Beeping Down Counter

Current Part:

Dual 7 Segment LED Display
NE556, Dual 555 Timer
2x 74-192, 4-bit Decade Counter
2x 4511, 7 Segment LED Driver
74-02, Quad 2-Input NOR Gate
2n2222 Transistor
100kΩ Resistor
4x 10kΩ Resistors
4x 1kΩ Resistors
2x 330Ω Resistors
2x 100Ω Resistors
2x 10uF Capacitors
0.1uF Capacitor
2x SPST Push Buttons
2x Green LEDs
Jumper Wire
9v Battery Clip
Battery Pack

Parts List Details
           Some of the parts listed above are a little more important than others, so I'll go ahead and describe them in more detail below.

           To give my wrists a break from all the wire-wrapping I did in the Masochist's Video Card and TTL Tilt Sensor projects, we'll use a breadboard and jumper wire to build this down counter. This breadboard is a standard size and to get the most out of it, we'll use almost every single row.

Dual 7 Segment LED
           The 7 segment LED display is a time honored tried and true display. It's been around for decades and I doubt it will be leaving us for decades. The display we'll use is a combined dual 7 segment display, since we want to count down starting from 25, requiring two digits.

74-192, 4 Bit Decade Counters
           This 4 bit counter is unique in that it counts from 0 to 9 and then starts over. This makes it a decade counter and not a true binary counter. Since we want to count in decades, this is perfect for counting down from 25 to 0. The 74-193 counter that I used in other projects, counts from 0 to 15 as it is a true binary counter.

4511, 7 Segment LED Driver
           The 4511 IC is made specifically for driving 7 segment led displays. It converts a 4 bit value into the proper A-G outputs to the 7 segment display for numbers 0-9. This means it will fit in perfectly with our decade counters that only count from 0-9. Since we are using 2 digits, we will need to use two 4511 drivers.

NE556, Dual 555 Timer
           The dual 555 timer in this project will have two functions. The first, master timer, will be used to generate a 1 Hz signal for down counting and also to enable the second timer in the NE556. This second timer will be in charge of driving a speaker to generate a tonal 'beep' sound. To make sure the beep is quick and sharp, the duty cycle for enable pulses from the master timer should be around 10%.

74-02, Quad 2-Input NOR Gate
           A three state machine will be created using this IC. The four NOR gates will be connected together to form two RS flipflops and then connected together so each of the 3 states can be triggered.