Help me finish a 7 segment display in Multisim 7.0

7 segment operation

i designed this 7 segment display by using multisim 7.0 but it didnt work, can you help me complite this display. i have to make this for writing a 6502microprocessor code in order to write "1 2 3 4 5 6" on this display. this is my project of year(for school) please help me.

Coution: to run file you must use Multisim 7.0

Re: 7 segment operation

There are several problems with this circuit.

1) Your switched don't connect to anything on one side so they cannot possibly do anything. For TTL logic, you need to pull each input High to VCC with a 4K7 or similar resistor. Your switch then switched that input to ground. Note, this ends up as opposite logi, ie switch closed = '0'. switch open = '1'.

2) Both of your driver ICs are of the type that SINK current, ie when the input in ON it looks like a swicth to GND. As your displays are Common Anode this is correct for the 74LS47, but the 74LS138 is incorrect, as you need to SOURCE current to the Anode of the displays. You could let the 74LS138 outputs drive a PNP transistor so that when it's output goes low it pulls current from the base (through a resistor, say 1K). With the emitter joined to VCC and the collector to your display Anode through a small resistor then current will flow from VCC through the PNP transistor to the display Anode.

To get you going, you could just use your switches straight from VCC to the Anode (through current limiting resistor).

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