This project shows how to use the Raspberry Pi to drive a 7 Segment Display with the help of a 74HC595 chip.
Parts used in this project:
- Breadboard
- Wiring cables (Male-Male, Female-Male)
- 7 Segement display (it actually has 8 segments because of the dot)
- 74HC595 Chip
- Raspberry Pi Revision 2
- some Python code
Here is a schematic made with fritzing of the project when it is ready. Here a link to download the 7-segment-display with 74HC595 and Raspberry Pi fritzing project.
the interresting thing here is the 74HC595 Chip. It takes some time to understand what it does. I think that the video from Kevin Darrah describes it fast and easy.
[youtube=http://www.youtube.com/watch?v=6fVbJbNPrEU]
Here is the Python code: rpi_7_segment_74hc595
#!/usr/bin/python import RPi.GPIO as GPIO import time #Data Pins needed on the RPi DATAIN=17 #DS LATCH=27 #STCP CLOCK=22 #SHCP CLEAR=25 #MR Low OE=11 #Output Enable Low #inter character sleep icsleep=0.06 #defining all the single LEDs led1=0x80 #10000000 led2=0x40 #01000000 led3=0x20 #00100000 led4=0x10 #00010000 led5=0x08 #00001000 led6=0x04 #00000100 led7=0x02 #00000010 led8=0x01 #00000001 #definition of all writeable letters and numbers letter={"0":0xFC, "1":0x30, "2":0xDA, "3":0x7A, "4":0x36, "5":0x6E, "6":0xEE, "7":0x38, "8":0xFE, "9":0x3E, "a":0xBE, "b":0xE6, "c":0xCC, "d":0xF2, "e":0xCE, "f":0x8E, "g":0x7E, "h":0xB6, "i":0x30, "j":0xF0, "l":0xC4, "n":0xBC, "o":0xFC, "p":0x9E, "s":0x6E, "t":0x38, "u":0xF4, "x":0xB4, "y":0x76, "z":0xDE } #loading function sequence load1=0x06 #00000110 load2=0x22 #00100010 load3=0x60 #01100000 load4=0xC0 #11000000 load5=0x82 #10000010 load6=0x12 #00010010 load7=0x18 #00011000 load8=0x0C #00001100 #up-down loading function sequence ud1=led8 ud2=led2 ud3=led1+led3 ud4=led7 ud5=led6+led4 ud6=led5 #left-right loading function sequence lr1=led1+led6 lr2=led2+led5+led7 lr3=led3+led4 lr4=led8 #rotational loading function sequence rot1=led2+led5 rot2=led1+led6+led3+led4+led7 #putting all segments of the sequences in a list letterrange=[] hexrange=[] for value in letter.values(): letterrange.append(value) for value in letter.values(): if value != "g": hexrange.append(value) loadrange=[load1,load2,load3,load4,load5,load6,load7,load8] udrange=[ud1,ud2,ud3,ud4,ud5,ud6] ledrange=[led1,led2,led3,led4,led5,led6,led7,led8] lrrange=[lr1,lr2,lr3,lr4] rotrange=[rot1,rot2] spinrange=[led1,led2,led3,led4,led5,led6] #GPIO definition def setup(): GPIO.setmode(GPIO.BCM) GPIO.cleanup() GPIO.setup(DATAIN,GPIO.OUT) GPIO.setup(CLOCK,GPIO.OUT) GPIO.setup(LATCH,GPIO.OUT) GPIO.setup(CLEAR,GPIO.OUT) GPIO.setup(OE,GPIO.OUT) GPIO.output(LATCH,False) #Latch is used to output the saved data GPIO.output(CLEAR,True) #Clear must always be true. False clears registers GPIO.output(OE,False) #Output Enable speaks for itself. Must be False to display GPIO.output(CLOCK,False) #Used to shift the value of DATAIN to the register GPIO.output(DATAIN,False)#Databit to be shifted into the register #Clean up GPIO, set display to no character def cleanup(): #Set all leds to off writenumber(0) #writeout stored in character writeout() #writeout "nothing" writeout() time.sleep(0.7) GPIO.cleanup() #shifts in a bit (but does not write it yet) def shift(input): if input == 1: input=True else: input=False GPIO.output(DATAIN,input) GPIO.output(CLOCK,GPIO.HIGH) GPIO.output(CLOCK,GPIO.LOW) GPIO.output(DATAIN,GPIO.LOW) #writes the stored data from register out to pins def writeout(): #Display LEDs GPIO.output(LATCH,GPIO.HIGH) #needed to read characters. otherwise the characters would be display to fast after each other time.sleep(icsleep) GPIO.output(LATCH,GPIO.LOW) #writes a character to the register def writenumber(number): for x in range(0,8): shift((number>>x)%2) #writes a range of character to the display def writerange(range): for x in range: writenumber(x) writeout() #additive and