This is a 2-LED light with pushbutton ON/OFF for playing disc (frisbee) golf at night. You should be able to tape the whole thing onto the bottom of your disc, and leave it there until the battery is dead. Battery life is increased by being able to turn it off when not in use, as well as by using resistors to limit the current.

Resistor value and LED brightness notes:

First, I have not built this circuit, and the LEDs might not be bright enough. Different resistor values down to 56 ohms may be tried, to make it brighter.
The LEDs are specified to have a brightness of 550 mcd at only 2 mA, over a 30 degree viewing angle, and the brightness rises linearly with current (the graph on the data sheet goes from zero to 10 mA). That seemed pretty bright to me, so I aimed for 1 mA of current. The datasheet says the LEDs drop about 1.75 volts at 1 mA. The battery will supply about 2.8 volts for the majority of its life. So, 2.8 - 1.75 = 1.05 volts for the resistor. 1.05 V/1mA = 1.05 kΩ. 1 kΩ is close enough.

Bill of Materials

All parts are in a Mouser shopping cart.
- Battery: CR2032 coin cell
- Holder: Linx Technologies BAT-HLD-001-THM
- Button: E-Switch TL2201EEYB DPDT latching pushbutton switch
- Resistors: Yageo MFR-12FTF52-1K 1 kΩ, 1/6 W, miniature resistors
- LEDs: Kingbright WP710A10LSECK/J3, 3mm low-current, 625nm red, 30° viewing angle
Total parts cost is currently $2.80, plus $7.99 shipping. Much savings will be achieved by building more than one board, since not only do you start to get volume discounts on the parts, but the shipping charge should remain constant.

Alternative parts

Any brand of CR2032 cell will work. The specified resistors are pretty small, almost a DIN0204 size, but the regular ¼ watt size (DIN0207) will also work. If you’re building more than two boards, Mouser currently has this resistor with a quantity discount of just one cent: KOA Speer MF1/4DCT26A1001F. The specified pushbutton switch (specification sheet: TL2201EEYB) comes without a cap, and maybe it’s better that way because it’s smaller, but Mouser also currently has a small quantity of them in stock with a big gray cap: TL2201EEYB.094GRY.