A dual 7-segment display

After spilling a box of electronic components and amusing myself by sorting the dusty debris, I came across a rather ancient dual 7-segment display and decided to see what a Velleman board could do to it. The result of this innocent amusement can be seen below.

Circuit description

The seven segments plus the decimal point leds are doubled up and connected to the eight output lines of the Velleman board. As the Velleman board has only eight output ports, I used the two digital to analogue converter outputs for multiplexing, hoping that they would be fast enough to make a usable display. In this assumption I was wrong.
Here is the circuit diagram of the project.

The doubled up cathodes of the diodes are connected to the output ports. The anodes of the two digits are driven by two BCY51 transistors. The type of transistor is not important, I simply used what I had in the box. The bases of the transistors are connected to the DAC outputs. By setting one or the other of the DACs to 5V, it is possible to select either one of the digits.

The Program

This is kept straight forward in keeping with the philosophy of this site. Four choices are offered. The first two simply use one or the other of the digits to count from 0 to 9. This is done by sending 255 to one of the DACs and the count sequence to the output ports. To change digits, the first DAC is turned off and 255 is sent to the second DAC. This nicely demonstrates the multiplexing technique in slow motion. The relevant procedure starts at line 440 in the listing below.

The third choice offers a 'walking' segment which uses both digits. Innocent fun and of no use whatsoever, but it looks very impressive. The relevant procedure is at line 620.

The fourth choice offered is a count from 0 to 99 - using both digits in multiplexing mode, i.e. switching so fast between the two that the persistence of our vision fools the brain into thinking that both displays are on at the same time. The relevant procedure starts at line 920.

This unfortunately did not work very well. The oscilloscope revealed why.

Above is the output of one of the DACs driven as fast as BBC BASIC and the response time of the Velleman K-8055 board allow. As can be seen, there is a rather long rise and fall time. As a result, the display is very dim because most of the time there is not enough voltage to produce a bright output. I had to insert the time delay into the code at lines 1030 and 1070 . This made the display much brighter, but it also produced a very noticeable flicker. Ah well, you can't win them all and at least it demonstrates the multiplexing principle in a very graphic way.

Below is the usual link to the archive with the BBC BASIC file and an executable file of the below.

K8055_7_seg-x2_1_4.zip

Here is a YouTube video of the project

I have also converted this program for the K8061 interface board. This board is faster than the K8055 and the multiplexing works much better.

