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. 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). Connect a SPST switch (or pushbutton) between pins2,3 and ground. 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. The 'Reset' state is when the switch is open. To blank the display: Remove the connection between Bl//RB0 (Pin 4) and Vcc. Connect a 'pullup' resistor between pin 4 and Vcc. Connect a SPST switch between pin 4 and ground. To get you going, you could just use your switches straight from VCC to the Anode (through current limiting resistor).
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