Low Ohm Continuity Tester Using LM358 Op-Amp

Hello everyone. Thanks for taking the time to check out my Instructable.

I created a low ohm continuity tester based on the design from author Jim Keith which is a fairly simple design that works well. The website is www.electroschematics.com/continuity-tester-differentiates-resistance/. I modified Mr. Keith's circuit to suit my needs. He stated that if the bias voltage to Pin 3 on the LM358 is set at around 18 mV, then 1 ohm and under continuity could be achieved. When I tested my device, I had achieved 18.4 mV bias voltage. The prototype I made has proved quite useful and has produced consistent results. This tester will only give a "passing" beep and light up the LED if the resistance is lower than 1.0 ohm. During the development, I wanted more than a LED to reflect results. It needed to have a beep, but not just any beep. It had to sound like a Fluke or any other multimeter when continuity is achieved. I was challenged to make this device after an issue with one of our wiring harnesses. We had to setup emergency engine testing using previously made harnesses. Using our trusty Fluke, we pinned out our engine harnesses and found no issues. The problem that arose is that when you hear the "beep", you think you are good because you have continuity. But unless you actually look at the display, you could have a resistance in the mega ohm range depending on your meter's specifications. This is a big issue when it comes to engine sensor operation or connector end shield wiring. In this particular case, the sensor wire was only attached by one strand of a 20 gauge wire. This caused high resistance even though it had continuity. Once we fixed the wire, the testing was resumed. Although technically this tester measures resistance it is used to measure continuity under 1.0 ohm. It has been useful to have a tester with a small continuity range that signals an alert without the need to look at a display.

I wanted to make the tester custom by 3D printing the enclosure with all the necessary fixtures for mounting without using glues, epoxies, or two sided tape. Even though those methods are totally acceptable, I wanted to make this device as professional as possible. Please know that I am a beginner when it comes to modeling in CAD. So this box was created by using three different Thingiverse models. I made some of the modifications using Tinkercad and some with my supervisor's modelling software. If you don't have a 3D printer or access to one, this tester can still be made using a store bought project box. My prototype of this tester used a project box and it works just fine. This 3D printed box is 120mm x 80mm x 45mm.

So if you made it this far and are not to bored, lets get started.

I will break this section out into supplies, tools, and optional items. I will add links for many of the items, but some items such as tools are from various suppliers in which i don't have links.

Supplies:

• Qty 3 470 Ohm resistors
• Qty 1 1 Mega Ohm resistor
• Qty 1 10K Ohm resistor
• Qty 1 2.2K Ohm resistor
• Resistors - www.amazon.com/gp/product/B07D54XMFK/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
• Qty 1 LM358 Op-Amp
• Qty 1 8 Pin Chip Holder
• LM358 and Chip Holder
• www.amazon.com/gp/product/B07Z6HRTGS/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
• Qty 1 1N4148 Diode
• www.amazon.com/gp/product/B07Q4F3Y5W/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
• Qty 1 Green 5mm LED
• www.amazon.com/gp/product/B07PG84V17/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
• Qty 1 LED Holder
• www.amazon.com/dp/B07WNMNS9P?ref=ppx_yo2ov_dt_b_product_details&th=1
• Qty 1 Buzzer
• www.amazon.com/dp/B07SQ8GDPZ?psc=1&ref=ppx_yo2ov_dt_b_product_details
• Qty 1 Red 4mm panel mount banana jack
• Qty 1 Black 4mm panel mount banana jack
• Banana Jacks - www.amazon.com/dp/B07C7WG23G?psc=1&ref=ppx_yo2ov_dt_b_product_details
• Qty 1 Set of Multimeter leads - I don't have the exact supplier. Amazon has leads, but most leads should work.
• Qty 1 USB C Lithium Ion 9 Volt Rechargeable Battery - I bought the one I used from TEMU, but it is not available anymore. I have also gotten batteries with micro USB charge ports from Amazon, but the 3D printed box would need modification to use this type. Any USB C battery should work. However, a regular 9 volt battery would work as well. The box has been created in such a way that changing the battery would not be difficult.
• Qty 1 9 Volt Battery Connector
• www.amazon.com/gp/product/B08SL9X2YC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
• Qty 1 2N3906 Transistor
• www.amazon.com/gp/product/B095NMT2SL/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
• Qty 1 Lighted Rocker Switch - I bought this one was also bought from TEMU. This one is also no longer available. Any rocker switch will work as long as it around 22mm mounting diameter. It also does not need to be lighted.
• Qty 1 ElectroCookie circuit board 2" x 1.5" www.amazon.com/gp/product/B081MSKJJX/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
• Qty 4 M2 x 4mm screws for circuit board
• www.temu.com/goods.html?_bg_fs=1&goods_id=601099528802111&sku_id=17592269973981&_x_sessn_id=pugcqqtvag&refer_page_name=bgt_order_detail&refer_page_id=10045_1717639207915_xohdmih83r&refer_page_sn=10045
• Qty 2 M3 x 2mm screws for Buzzer
• www.amazon.com/dp/B0B6PVJTHY?psc=1&ref=ppx_yo2ov_dt_b_product_details
• Qty 4 M3 x 12mm screws for Box Lid - Same kit for screws as above
• PLA Filament in any color you choose. I used Elegoo Grey from Amazon
• Misc. 22 gauge wire
• 3D Printed or project box