10 REM K8055_7-seg 20 REM Multiplex two 7-segment displays 30 REM Jochen Lueg 40 REM http://roevalley.com 50 REM Limavady, February 2012 60 REM Version 1.2 70 80 90 ON ERROR PROCerror 100 PROCK8055_init 110 PROCinit 120 SYS K8055_CloseDevice%,Board% 130 SYS K8055_OpenDevice%,Board% 140 PROCscreen(480,250) 150 OFF 160 PRINT 170 PRINT " Select from the following:" 180 PRINT 190 PRINT " 1 . . . . . . . . Display 1 counts from 0 to 9" 200 PRINT " 2 . . . . . . . . Display 2 counts from 0 to 9" 210 PRINT " 3 . . . . . . . . Walking segment" 220 PRINT " 4 . . . . . . . . Count from 0 to 99 on both displays" 230 PRINT 240 PRINT " R . . . . . . . . Return to this menu" 250 PRINT " Q . . . . . . . . Leave this program" 260 PRINT 270 REPEAT 280 REPEAT 290 Key$=INKEY$(0) 300 UNTIL Key$="1" OR Key$="2" OR Key$="3" OR Key$="4" OR Key$="Q" OR Key$="q" 310 IF Key$="1" PROCcount_display(2) 320 IF Key$="2" PROCcount_display(1) 330 IF Key$="3" PROCwalking_segment 340 IF Key$="4" PROCmultiplex 350 UNTIL Key$="Q" OR Key$="q" 360 370 SYS K8055_ClearAllDigital%,1 380 SYS K8055_ClearAllAnalog%,1 390 SYS K8055_CloseDevice%,1 400 SYS "FreeLibrary",K8055_Board% 410 REPEAT UNTIL INKEY(0)=-1 420 *QUIT 430 END 440 450 460 DEFPROCscreen(ScreenWidth%,ScreenHeight%) 470 SYS "SetWindowPos",@hwnd%,0,0,0,ScreenWidth%,ScreenHeight%,6 480 VDU 26 490 COLOUR 128 500 CLS 510 COLOUR 15 520 ENDPROC 530 540 550 DEFPROCcount_display(d%) 560 SYS K8055_ClearAllDigital%,1 570 SYS K8055_ClearAllAnalog%,1 580 SYS K8055_OutputAnalogChannel%,d%,255 590 REPEAT 600 FOR J%=0 TO 9 610 SYS K8055_WriteAllDigital%,N%(J%) 620 TIME=0 630 REPEAT 640 Key$=INKEY$(0) 650 UNTIL Key$="R" OR Key$="r" OR Key$="Q" OR Key$="q" OR TIME >50 660 NEXT 670 UNTIL Key$="R" OR Key$="r" OR Key$="Q" OR Key$="q" 680 SYS K8055_ClearAllAnalog%,1 690 SYS K8055_ClearAllDigital%,1 700 ENDPROC 710 720 730 DEFPROCwalking_segment 740 SYS K8055_ClearAllAnalog% 750 SYS K8055_ClearAllDigital% 760 REPEAT 770 SYS K8055_ClearAllAnalog% 780 SYS K8055_OutputAnalogChannel%,2,255 790 800 FOR J%=1 TO 4 810 SYS K8055_WriteAllDigital%,W%(J%) 820 TIME=0 830 REPEAT 840 Key$=INKEY$(0) 850 UNTIL Key$="R" OR Key$="r" OR Key$="Q" OR Key$="q" OR TIME >2 860 NEXT 870 880 SYS K8055_ClearAllAnalog% 890 SYS K8055_OutputAnalogChannel%,1,255 900 FOR J%=5 TO 8 910 SYS K8055_WriteAllDigital%,W%(J%) 920 TIME=0 930 REPEAT 940 Key$=INKEY$(0) 950 UNTIL Key$="R" OR Key$="r" OR Key$="Q" OR Key$="q" OR TIME >2 960 NEXT 970 UNTIL Key$="R" OR Key$="r" OR Key$="Q" OR Key$="q" 980 SYS K8055_ClearAllAnalog% 990 SYS K8055_ClearAllDigital% 1000 ENDPROC 1010 1020 1030 DEFPROCmultiplex 1040 LOCAL N1%,N2% 1050 N1%=0: N2%=0 1060 SYS K8055_ClearAllAnalog% 1070 SYS K8055_ClearAllDigital% 1080 REPEAT 1090 FOR J%= 0 TO 9 1100 FOR I%=0 TO 9 1110 FOR Waste%=1 TO 8 1120 SYS K8055_WriteAllDigital%,N%(J%) 1130 SYS K8055_OutputAnalogChannel%,1,255 1140 TIME=0:REPEAT UNTIL TIME>2 1150 SYS K8055_ClearAnalogChannel%,1 1160 SYS K8055_WriteAllDigital%,N%(I%) 1170 SYS