I 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.
It’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.
With 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.