Tools:

• 3D Printer - I used a Elegoo Neptune 3 Pro
• Soldering Iron
• Solder
• Wire Strippers
• Wire Cutters
• Needle Nose Pliers
• Circuit Board Holder or Helping Hands
• Small Phillips screwdriver - PH00
• Small Allen Wrench - 1.5mm
• Hobby Knife
• Assorted Files
• Deburring Tool
• Multimeter
• Kitchen Sink - Just Kidding

Optional:

The resistors listed are used for testing the range of the tester. I will show my tests later in this Instructable.

• 1 Ohm Resistor
• 1.1 Ohm Resistor - I didn't have one, so I used Qty 2 2.2 Ohm resistors in parallel.
• 1.2 Ohm Resistor
• 1.5 Ohm Resistor
• Resistor Kit -www.temu.com/goods_snapshot.html?goods_id=601099538694207&title=Details&_x_sessn_id=pugcqqtvag&refer_page_name=goods&refer_page_id=10032_1717639926344_wo8azk91mi&refer_page_sn=10032
• Needle Point Multimeter push on leads - I only use the full metal gold plated ones.
• www.amazon.com/dp/B09BFBQFJP?psc=1&ref=ppx_yo2ov_dt_b_product_details

Since I am not an expert on 3D printing or CAD modeling, I used my go to of finding existing models. Then I modify them in Tinkercad to fit my needs. I think the term is Monkey see Monkey do along with a dose of Cherry Picking. I have found Tinkercad to be helpful for an inexperienced person such as myself. However, there are some things I still just haven't figured out how to do with the software. For example, being able to place holes in exact locations based on the centerline of the main part. I fused three models together in Tinkercad, but the holes and stand offs were all done with different modelling software.

The project box, 9 volt battery holder, and multimeter lead holders were all models I found on Thingiverse which I fused together in Tinkercad. Please see the following links for each model.

• 9 Volt Battery Holder by ahmetcemturan https://www.thingiverse.com/thing:336319
• Multimeter Leads Holder by Mushka https://www.thingiverse.com/thing:3814583
• Project Box by zx82net https://www.thingiverse.com/thing:4801005

I have included the STL files needed to print the box. I provided two files for the main body of the box with and without lead holders. Since many types of leads can be used, you might not want the holders reserved for the more traditional style.

