DIY Dirt Cheap Continuity Tester With LED + 555 IC


Introduction: DIY Dirt Cheap Continuity Tester With LED + 555 IC

Hi, i wanted to have a continuity tester (my multimeter doesn't have a buzzer) so i don´t have to look away from the circuit I am testing to see the multimeters display. Although i wanted a buzzer i also added a LED and an on/off switch for late night tinkering "stealth mode"
I don´t have any engineering or electronics background (actually i am a surgeon), but i love to make stuff and i am self teaching electronics with the help from Google and . This is my first instructable and English isn´t my native tongue, so bear with me please...

DISCLAIMER: If following this guide anything/anyone/yourself damages i shall not be held responsible.Do it at your own risk. Take all the safety measures (goggles, gloves, etc)

Step 1: Bill of Materials:

-Highlighter (worn out)
-Blue LED
-555 IC
-0.01uF ceramic capacitors= 2
-1uF electrolitic capacitor = 1
-180 k resistor = 1
-56 k resistor= 1 (i did it on the weekend with no opened stores, so i used 47k + 10k series resistors)
-Piezo buzzer= 1 (i used a Hycom HY-07 that i took from an old modem)
-Button cell batteries= 3
-Battery holder (from a cheap-o pen/flashlight i had)

-Soldering Iron
-hot glue gun
-xacto knife

Step 2: Let the Stripping Begin..

First of all, i took apart the highlighter, threw away the tip and the ink cartridge by cutting the back end with an xacto knife. And i did the same to the flashlight/pen in order to take the battery holder away.

Step 3: Test 1,2,3

I found a buzzer circuit ( ) the easiest and simple circuit i could find and tried it on the breadboard. It worked out fine, except i changed de 10k resistor (pins 7 and 8 of the 555 IC) for a 180k resistor because i liked the tone better.

Step 4: Avengers Assemble

i soldered all together over the 555 directly trying to make it as compact as possible (although i had plenty of room inside the highlighter, i did that way just in case i want to house it in a smaller pen someday)

Step 5: Adding the LED, Buzzer and Switch

-I stuffed some solder with the wires into a BIC pen tip (it´s a little messy because of the ink) and heated them with the soldering iron. -Then wired the tip to the (-) leg of the led and the pin 1 of the 555 IC. 
-The (+) leg of the LED and the pin 4 of the 555 (through the on/off switch) to the positive pole of the battery pack.
-The secondary probe (another used pen) with a wire wrapped around the tip to the negative pole of the battery pack
When the two probe tips touch each other (or through a conductive material such as a wire or a circuit track) the circuit closes and the LED lights up and the buzzer vibrates with a nice tone.
The buzzer can be turned off with the switch, while the LED still lights up with continuity.

Step 6: Mounting the Switch, the Buzzer and the Battery Holder

-I made a hole at one side of the highlighter so i can install the small switch i had
The buzzer and the battery holder are housed in the cap that clips on the end (that was reattached with a hole so wires can pass through and hot-glued). This allows me to change the batteries when they die. 

Step 7: Continuity Tester Completed

Everything in place, time for the last step

Step 8: It�s Alive!!

Testing, everything do what it should be doing.
You can see the LED light (obviously you can´t hear the buzzer), maybe later i upload a video.

Step 9: Final Thoughts

You could use smaller components in order to fit everything into a smaller pen o skip the buzzer part and built a led crcuit.



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    Please be positive and constructive.




    You are brave to solder directly to the IC. I tend to destroy ICs and try to use a socket to avoid static electricity damage. The US Navy Electronics course is available free for download (1998 edition) here. It is a great resource for anyone who wants to learn basic electronics from beginning to end. Thank you for your very practical Instructable.

    1 reply

    me shahbaz from lahore student of laptop repairing also basic electronic ,plz add me

    in ur friend my email is

    skype id . shahbaz007761

    If you use a piezo buzzer module, rather than a piezo speaker,
    then you can omit the 555 and associated components. A piezo
    buzzer needs only current applied to buzz, and requires no oscillator
    to drive it.

    Nicely done project, though.

    1 reply

    thanks, i´ll have that in mind for next time.

    Thanks, the 555 timer is like the swiss army knife of electronics, and internet is a great powerful tool for courious people like us..

    Great Idea! Got to love the versatility of the 555 timer. Also, how great is the internet? I'm teaching myself electronics through the same method.