Friday, October 2, 2009

Blinkin' Basics: The 555 timer.


I recently took the electronics safety and basic usage (SBU) class at TechShop so I could make use of the soldering equipment. After making some paperclip men we built a simple blinky using a 555 timer, an assortment of resistors and capacitors, and an LED.

The 555 is a basic building block of many digital circuits. According to wikipedia (555_timer_ic), over a billion were manufactured in 2003 alone. The timer can be used to implement basic logic functions like an inverter or a flip-flop. It can also be used to blink an LED or drive a speaker.

Its operation as an LED blinker is pretty straight forward. The 555 monitors the voltage across a capacitor as it charges and discharges. When the voltage across the capacitor reaches 2/3 the power supply voltage, the 555 flips its output state and also discharges the capacitor. When the capacitor's voltage drops to 1/3 the power supply voltage, the output is flipped again and the capacitor starts charging. The values of the capacitor and some current limiting resistors (to slow down the charging and discharging) control how fast the 555 changes state and blinks the LED. "The Electronics Club" has a nice write up of 555/556 timer circuits.

One drawback of the 555 is that it isn't very power efficient. It's constantly charging and discharging the capacitor and all that energy is thrown away. My blinky ran for about 12 days off of a 9V battery. These days a cheap 8-pin micro-controller (like the ATTiny13A) could perform the same function, with only one resistor to limit the current through the LED (and if you used pulse-width modulation you wouldn't even need that resistor) for a much longer time. You could also control 5 LEDs with the microcontroller like the Tiny Cylon Kit. Still the 555 has been a workhorse component since it came out in 1971 and many blinking things wouldn't exist without it....and that would be sad. :(

Incidentally, the battery clip in the picture was made from an old 9V battery. I soldered a couple wires to the connections (remember, they are backwards on the clip, the big one is positive) then I used a hot glue gun to stick the bottom of the dead battery to cover up the solder connections.

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