tan in the middle of winter.

The circuit is as follows:

The timer part - just providing a square wave out of Pin 3

More power on this side with the signal coming in to Pin 8

In real life

There are a couple of cute "gotchas" with this circuit. The first is that the positive or anode side of the LED connects directly to the "COM" pin on the ULN2803 and the ground lead plugs (via a resistor) into the "output" pin. You'd expect given the pins are called output that it would be the other way around.

The second is that those LEDs I've plugged in get pretty warm - it's OK for intermittent flashing but really they should have some form of heatsink if the current is large and constant.