K8055_OutputAnalogChannel%,2,255 1180 TIME=0:REPEAT UNTIL TIME>2 1190 SYS K8055_ClearAnalogChannel%,2 1200 Key$=INKEY$(0) 1210 IF Key$="R" OR Key$="r" OR Key$="Q" OR Key$="q" ENDPROC 1220 NEXT 1230 NEXT 1240 NEXT 1250 UNTIL Key$="R" OR Key$="r" OR Key$="Q" OR Key$="q" 1260 ENDPROC 1270 1280 1290 DEFPROCerror 1300 PRINT REPORT$;" at line ";ERL : 1310 SYS K8055_ClearAllDigital%,1 1320 SYS K8055_ClearAllAnalog%,1 1330 SYS K8055_CloseDevice%,1 1340 SYS "FreeLibrary",K8055_Board% 1350 END 1360 ENDPROC 1370 1380 1390 DEFPROCinit 1400 Board%=0 1410 T%=-3 1420 1430 DIM N%(9) : REM Pattern for numerals 1440 N%(0)=%11101110 1450 N%(1)=%01000010 1460 N%(2)=%11011100 1470 N%(3)=%11011010 1480 N%(4)=%01110010 1490 N%(5)=%10111010 1500 N%(6)=%10111110 1510 N%(7)=%11100010 1520 N%(8)=%11111110 1530 N%(9)=%11111010 1540 1550 DIM W%(8) : REM Pattern for walking segment 1560 W%(1)=%10000000 1570 W%(2)=%01000000 1580 W%(3)=%00000010 1590 W%(4)=%00001000 1600 W%(5)=%00001000 1610 W%(6)=%00000100 1620 W%(7)=%00100000 1630 W%(8)=%10000000 1640 ENDPROC 1650 1660 1670 DEFPROCK8055_init 1680 REM Typing errors in routine name do not generate an error message - they just hang up the program. 1690 REM These are all the system calls in the order found in the manual 1700 SYS"LoadLibrary","K8055D.dll" TO K8055_Board% 1710 SYS"GetProcAddress",K8055_Board%,"OpenDevice" TO K8055_OpenDevice% 1720 SYS"GetProcAddress",K8055_Board%,"CloseDevice" TO K8055_CloseDevice% 1730 SYS"GetProcAddress",K8055_Board%,"ReadAnalogChannel" TO K8055_ReadAnalogChannel% 1740 SYS"GetProcAddress",K8055_Board%,"ReadAllAnalog" TO K8055_ReadAllAnalog% 1750 SYS"GetProcAddress",K8055_Board%,"OutputAnalogChannel" TO K8055_OutputAnalogChannel% 1760 SYS"GetProcAddress",K8055_Board%,"OutputAllAnalog" TO K8055_OutputAllAnalog% 1770 SYS"GetProcAddress",K8055_Board%,"ClearAnalogChannel" TO K8055_ClearAnalogChannel% 1780 SYS"GetProcAddress",K8055_Board%,"ClearAllAnalog" TO K8055_ClearAllAnalog% 1790 SYS"GetProcAddress",K8055_Board%,"SetAnalogChannel" TO K8055_SetAnalogChannel% 1800 SYS"GetProcAddress",K8055_Board%,"SetAllAnalog" TO K8055_SetAllAnalog% 1810 SYS"GetProcAddress",K8055_Board%,"WriteAllDigital" TO K8055_WriteAllDigital% 1820 SYS"GetProcAddress",K8055_Board%,"ClearDigitalChannel" TO K8055_ClearDigitalChannel% 1830 SYS"GetProcAddress",K8055_Board%,"ClearAllDigital" TO K8055_ClearAllDigital% 1840 SYS"GetProcAddress",K8055_Board%,"SetDigitalChannel" TO K8055_SetDigitalChannel% 1850 SYS"GetProcAddress",K8055_Board%,"SetAllDigital" TO K8055_SetAllDigital% 1860 SYS"GetProcAddress",K8055_Board%,"ReadDigitalChannel" TO K8055_ReadDigitalChannel% 1870 SYS"GetProcAddress",K8055_Board%,"ReadAllDigital" TO K8055_ReadAllDigital% 1880 SYS"GetProcAddress",K8055_Board%,"ResetCounter" TO K8055_ResetCounter% 1890 SYS"GetProcAddress",K8055_Board%,"ReadCounter" TO K8055_ReadCounter% 1900 SYS"GetProcAddress",K8055_Board%,"SedtCounterDebouceTime" TO K8055_SetCounterDebounceTime% 1910 ENDPROC

Return to interfacing index