## Wednesday, September 5, 2012

### Complete Precision

Project Precision has reached its successful conclusion. I now have a physical hardware clock with an hour, minute, second, and third hand, and enough accuracy to justify needing a third hand.

As I have mentioned before, I noticed that an Arduino Nano has precisely the pinouts necessary to drive a charlieplex with 240 lights. The interesting thing is how few leftover resources there are. There are two analog inputs that are useless in this design. Every single other pin is used. I even considered giving up the crystal inputs to get two more digital pins.I had to include a digital multiplexer since the ATMega only has one serial port, and it needs to listen to both the USB port and the GPS.

As I said before, I will not make one for you for less than $300. The parts alone cost almost that much. However, I am going to publish everything you need to make one yourself. This is the Digikey part list:  Quantity Digikey Part Number Part Value Case Placement Price per Min quantity Ext Price 2 445-7483-1-ND Capacitor 4.7uF Ceramic 0603 C010 C418$0.24000 1 $0.48 2 478-6025-1-ND Capacitor 18pF 2% NP0 Ceramic 0603 C407 C408$0.40000 1 $0.80 2 445-1316-1-ND Capacitor 100nF Ceramic 0603 C420 C502$0.10000 1 $0.20 61 754-1359-1-ND LED Red 320mcd LED 0603 D000-D059 D502$0.14040 1 $8.56 60 754-1124-1-ND LED Yellow 150mcd LED 0603 D100-D159$0.11160 1 $6.70 60 350-2036-1-ND LED Green 300mcd LED 0603 D200-D259$0.51840 1 $31.10 61 350-2037-1-ND LED Blue 140mcd LED 0603 D300-D359 D501$0.48960 1 $29.87 4 CRA4S847CT-ND Resistor Pack 47 CRA04 R1 R2 R3 R4$0.04300 10 $0.43 2 P680GCT-ND Resistor 680 SMD 0603 R501 R502$0.10000 1 $0.20 1 CRA4S810KCT-ND Resistor Pack 10k CRA04 R606$0.04300 10 $0.43 1 SW1021CT-ND Switch SPST B3U-1100P S429$1.03000 1 $1.03 1 ATMEGA328P-15AZCT-ND Microcontroller ATMEGA328P 32-TQFP U401$6.45000 1 $6.45 1 768-1007-1-ND USB interface FT232RL 28-SSOP U501$4.50000 1 $4.50 1 NC7SZ157P6XCT-ND Multiplexer Noninv 2 input SC-70-6 U602$0.41000 1 $0.41 1 887-1319-1-ND Crystal 16MHz 7M Y401$1.69000 1 $1.69 Lights$76.23 Rest $16.62 Total$92.85
This costs on the order of \$100, but the vast majority of the cost is in the 242 lights (240 for the hands, 2 for the TX/RX indicator). I get the above prices today from Digikey with no tax or shipping added on.You can get cheaper lights if you are satisfied with not using green or blue.

You will also need some connectors for the boards:
Sparkfun Female Header Pack - a set of two 6-pin and two 8-pin sockets. You will need this complete set, plus another 6- or 8-pin that will be cut down to 4 pins. You might as well get two of these sets, since they are cheap