Controlling Relays using Shift Registers

Projects Interfacing w/ Software on the Computer
1

I have been trying to use shift registers (74HC595) in one of my projects and I am using daisy chaining in the same since I have to control switching of 9 devices.

I only have 3 GPIO available on my ESP32 board, need to control 9 relays and 3 LEDs using 2x74HC595 Shift registers.

The issue I have is, no matter what I try, the relay switches do not seem to work as expected.

Ideally, a shift register works in this way.

//Pin connected to ST_CP of 74HC595
int latchPin = 1;
//Pin connected to SH_CP of 74HC595
int clockPin = 12;
////Pin connected to DS of 74HC595
int dataPin = 0;


void setup() {
  pinMode(latchPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(dataPin, OUTPUT);
}

void loop() {

    digitalWrite(latchPin, LOW);
    shiftOut(dataPin, clockPin, MSBFIRST, B10010100);
    digitalWrite(latchPin, HIGH);
}

When I use "B10010100" as the value, my understanding is out of 9 relays, I should be able to control 8 using above code.

With B10010100, I should have below configuration.

  • Relay 1 - ON
  • Relay 2 - OFF
  • Relay 3 - OFF
  • Relay 4 - ON
  • Relay 5 - OFF
  • Relay 6 - ON
  • Relay 7 - OFF
  • Relay 8 - OFF

But, I have spend atleast 2 days trying to figure out, why am I not able to control them in the way I expect. I am still able to control them using 595's, using different codes available on internet (mostly LED), which means the schematic is right.

I have attached a wiring diagram, with just one Shift Register, to make it simple to understand, and finally the three transistor on the left have their Emitter connected to relays. I have 9 of such transistor and 9 relays connected.

ESP32-595.JPG
ESP32-595.JPG1467×616 146 KB

ESP32-595.JPG
ESP32-595.JPG1467×616 146 KB

2

the relay switches do not seem to work as expected.

Please describe exactly what does happen?

3

The order of the relay being switched off/on is just unpredictable.

11001100

Should have first, second, fifth and sixth switched ON, but in reality, I have 3,4,7 and 8 switched ON. Which does look straight forward, where 0 means relay is ON.

However,

  • if I used, 00110011, then I only have 1,2 and 5th switched ON.
  • if I used, 01111111, then I only have 1,2,3,4,5 and 7th switched ON.
  • if I used, 00000001, then I have 1 switched ON, which is right, but then first logic becomes wrong
4

cyberpks:
11001100

Should have first, second, fifth and sixth switched ON, but in reality, I have 3,4,7 and 8 switched ON. Which does look straight forward, where 0 means relay is ON.

I looks to me as if "1" means "relay is ON", and they are numbered 8,7,6,5,4,3,2,1 !!

cyberpks:

  • if I used, 00110011, then I only have 1,2 and 5th switched ON.
  • if I used, 01111111, then I only have 1,2,3,4,5 and 7th switched ON.

Looks like 6th is not working.

cyberpks:

  • if I used, 00000001, then I have 1 switched ON, which is right, but then first logic becomes wrong

Which is correct: relay 1 is connected to the least significant bit (LSB)!

You should check this by activating one relay at a time, like this (pseudo code): send 00000001 to the shift register, delay(1000), send 00000010, delay(1000), etc. up to 10000000.

5

Erik_Baas:
Looks like 6th is not working.

Actually, yes!! :open_mouth: I did have that feeling earlier but I checked most components and they worked well. However, the transistor was faulty. Thanks a lot for pointing this out. I changed it and its working now.

1 last thing, if you can please help.

How do I control the other 595 with this code,

I can only send 8 bit at a time. But I need to send it to next chip now. Its connected in daisy chaining format.

6

In pseudo code:

latchPin LOW
shiftOut 8 bits for the second(!) 595
shiftOut 8 bits for the first(!) 595
latchPin HIGH

At least, I think that's how it works, I never actually used it. :wink:

7

Yeah, other people on the internet have used it in the same way, but for me, it seems like its just overwriting the data on first one.

8

Exactly how are the 595's connected?

9

Exactly this way - daisy chaining 74HC595's - General Electronics - Arduino Forum (credits to the OP)

10

Really? I see 3 wires connected to pins 8, 9 and 10, but your code says:

int latchPin = 1;
int clockPin = 12;
int dataPin = 0;

Oh well, as long as clock and latch are connected to both 595's, and data_out from the first is connected to data_in of the second 595.

Are you aware that the first 8 bits go to the second shift register? Just checking... :wink:

11

Oh wait, that's an Uno. Now I'm lost...

12

Erik_Baas:
Really? I see 3 wires connected to pins 8, 9 and 10, but your code says:

Yes, I mean the code connections were similar, but the pins on the MCU would be different, since the OP used an Arduino but I'm using ESP32 :slight_smile:

Anyways, I figured out the issue. I missed one connection on Pin 8 (GND) of the second 595.

This fixes all of it!! Awesome!

Really appreciate your help @Erik_Baas :art: . You've been a great :). Mostly it was my ignorance, but somehow posting it here helped a lot. I already wasted 2 days trying to figure out the issue, but I fixed it within a few hours here.