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LM555 Tick tick tick

without batteryI wanted to see how few parts I could use, from what I had on hand, to build this clicking blinking LED device based on a 555 chip.

This article was submitted by Lee Bornifield as part of the “Hobby parts for articles ” program. Write something of interest to electronic hobbyist and receive parts for your next project.

Probably the most expensive part is the 9V battery. There is no circuit board, and no additional wires; just bending the leads of the components provides enough connections.

The earphone clicks and the red LED blinks at a constant 137 times a minute. The exact rate depends on the value of the resistor and capacitor. That rate will be maintained very precisely for the life of the battery. It’s not forever, but listening closely I could still hear the earphone faintly clicking after 6 days of continuous operation.

with batteryIt’s much brighter and louder when the battery is new. Don’t stick it in your ear with a fresh battery, it is TOO loud. If you use a small capacitor (picofarads) I think the discharge is too small to make an audible click. I used a 20 mFd capacitor and a 27K resistor. A larger capacitor or a larger resistor will slow down the pulse rate.

The 9 volt battery clip red wire goes to pin 8 of the 555 chip, and the black wire goes to pin 1. The resistor is between pins 2 and 8. The capacitor is between pins 1 and 2. The leads of those components are bent so that pins 4 and 8 are connected, and pins 2,6,7 are connected. The LED and earphone are in series between pins 3 & 4.

Actually I don’t recommend this construction technique for an expensive LED, because I didn’t measure the current, it may be too high. A 555 can output 200mA. Usually you’d put a resistor in series with the LED to limit that current to a safe level like 30mA. I only used the small resistance that comes from having the earphone in series with the LED.

layoutWith a fresh 9 volt battery the click can easily be heard across the room. This is a good use for “dead” 9v batteries. They still have plenty of ticks when they’re down to 6 volts. The battery was down to about 2.1 volts when it finally stopped ticking. If you unplug the battery for a day it will recover a low voltage and start ticking again.

This device also simulates a faucet dripping. Hide it near a sink to frustrate a handyman. :-)

Posted in Hacks, Parts, Projects, Scrounging Parts.

12 Responses

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  1. NGinuity said

    You can decrease the total volume, should you want to put it in your ear. Alls you need is a potentiometer (I use a single turn 500 ohm), and tie the center and one of one of the leads together in series between the LED and speaker….You can regulate the volume when the battery is high.

  2. NGinuity said

    Oh, it would take care of regulating the current to the LED too.

  3. There is an artistic look to these free-form circuits that I love. A lot of the BEAM bots have the same kind of look to them. Nice job!

  4. I love the versatility of the 555 timer, but when it comes to long-running low-power circuits, it doesn’t do nearly as well as the LM3909 oscillator. Add a capacitor, LED, and battery to the 3909 and you have an extremely simple LED blinker that will uses very little power. I made one such blinker, wired to a 1.5V alkaline battery, and gave it to my Dad to put on his office shelf. I chose a capacitor value for a blink every couple of seconds. Two years later, it was still blinking! I don’t know how long that one went before eventually giving up the ghost, but at two years I considered it a smashing success.

  5. Crudely Wrott said

    “This device also simulates a faucet dripping. Hide it near a sink to frustrate a handyman.”

    Hey! I’m a handyman! I get paid to come to your house and fix stuff, including drips. Do I look like I need further frustration?!! If I discover that you have fooled me with this silly contrivance . . . I’ll buy and fly!

    But why 137 beats per minute? Is this a cosmic joke?

  6. Rajah said

    That’s a fun quickie project. I did something similar back in 1964 with a used 90 Volt battery for portable tube type radios, a megohm resistor, capacitor and an NE-2 neon bulb that formed a relaxation oscillator. It ran for two years, binking the neon bulb about once every two seconds, in my college dorm room before someone “borrowed” it. Thanks for reminding me. :-)

  7. Alan said

    I was just thinking that with a time delay this could be a profitable device for plumbers. Set it up to go off 5 months after you leave. :) Just hope the device isn’t spotted…

  8. Lee said

    “But why 137 beats per minute? Is this a cosmic joke?”

    I’d like to say that constant was finely structured :-) but actually I was aiming at 120 bpm, like an aroused heartbeat, with a deep red LED, like a valentine of constant undying LUUUUVVVV. She thought it was a bomb. :-(

    Mainly I liked the shiny blue capacitor and picked a resistor to get close. 555 spec sheets give a simple formula for calculating the period of this astable multivibrator T= .693(Ra + 2Rb)C .
    The nominal component values I used were Ra=27000, Rb=0, C=.00002 .

  9. Lee said

    You can use this circuit to figure out the value of the capacitor, if you know precisely the value of the resistor. Display the ticks on an oscilloscope and measure the time between ticks. 137 beats per minute is .44 seconds between ticks, a frequency of 2.28 Hz. Capacitance in farads C = 1.44/(R*Hz). So (if my resistor were exactly 27000 ohms) the capacitor labeled “20 mFd” actually was 23 microfarads.

    .000023 = 1.44 / ( 27000 * 2.28)

  10. Lee said

    “1964…90 Volt battery…NE-2 neon bulb that formed a relaxation oscillator. It ran for two years”

    You remind me of a “drive somebody crazy” device I always wanted to build after seeing it on an old magazine — several NE-2s and a 90V battery, blinking for years. It was cast inside a clear plastic block so there was NO WAY to turn it off!

  11. dash said

    i think it works great. I don’t have any ear phones though, so i used another led to replace the ear phone. It would be very cool if you could make it so one led blinks and then the other.

  12. Hi Jim. Photos i received. Thanks