following substractive writeout of a range of characters def writeaddrange(range): character=0 for x in range: character+=x writenumber(character) writeout() for x in range: character-=x writenumber(character) writeout() #additive and following substractive writeout with reversed call of a range of characters def writeaddremrange(range): character=0 for x in range: character+=x writenumber(character) writeout() for x in range: character-=x writenumber(character) writeout() for x in reversed(range): character+=x writenumber(character) writeout() for x in reversed(range): character-=x writenumber(character) writeout() #additive and following reversed substractive writeout of characters def writeaddbackrange(range): character=0 for x in range: character+=x writenumber(character) writeout() for x in reversed(range): character-=x writenumber(character) writeout() #chained XORed and reversed XORed writeout of characters def writexorrange(range): #close the chain to have no interrupts while displaying character=range[0]&range[-1] for x in range: character^=x writenumber(character) writeout() for x in range: character^=x writenumber(character) writeout() for x in reversed(range): character^=x writenumber(character) writeout() for x in reversed(range): character^=x writenumber(character) writeout() print("####Setup####") setup() #Tryout of most ranges and displayfunctions try: print("####Write Stuff####") while True: writerange(letterrange) writerange(hexrange) writerange(loadrange) writeaddremrange(udrange) writexorrange(loadrange) writexorrange(udrange) writexorrange(ledrange) writexorrange(lrrange) writexorrange(rotrange) writexorrange(spinrange) writeaddrange(ledrange) writeaddbackrange(ledrange) #Wait for KeyboardInterrupt or SystemExit (called by kill or others) except (KeyboardInterrupt, SystemExit): print("Exit...") finally: cleanup()
Have fun driving your 7 Segment display with the Raspberry Pi.
Thanks fpr sharing this project!
When I implemented this code on my Pi there was a little error: The numbers were not displayed in correct order. I figured out, that I have to remove the quotation marks in the letter array, then the numbers will show up in perfect order.
Thank You for this tutorial!
But I think You need a 220 Ohm resistor for every led in the segment for current protection. Without the resistors, this circuit doesn’t work reliably on my own raspberry pi.
I agree. At least the segment display I’m using require something like 15mA. The output current of RPi’s 3.3V is however 50mA (as far as I remember). This is more than 3 times the current that is required.
This is a very cool project!
But I’ve got a question:
Can it display things you want?
Mine is just blinking… 😉
Sorry for bad english…
Thank you, Bennett
Just a big fat warning for all who try to recreate this project: since the model of the segment display is not given if you are new to this it is quite possible to screw things up big time. Segment displays such as this usually have a common anode or cathode (this means that you use 2 of the pins to supply the display with a + or – and control the segments with the opposite). If you are not careful you might actually get a segment display that has a common + for its segments which is the exact opposite of what the Fritzing project shows. Just be careful and do some research on WHICH display you are using and change the project a little bit according to its specification.
@Red Baron (Or anyone else reading a year later)
My segment is indeed one that has the voltage, not the ground going to the two common pins. This is leaving my display lit up immediately when I wire this up. I’ve got resistors in place on all the segments so I haven’t killed anything… that I know of anyway. Not sure what to change in the program to have it not lit up by default.
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