ULN2803: all about the Darlington transistor pair

ULN2803

If you are working with transistors, there is probably a combination of these semiconductor devices that will interest you. This is the pair of transistors known as Darlington. This configuration is quite interesting for many DIY electronic projects, and you can find them cheaply in the IC ULN2803.

You will be able to find the ULN2803 manufactured by different companies, like the mythical Texas Instruments, or the European STMicroelectronics, etc. And in this guide article, I will try to solve all the doubts about this product, tell you where you can buy it, and how to work with it…

What is the ULN2803?

The ULN2803 is a chip, an integrated circuit with a traditional DIP packaging, like so many others. That is, with two stacks of pins at its sides. Well, up to here it may seem like so many others, but inside it does not have conventional logic gates, multiplexers, filters, current sensor modules, shift register, nor a microcontroller…

Inside the ULN2803 you will find drivers, with a series of transistors, some devices of which I have already spoken in other occasions with different types like: MOSFET, BC547, 2N3055, 2N222, etc.

What is the transistor or Darlington pair?

The Darlington transistor is not a transistor as such, but a pair of them connected in a very specific way. Two connected bipolar transistors will form the Darlington pair, which allows the current amplified by the first transistor to enter the base of the second transistor and be amplified again.

This type of amplification was used with two separate transistors, but a Bell Labs engineer named Sidney Darlington patented the combination in 1952. The idea was to put two or three transistors in a single monolithic pickup. A similar idea to that of creating a chip or integrated circuit, although this achievement is not recognized to him, as you well know…

The Darlington pair behaves as a single conventional transistor, i.e. after the combination of the two transistors it still has a single base, a collector and an emitter. Only the current gain will be the combined one, and therefore higher than using only one transistor. Specifically, the gain on the Darlington is considered to be approximately the result of the product between the two gains of the transistors used separately.

The advantages of using this Darlington pair is clear, you get a great current gain. This allows you to control higher magnitude currents with a small base current. But it also has its disadvantages, such as a higher phase shift at high frequencies than when using a single transistor, which makes them somewhat unstable for use in circuits with negative feedback.

And it is not the only problem associated to the Darlington pair, since the voltage drop between the base and the emitter is greater due to the double junction (equivalent to the sum of both drops of both junctions).

The saturation voltage they have is also another limitation. On a practical level, this means more power dissipation, i.e. more heat. And continuing with the disadvantages, the reduction of switching speed is another limiting factor, and cannot be used in circuits where more agility is needed. The first transistor cannot actively inhibit the base current of the second one, which slows down the shutdown .

These Darlington transistors can be found either encapsulated separately, i.e. only one pair, or in integrated circuits with several Darlington transistors as is the case with ULN2803.

ULN2803 datasheet and pinout

The operation of the ULN2803 is very simple and its assembly is also very easy. This integrated circuit has a set of 8 inverter doors implemented by a Darlington transistor, in this case using NPN transistors. This makes it possible to connect other devices that have a strong current demand, such as stepper motors, to its pins, as the motors, driver, relays, etc.

Therefore, the ULN2803 is a very versatile c circuit that can be seen in many manufacturers‘ projects as an output of digital circuits to drive actuators, motors of various types, and other components. All of them can be handled with a low current, admitting high demand currents like 500 mA or 0.5A, which for electronics is a very high value.

It admits power supply and digital output voltages up to 50v, to transform 5v TTL digital signals to any voltage up to 50 volts. This practical function is what is known as a driver, that is, it acts as a type of element that isolates as if it were an electronic barrier, protecting digital logic circuits from others that require higher voltages and currents.

You can see all the complete features and pinout in the datasheet of the manufacturer. For example, here are two of the most common ones

Price and where to buy

It’s not too complicated to find if you know where to look. Its price is cheap, and you can even buy it in ULN2803 chipsets if you need several. For example, one of the cheapest is the that you can buy here for about 1

First project with a ULN2803

Only 3 of the ULN2803 drivers have been used in the video, but you can use all 8 to get more levels or greater accuracy from your home level meter. Although less might be enough for your project…

One of the main simple circuits that are usually made with the ULN2803 to show its behavior is a home water level meter. It is very simple, thanks to its 8 inverter doors made by Darlington and using about 8 resistors of 10k and other 560 ohms, and also other 8 LEDs, you can have the meter ready. You can add a buzzer, or an Arduino board so that when it reaches a certain level you can program the microcontroller to do some action, like cutting a valve, etc. The combinations are very high.

The assembly as you can see is very simple too. The low current demand at its input (high impedance) of the ULN2803 to activate the conduction of the transistors makes it possible to immerse the conductors connected to the chip in a water tank and the conductivity of the water itself is sufficient to use the electrical signal to activate them.

This would not work with distilled water, i.e. pure, it needs to have dissolved minerals like tap water to have some conductivity. Contrary to what many people think, water is a bad conductor of electricity, it is its dissolved minerals that conduct. Therefore, the more impure the water, the more it conducts… [/ highlighted]

That way, as the water reaches the different levels of each of its 8 drivers, it will activate the LEDs at its output and the buzzer will beep when the water tank is filled.

As an additional idea, you can connect each of the outputs to the Arduino input pins, so that when it reaches the first level you can program a sketch to do X action, when it reaches the second level Y action, and so on. Another option is to use relays instead of LEDs at the output, which would allow you to drive or control higher power circuits or devices depending on the level you reach, such as electrovalves or electrically controlled valves.

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