Beeping Down Counter

Schematic Overview
           The schematic for this project isn't too, too bad to look at even though it's not the best drawing/layout for a schematic that I've ever done. The basic flow is from left to right. Left side is the state machine, then the 555 timers, then counter circuitry and finally the 7 segment LED output circuitry.

View Full Schematic

Schematic Specifics

Power Regulator
           No tricks here, just a standard battery pack and a 7805 +5 volt regulator for all the digital IC's in this project.

74-02 As A State Machine
           The theory section explained this already a little bit, but the 4 NOR gates will act like a 3 state machine, with the buttons triggering between set and start states and the count down value of 00 moving the system to the 'trigger' state.

74-192 Decade Counters
           The decade counters are hardwired to load the value of '25' into the system when the set state is activated. The outputs from these counters go directly into the 4511 LED drivers to set the count value on the display.

NE556 Dual Timer IC
           The 556 IC appears as two 555 timers in the circuit. As discussed before one is used as a master or system clock at about 1 Hz and the other is used for generating the 2-5 KHz beep sound.

Dual 7 Segment LED
           This system is made for common-cathode 7 segment displays, so be sure that's what you've got. Similarly, it would be wiser to add resisters before the A-G inputs, but I got lazy and added a single resistor between the common cathode pin and ground.