The 3D printer I use is the Elegoo Neptune 3 Pro. It is very user friendly for inexperienced users such as myself. The settings I used are listed below. I was slightly disappointed with the lead holders. They came out a bit messy partly due to the supports. But, they are completely functional. Please note that cleanup of the holes and leads holders will be necessary for a perfect fit.

• Layer Height: 2mm
• Infill: 25% (10% would probably be fine and speed up printing time)
• Print Speed: 60mm/s (This speed could also probably be increased, but print quality could suffer)
• Supports: Yes - Normal Everywhere
• Rafts: No Print to Table with Skirt only.

Now the real fun begins. When I first started working with electronics I found reading schematics and applying them to circuit boards or breadboards difficult. Once I got more experienced, I found the ElectroCookie circuit boards easy to use since they are setup just like a breadboard. It was easy to take my project from the breadboard and solder it to these circuit boards. The smaller one (1/4 breadboard) that I used in this project doesn't have power and ground rails incorporated, but each row is tied together so I pick a row for positive and a row for negative. The board has rows of numbers and letters which is also similar to a breadboard. Honestly, I believe my skills have advanced enough to not need to use this type of board, but I have many of them and it does help in illustrating the circuit.

Although a chip holder is not necessary, I usually install one because I have found bad LM358s in the past. Also, but I don't want to admit it, I do make mistakes that sometimes destroys an IC or two. In this case, I actually got it right the first time which is pretty rare. It helps that I had a protype to reference.

Lets get started. I will list the connections made by referencing the row letters and numbers. You might have a better arrangement than me.

The steps are as follows.

1. I first placed the 8 pin chip holder on the by straddling the center of the board.
2. E12 through E15 which correspond to Pins 1-4 on the LM358.
3. F12 through F15 which correspond to Pins 5-8 on the LM358.
4. The 1N4148 Diode was placed.
5. Pins J1 and J10 with J1 being the positive side.
6. Transistor 2N3906 was placed.
7. Pins I10, I11, and I12 with I10 being Pin one on the transistor. Please note that Pin 3 is on the same rail as Pin 8 of the LM358.
8. The 1 Mega Ohm resistor was next.
9. Pins H12 (Pin 1 of transistor) and E9
10. Next was the 2.2K resistor.
11. Pins C9 and A17
12. Then I placed the first of the three 470 Ohm resistors.
13. I1 to I7
14. Then I added a 22AWG green wire from H7 to H11(Pin 2 of transistor).
15. Another 470 Ohm resistor was added.
16. G7 to C7
17. The last 470 Ohm resistor which will supply power to the green LED and buzzer when activated.
18. Pin 1 of LM358 (D12) to D3
19. 10K resistor which goes out to one of the two test leads.
20. LM358 Pin 2 (D13) to D7
21. LM358 Pin 3
22. A 22AWG white wire from LM358 Pin 3(C14) to D9. This happens to fall between the 1 Mega Ohm and 2.2K resistors.
23. LM358 Pin 4
24. A 22AWG black wire from LM358 Pin 4(D15) to E17.
25. 22AWG Red wire was put on Pin A7 which will goes to the Red Banana Jack.
26. 22AWG Black wire was put on Pin B17 which goes to the Black Banana Jack.
27. 22AWG Red wire was added to Pin E3 for the positive side of the green LED.
28. The Red wire from the positive side of the Buzzer was added to Pin C3.
29. 22AWG Red wire was placed at H1 for Power from Pin 1 of the Rocker Switch.
30. 22AWG Black wire was place at C17 for Ground from Pin 3 of the Rocker Switch.

That is quite a bit to take in even for a fairly simple circuit. It would seem that the breadboard style circuit board adds some complexity to the build. On to the final assembly.

