| This system uses a simple 7 segment LED (like you've seen on old digital equipment) to show the CAB number selected (see diagram 21). The 7-Segment Decoder IC chip drives the 7 segement LED to turn the proper segments on and off the indicate the the desired number. The driver is fed a binary number representing 1 - 7 and it decodes the correct segments it needs illuminate on the display. (The driver can actually decode numbers 0 through 15 for other computer / digital counting systems but we're sticking with the common decimal system for our purposes.) I do NOT cover numbers above 7 as that introduces another level of complication to the circuit but you can modify this design for more CABs. I used simple (cheap) switching diodes to set up the propper binary code to represent the numbers 0 - 4. |  Diagram 21 |
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| The 7-Segment Decoder IC chip is fed the binary number representing the CAB number (see table 1). E.G. for the number 1, use binary number 0001 and for the number 3, use binary number 0011. In digital equipment, 5VDC represents a binary 1 while ground (0VDC) represents a binary 0. By using the resisitors between the ABCD inputs of the 7 segment LED and the +5VDC source, the circuit registers a '1' on the line unless the line is grounded (by the driver IC). These resistors are referred to as 'pull-up resisitors' since they pull the voltage up to 5V. The D input of the 7-Segment Decoder IC chip is connected to ground to keep a binary '0' there at all times since we never need a '1' on the D input unless we go over the number 7. Simply connect the 7 segment lines (a - g) from the driver to the display and all is taken care of by the IC chip. As an added bonus, the entire display goes blank any time a low (ground) is applied to the BL (blanking) pin of the 7-Segment Decoder IC chip allowing you to 'blank out' the display when the rotary switch is in the OFF position. |
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| The best way to construct these circuits is to use IC sockets (14 & 16 pin dip sockets) so you can change bad parts (if they go bad) and so you don't have to worry about 'burning up' the IC chips when you solder the connections. Both the driver and the display will fit into these sockets (14 pin for the display and 16 pin for the driver). The 7-Segment Decoder IC is a 74LS47 BCD TO 7Segment Decoder/Driver IC chip. The 7 segment LED is a 1 DIGIT .3" LED Display. The diodes can be any cheap low current diode. I used 1N914 switching diodes. These are all available at most electronics stores or through catalogs. I cut up and used IC perfboards (protoboards) to construct a circuit board for each driver-display pair so I could position it right by the switch on my CTC but this may be a problem if you are building a very compact CTC panel. Now all you need to do is wire the components as shown in digram 22. Be careful to conect to the correct pin. The pin numbers start with 1 at the top-left (looking from the top of the socket or IC) and go counter-clockwise. |
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| Wiring the driver and display 1. Connect 330 ohm 1/8 watt pull-up resisitors between +5 VDC and pins 1, 2, 4, & 7 of the 74LS47 driver. 2. Connect +5VDC to pin 16 of the 74LS47 driver. 3. Connect pins 6 & 8 of the 74LS47 driver and pins 3 & 8 of the display to ground. 4. Connect pin 13 of the 74LS47 to pin 7 the display. 5. Connect pin 12 of the 74LS47 to pin 6 the display. 6. Connect pin 11 of the 74LS47 to pin 4 the display. 7. Connect pin 10 of the 74LS47 to pin 2 the display. 8. Connect pin 9 of the 74LS47 to pin 1 the display. 9. Connect pin 15 of the 74LS47 to pin 9 the display. 10. Connect pin 14 of the 74LS47 to pin 10 the display. |
 Diagram 22 |
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| Wiring the diodes to the display assembly. 1. |
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| Wiring the complete display assembly to the CAB control switch. 1. |
