Make a Cigar Tube Circuit Tester (Breadboard Prototype)

In this project, we’re going to make a low voltage DC circuit tester — like the ones you get for testing automotive circuit points — but the one we will be making we'll also be able to easily use on smaller circuit points, such as for breadboard prototype circuits and PCB circuits. This kind of project is super-easy and is a great way to learn how to use basic electronic components to make something useful.

The following steps are to show you how to set up the prototype of the Cigar Tube Circuit Tester on a breadboard. To see the entire build process, from start-to-finish, visit our page titled "Make a Cigar Tube Circuit Tester".

The design of the cigar tube circuit tester has two LEDs within the handle that lights up to tell you which direction or orientation you have the probes connected across a voltage potential:

1. The white LED lights up when the paper clip probe is touching the positive point in a circuit and the alligator clip is connected to ground or a negative point in a circuit.
2. The red LED lights up when the paper clip probe is touching the negative point or ground in a circuit and the alligator clip is connected to a positive point in the circuit.
3. The cigar tube circuit tester is designed for either automotive, breadboard prototype circuits, or PCB circuits.
4. The cigar tube circuit tester has a durable, thin pointed probe at the end of its handle - perfect for smaller circuit points!
5. The cigar tube circuit tester is rated for a test voltage range of 6VDC to 24VDC.
6. I actually tested it as low as 3V and both LEDs were still able to light up. I also tested at 2V on my voltage supply, but neither LED was able to light up – this is probably because of the use of diodes in the cigar tube circuit which have a forward voltage drop of around 0.7V.

Breadboard Prototyping Parts List

1. 2x 1.2 kΩ Resistor
2. 2x 1N4001 Diode
3. 1x Red LED 5mm Diffused
4. 1x White LED 5mm Diffused
5. 1x Solderless Breadboard
6. Several miscellaneous sizes of Jumper Wires

As you can see from the image above, I’ve started off with placing the two diodes onto the breadboard. Diode (D1) is placed at points d10 and f10, with its cathode end (the gray mark) facing down row 10 through columns a to e, as shown by the purple arrow. Diode (D2) is placed at points d19 and f19, with its cathode end (the gray mark) facing down row 19 through columns f to j, as shown by the teal arrow.

Referring to the image above, I’ve placed a 1.2kΩ resistor (R1) at point c10 on the breadboard — in-line with the cathode (-) of diode D1 — and the other terminal end of resistor R1 at point c14. Another 1.2kΩ resistor (R2) is placed at point c19 on the breadboard — in-line with the anode (+) of diode D2 — and the other terminal end of resistor R2 at point c16.

In the image above, the white LED’s anode (+) is placed in-line with the resistor (R1) in row 14 at point a14 on the breadboard, and its cathode (-) is placed in a point on the negative supply column on the breadboard. The red LED’s cathode (-) is placed in-line with the resistor (R2) in row 16 at the point a16 on the breadboard, and its anode (+) is placed in a point on the negative supply column on the breadboard. Note that the orientations of the white and red LEDs are opposite each other. Be sure that you’ve placed each LED in their correct orientation as previously described.

As you can see in the image above, I’ve placed a small orange jumper wire leading from a point on the positive supply column of the breadboard to point j10 on the breadboard – a point on row 10, in-line with the diode (D1) that we’ve placed on the breadboard previously. I’ve also placed a larger orange jumper wire that goes from point i10 to point i19, each point in-line with the rows of the diodes D1 and D2, respectively.

Next, we add the probes to our breadboard circuit. As you can see in the image above, I’ve added an orange jumper wire as my positive-side probe, and a black jumper wire as my negative-side probe.

Now it’s time to test our prototype breadboard circuit! Just grab a 9V battery as a test voltage source and touch your circuit probes to the battery terminals. You’ll see that if the orange jumper wire probe touches the positive terminal of the battery and the black jumper wire probe touches the negative terminal of the battery, then the white LED lights up. If the probes are switched across the battery terminals, then the red LED lights up.

If you're interested in continuing this fun project and are looking to transfer your prototype circuit to a PCB, to then complete the project as a Cigar Tube Circuit Tester, then check out the full tutorial here.

At our page titled "Make a Cigar Tube Circuit Tester", you'll find a more detailed version for the steps of the breadboard prototype build, more close up images, and documents, as well as a more thorough parts list and a tool list! You'll also find the steps and accompanying videos to instruct you to complete this project in its entirety.

Thanks for trying out this neat project. Hope you had fun! Remember to keep at it and stay motivated!