The final assembly can now begin. The installation steps are as follows:

1. I first installed the battery into the holder by sliding it into place. But I needed to add a 0.2mm shim in the form of a manilla tag otherwise the battery would side around a bit. Unfortunately, I 3D printed the lid three times trying to get it right. I had another battery from a different brand, but it would not fit and would require filing the tabs at the top of each post. I really wanted to use the USB C style battery so a shim was necessary. It seems that different rechargeable 9 volt batteries has varying dimensions.
2. Push the Rocker Switch into place.
3. The battery connector red positive wire is soldered to the Pin 2 (middle) of the Rocker Switch. The black wire is soldered to Pin 3 of the Rocker Switch.
4. Push in the Black and Red Banana Jacks and use the washers and nuts to tighten. During deburring of the box I worked slowly so that the jacks would be tight so they would not spin. It helps they have two flat sides to keep them from moving. The red wire from Pin A7 of the circuit board goes is soldered to the red banana jack. The black wire from Pin B17 of the circuit board is soldered to the black banana jack.
5. I then pushed in the Buzzer which also had a tight interference fit due to careful deburring. I used the two M3 x 8mm screws to secure into place using a 1.5mm Allen wrench. Honestly the fit was so tight the screws are not necessary or bit of glue could be used.
6. I then assembled the LED holder. Push the LED leads through the black grommet and insert. As many reviewed on Amazon, the LED wanted to fall out. I used a pair of pliers to squeeze the bottom so the insert would stay put. A bit of hot glue would also work. Then solder on a 22AWG gauge Red and Black wire to the positive and negative posts of the LED. The red wire goes to Pin E3 on the circuit board. The black wire goes to Pin 3 on the rocker switch.
7. Pin 1 of the Rocker Switch is soldered to H1 on the circuit board.
8. Pin 3 of the Rocker Switch is soldered to C17 on the circuit board.
9. Screw the circuit board to the bottom lid using four M2 x 4mm and a PH00 screwdriver.
10. Install the Lid to the body of the Box using four M3 x12mm screws with a 1.5 mm allen wrench.
11. The multimeter style leads can be installed and wrapped around the tester and use the molded in holders. The holders requires a lot of clean up and filing to get my leads to fit. Different leads will require custom clean up. Take your time and check fitment often. If you go too far, you will be printing another box which took me about 10 hours. Even though I listed the needle point lead push on adapters as optional, I find that they give much more consistent results.

Now its done. It may seem like a lot, but it's not too bad. On to testing to verify correct operation and at what Ohm range it works within. It should be 1 Ohm and under. Anything over 1 Ohm should not light the LED or sound the Buzzer. Lets see if I'm correct. Kind of crazy comment because I already know, but I wanted to create some drama.

I used three different resistors with values of 1.0 Ohm, 1.1 Ohm (made using two 2.2 Ohm resistors in parallel, and a 1.2 Ohm resistor. As expected, the tester only lit up the LED and sounded the buzzer when testing the 1 Ohm resistor. It has the same functionality as my prototype, but looks much more professional and less likely to fall apart.

I'm very pleased with this tester. It will be a useful tool in the workshop. As you can see in the picture, the prototype is not very polished. I think I have learned a lot in the past year. Since adding a 3D printer, I have been able to up my game and make more professional devices. Packaging has always been a problem for me, but I feel this tester is an improvement of my skills. However, this box is 120mm x 80mm x 45mm so it is quite big. Even so, I feel that it is very functional. The large size allows it to sit well on the table. I also believe I was able to mimic the continuity sound that most multimeters output which was important to me because a regular buzzer sound is obnoxious. I wanted to state again that I believe the tester works better with the push on needle probes. I'm not sure why, but I think it may be the additional surface area. I would think it would add to the possible resistance stack up of the tester.

I thought about possible improvements and have a few. First, I would try to get the charge port the right size without too much cleanup. But, since I found that different brand 9 volt rechargeable batteries have different dimensions it may not be possible to make the charge port universal. Second, I would like to make the box a bit smaller especially the thickness. If it could be the same thickness of a multimeter, it would be perfect. Finally, adding magnets to the base might make it easier when working in the lab or remote area.

Thanks for checking out my Instructable.

*Updated on 6/8/24 to fix grammatical and typographical errors.