Relays are devices which allow low power circuits to switch a relatively high Current/Voltage ON/OFF. For a relay to operate a suitable pull-in & holding current should be passed through its coil. Generally relay coils are designed to operate from a particular voltage often its 5V or 12V.

The function of relay driver circuit is to provide the necessary current (typically 25 to 70ma) to energize the relay coil.

Figure 1

Figure 1 shows the basic relay driver circuit. As you can see an NPN transistor BC547 is being used to control the relay. The transistor is driven into saturation (turned ON)  when a LOGIC 1 is written on the PORT PIN thus turning ON the relay. The relay is turned OFF by writing LOGIC 0 on the port pin.

A diode (1N4007/1N4148) is connected across the relay coil, this is done so as to protect the transistor from damage due to the BACK EMF generated in the relay's inductive coil when the transistor is turned OFF.When the transistor is switched OFF the energy stored in the inductor is dissipated through the diode & the internal resistance of the relay coil.

As you can see we have used a pull up resistor at the base of the transistor. AT8951/52/55 has an internal pull up resistor of 10k so when the pin is pulled high the current flows through this resistor so the maximum output current is 5v/10K = 0.5ma, the DC current gain of BC547 is 100 so the maximum collector current we can get is 0.5ma x 100 = 50ma, but most of the relays require more than 70ma-130ma current depending on the relay that we have used, 0.5ma of base current is not suitable enough for turning ON the relay, so we have used an external pull up resistor. When the controller pin is high current flows through the controller pin i.e. 5v/10k=0.5ma as well as through the pull up resistor. We have used 4.7k pull up resistor so 5v/4.7k=1.1ma so maximum base current can be 0.5ma + 1.1ma=1.6ma i.e. collector current =1.6ma x 100 = 160ma which is enough to turn ON most of the relays.

NOTE: This relay driver circuit is to be used only with controllers for using this circuit with other digital IC's like LM 555 use a resistor should be used between that IC's output & the base of transistor.No need of  pull up resistor in that case. 

-Amol Shah

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