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 |
Top Comments
Really Nice explanation of a 555
just a note there is a very nice 555 Timer - Frequency and Duty Cycle Calculator here!
>>>> remember what an engineer does: Always know where to find the answer! Not necessarily…
Another great Learning Circuit episode.
The original design looks like it uses the TI TLC555CP CMOS version of the 555 timer which looks like it works down to a supply voltage of 2v, which should work off…
Thanks makerkaren for a great leaning episode and fun too.
Clem