In the last lesson, we learned what happens inside a 555 timer to make it work. In today’s episode, Karen uses 555 timers to make LED “dominos”. Each unit has an LED connected to the output of a 555 timer and an LDR attached to the trigger. When the LDR senses light, the 555 is triggered turning the light on for a brief period of time. When multiple units are placed in a row, each one triggers the next. The duration that each light is on is determined by an “RC circuit” made of a resistor and capacitor. Changing their values can change the duration that the light is on.
The idea for this project came from a video made by Randy Elwin back in 2011. Karen uses his idea to make a similar device to show off using a 555 timer in a monostable circuit. (Learn more about monostable circuits in the NEXT lesson about 555 timers.) Shining a light on the LDR triggers Pin 2 and the 555 is “set” on. The RC circuit connected to pins 6 & 7 eventually cause the voltage at Pin 6 to go High enough to reset the 555, turning the LED off.
Bill of Material:
| Product Name | Manufacturer | Quantity | Buy KitBuy Kit |
|---|---|---|---|
| Electrolytic Capacitor, Miniature, 100 µF, 50 V, HE Series, ± 20%, Radial Leaded, 8 mm | nichicon | 1 | Buy NowBuy Now |
| Ceramic Disc Capacitor, 0.01 µF, 100 V, ± 20%, 5 mm | Vishay | 1 | Buy NowBuy Now |
| LM555CN - Timer, Oscillator & Pulse Generator IC, TTL Compatible, Astable, Monostable, 4.5 V to 16 V, DIP-8 | On Semiconductor | 1 | Buy NowBuy Now |
| Through Hole Resistor, 220 ohm, 400 mW, ± 1% | Vishay | 1 | Buy NowBuy Now |
| Through Hole Resistor, 10 kohm, 250 mW, ± 1% | TE Connectivity | 2 | Buy NowBuy Now |
| Photo Resistor | Adafruit | 1 | Buy NowBuy Now |
| Prototype Board, 25 x 15 grid, 1.6 mm, 72 mm, 47 mm | Multicomp | 1 | Buy NowBuy Now |
| LED, Cool White, Through Hole, T-1 3/4 (5mm), 20 mA, 3.2 V | Cree | 1 | Buy NowBuy Now |
| 9V battery snap, flexible | Multicomp | 1 | Buy NowBuy Now |
| Battery, 9 V, PP3, Alkaline, 565 mAh | Pro Elec | 1 | Buy NowBuy Now |
| Hook-up Wire Spool Set - 22AWG Solid Core - 10 x 25ft | Adafruit | 1 | Buy NowBuy Now |