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. switch open = '1'.Ģ) 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.

Note, this ends up as opposite logi, ie switch closed = '0'.

Your switch then switched that input to ground.

For TTL logic, you need to pull each input High to VCC with a 4K7 or similar resistor. There are several problems with this circuit.ġ) Your switched don't connect to anything on one side so they cannot possibly do anything.