Arduino : How To Connect Relay With 74HC595 Shift Register

The reason of publishing this article is to warn. It is common to join ideas for a new thing. It is more common to ask how to connect relay with 74HC595 shift register in Arduino circuit. It demands good knowledge with basic components to control AC things like multiple AC operated LED bulbs. It is not really joining our Arduino relay guide to blink single AC bulb and Arduino 74HC595 Shift Register guide to blink multiple DC LED. Output current from 74HC595 is too high for the magnet of the relay.

How To Connect Relay With 74HC595 Shift Register?

There are few options depending on safety, complexity of circuit etc :

1. Using full DC circuit and avoiding AC (less complex, most safe)
2. Using 1-2 relays (less complex, safe)
3. Using ULN2803A chip with 74HC595 (slight complex for the beginners, somewhat safe)
4. Using 74HC595 and full calculation to determine the values (highly complex, not safe for the beginners)
5. Using ready to use boards with relays and 74HC595 (easiest and safe for the beginners, suggested)

What is ULN2803? As the output from 74HC595 is high, we need to add a transistor / resistor to the output of the 74HC595. That is not complex matter when the number of relay is 1-3. It becomes complex as the number increases. Also, increasing number increases the chance of accidental error. ULN2803 is a low cost chip which have eight channels ready to the work of resistor, diode and the transistor. If you want to create circuit with basic components instead of ULN2803, then this website and the calculator will be helpful :

Does it means blinking a chain of AC operated bulb or AC operated LED impossible by a beginner? No. The safest way is to find ready to use boards which has relay, 74HC595 and the components.

There are one type of 8 channel relay boards has 10 pins (8 + 5v + GND) to directly connect with 74HC595. These are common. Like this :

Some of the 8 channel relay modules has 74HC595 already mounted on the relay board decreasing the need of lot of wiring and calculation. They need to connect with Arduino with 3 + 5v + GND. They have a 16 on-board relay variant. There are hobbyists in some countries who sell custom things (we found from Arduino forum) :

Even if you do not live closer to them, you can ask them to arrange such. There are PCB fabrication companies who will bride the geographical gap. Some of those peoples these days contact them and tell you total payable bill.

Conclusion

8 channel relay boards with 74HC595 are quite safe and easy to use. Not only they are easy and safe, their components are tested. It is practical to think to control bigger AC operated LED for decoration.