Chapter 15 74HC595 & LED Bar Graph

We have used LED bar graph to make a flowing water light, in which 10 GPIO ports of ESP32 is occupied. More GPIO ports mean that more peripherals can be connected to ESP32, so GPIO resource is very precious. Can we make flowing water light with less GPIO? In this chapter, we will learn a component, 74HC595, which can achieve the target.

Project 15.1 Flowing Water Light

Now let’s learn how to use the 74HC595 IC chip to make a flowing water light using less GPIO.

Component List

ESP32-WROVER x1		GPIO Extension Board x1
Breadboard x1
74HC595	Resistor 220Ω x8	Jumper M/M x15	LED Bar Graph x1

Related knowledge

74HC595

A 74HC595 chip is used to convert serial data into parallel data. A 74HC595 chip can convert the serial data of one byte into 8 bits, and send its corresponding level to each of the 8 ports correspondingly. With this characteristic, the 74HC595 chip can be used to expand the IO ports of a ESP32. At least 3 ports are required to control the 8 ports of the 74HC595 chip.

The ports of the 74HC595 chip are described as follows:

Pin name	GPIO number	Description
Q0-Q7	15, 1-7	Parallel data output
VCC	16	The positive electrode of power supply, the voltage is 2~6V
GND	8	The negative electrode of power supply
DS	14	Serial data Input
OE	13	Enable output, When this pin is in high level, Q0-Q7 is in high resistance state When this pin is in low level, Q0-Q7 is in output mode
ST_CP	12	Parallel Update Output: when its electrical level is rising, it will update the parallel data output.
SH_CP	11	Serial shift clock: when its electrical level is rising, serial data input register will do a shift.
MR	10	Remove shift register: When this pin is in low level, the content in shift register will be cleared.
Q7’	9	Serial data output: it can be connected to more 74HC595 in series.

For more detail, please refer to the datasheet on the 74HC595 chip.

Circuit

The circuit of this project is similar to the one in the last chapter. The only difference is that the photoresistor is replaced by the thermistor.

Schematic diagram

Hardware connection

If you need any support, please contact us via: support@freenove.com

Code

In this project, we will make a flowing water light with a 74HC595 chip to learn about its functions.

Move the program folder “Freenove_Ultimate_Starter_Kit_for_ESP32/Python/Python_Codes” to disk(D) in advance with the path of “D:/Micropython_Codes”.

Open “Thonny”, click “This computer” -> “D:” -> “Micropython_Codes” -> “15.1_Flowing_Water_Light”. Select “my74HC595.py”, right click your mouse to select “Upload to /”, wait for “my74HC595.py”to be uploaded to ESP32-WROVER and then double click “Flowing_Water_Light.py”.

15.1_Flowing_Water_Light

Click”Run current script” and you will see that Bar Graph LED starts with the flowing water pattern flashing from left to right and then back from right to left. If it displays nothing, maybe the LED Bar is connected upside down, please unplug it and then re-plug it reversely.

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The following is the program code:

import time
from my74HC595 import Chip74HC595

chip = Chip74HC595(14,12,13,5)
# ESP32-14: 74HC595-DS(14)
# ESP32-12: 74HC595-STCP(12)
# ESP32-13: 74HC595-SHCP(11)
# ESP32-5 : 74HC595-OE(5)

while True:
    x=0x01
    for count in range(8):
        chip.shiftOut(1,x)
        x=x<<1;
        time.sleep_ms(300)
    x=0x01
    for count in range(8):
        chip.shiftOut(0,x)
        x=x<<1
        time.sleep_ms(300)

Import time and my74HC595 modules.

import time
from my74HC595 import Chip74HC595

Assign pins for ESP32-WROVER to connect to 74HC595.

chip = Chip74HC595(14,12,13,5)

The first for loop makes LED Bar display separately from left to right while the second for loop make it display separately from right to left.

x=0x01
for count in range(8):
    chip.shiftOut(1,x)
    x=x<<1;
    time.sleep_ms(300)
x=0x01
for count in range(8):
    chip.shiftOut(0,x)
    x=x<<1
    time.sleep_ms(300)

Reference

Class Chip74HC595

Before each use of the object Chip74HC595, make sure my74HC595.py has been uploaded to “/” of ESP32, and then add the statement “from my74HC595 import Chip74HC595” to the top of the python file.

Chip74HC595():An object. By default, 74HC595’s DS pin is connected to Pin(14) of ESP32, ST_CP pin is connected to ESP32’s Pin(12) and OE pin is connected to ESP’s Pin(5). If you need to modify the pins, just do the following operations.

chip=Chip74HC595() or chip=Chip74HC595(14,12,13,5)

shiftOut(direction, data): Write data to 74HC595.

direction: 1/0. “1” presents that high-order byte will be sent first while “0” presents that low-order byte will be sent first.

data: The content that is sent, which is one-byte data.

clear(): Clear the latch data of 74HC595.