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. 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). 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.
