I used a PIC. It's one of the newer XLP (eXtremely-Low-Power) chips they make and has more pins than the original AVR. How is it different? Well...
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PROS:
* Has both start and stop buttons
* More energy efficient
Uses 80% less power in (deep) sleep mode
Stop button saves running power
CONS:
* PIC footprint is larger
* Less community support for PICs
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Recommended list of parts:
Ex. PART # DESCRIPTION Price
------------- ------------------------------------- -------
PIC24F08KA101 Microchip low-power MCU $2.94
1-390261-6 20-position IC socket (RECOMMENDED) $0.23
SR295E104MAR 0.10 uF, 20+V ceramic capacitor $0.13
(stock) 100 uF capacitor $0.11
(stock) 160 Ohm resistor (x2) $0.08 x2
MPSW01AG NPN 1W transistor (x2) $0.40 x2
751-1203-ND IR LED (wide output) $0.55
SFH 4511 IR LED (narrow output) $0.53
FCR8.0MC5 8 MHz resonator $0.33
BC22AAL 2-AA battery holder $0.71
PTS645SL43 6mm SPST tact switch, black $0.13
PTS645SK43 6mm SPST tact switch, red (OPTIONAL) $0.13
(stock) 100 Ohm resistor (OPTIONAL) $0.08
LTL-307GE LED (~2.1V forward bias) (OPTIONAL) $0.09
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$6.92
Variant: This project was designed to run off of 3V (2-AA batteries) or 4.5V (3-AA batteries). 4.5V will offer increased power output, but I haven't tested it.