Introduction: Amplified Automotive Stethoscope and Other Scope Compatible Probes

I have an old HarborFreight Stethoscope but it was pretty sad looking and crumbly after sitting on a shelf for a few years so I decided to make a better one.

I used a piezo element and a 16p nail for the pickup and a small LM386 amplifier I designed to directly drive headphones.

Not wanting to throw anything out I also 3D printed an adapter for the old HarborFreight body to fit a small electret microphone.

I added a Low Pass Filter option so it can be used with medical pickups and Phantom Power so it can power the electret microphone.

Still not enough options so I then made a mini C-clamp style pickup from a standard Automotive Knock sensor for really tough to find noises.

The various options helped identify and confirm a Toyota with a bad timing belt idler pulley, an Envoy with bad water pump bearings, a Geo with an AC Clutch bearing that was on its way out, and finally Jeep with noisy rear wheel bearings....


I used my own design LM386 amplifier but any general purpose amp should work.

If you already have something just skip this step.

I had a big bag of Piezo elements on hand...

I use communication headphones as they seem to have a crisper sound that matches my own preferences. Use what suits you best.

Step 1: Building the Amplifier

I used an old white case I had laying around. It had already been used in another project so it was a little beat up but I was in a hurry. I later found the nicer black case shown above which even has a 9V battery door.

The board was originally designed to use pin jumpers for the phantom power and low pass filter but I added switches to make them easily selectable.

I used a BNC for the input as I also use the same probes with my oscilloscope.

I went back and forth on the headphone jack. I really wanted to use a 1/4" style as they are much sturdier.

To make it slim I folded the potentiometers over and had to use a little Kapton tape to be sure the cases did not short out on the board. I probably didn't need to but it cant hurt..

I couldn't find a battery clip so for now I just soldered in the 9V battery. It will probably stay that way too..

If you want to buy the bare board it's available on Etsy...

Step 2: Printing and Assembling the Probe

I used OpenScad to design a simple slip fit housing to hold a piezo element with a 16p nail soldered to its center.

I used a 60W soldering iron and it didn't seem to hurt the crystal with the heat. Be careful about applying too much heat. tin the nail head and center of the disc first. Then heating only the nail head from underneath wait till the solder on top of the head flows out then immediately place the disc on top and continue to heat until the solder flows out to the disc.

Then remove the heat and allow to cool slowly. Resist the urge to blow on it or use canned air to cool it quicker....

Step 3: Clamp on Probe

I had a tough one on the Jeep as about 3 months after I replaced the rear wheel bearings it seemed to develop the same problem. It was fine and quiet when I jacked up the vehicle. it was only noticeable at higher speeds when loaded so I made this sensor to clamp to the trailing arm to "listen" to the bearing at speed and under the full weight of the Jeep...

Almost any Piezo sensor should work. I searched eBay and found this one for $4.99 with free shipping. I already had the I-Beam clamp as I pick them up at construction sites since the workers just like to sweep the fallen ones into the trash at the end of the day.

The clamp had 3/8 threads at both sides but the sensor I had took a 5/16" bolt. I just shoved a 5/16" carriage bolt in there and welded it in place. If you don't have a welder a nut insert or bushing would do or even just drill and cross pin the bolt to the clamp.

If you're wondering why the two nuts it's because then I can saw the bolt off and use those nuts to chase the threads and save both halves of the bolt.

Did I mention I hate to throw stuff out.....

Step 4: HarborFreight Probe Upgrade

After removing the two top hoses from the old HF probe I fitted this small body with a little E6000 glue.

Then pressed on the cap with a small electret microphone in sandwiched in the center...

Step 5: Bonus Ultrasonic Probe

While this probe wont really work well with this amplifier it does use the same body shell as the main piezo probe. I just removed the taper and enlarged the opening so the ultrasonic receiver would slide in freely.

I pulled an ultrasonic receiver element from an old alarm sensor. It already had a phono connector on the back so I just had to add a cable and handle..

This probe works well with a scope for quickly checking the backup sensors on a car for output.

If you want to actually hear the ultrasonic sounds you can check out the "Bat Whisperer" instructable here:

Step 6: Another Oddball Probe

As long as I'm sharing these 3D printed probes I might as well add this one too.

A Piezo based delta pressure sensor... Good for looking at pressure waves in the intake or exhaust systems...

For the intake you can connect it directly to the intake plenum.

For exhaust just use a 5' long piece of 3/8 hose and place the end about 6" in to the tailpipe.

You will see the combustion pulses while the car is running. A misfiring cylinder would show up as a missing exhaust pulse….

Old school way was to place a piece of shirt cardboard over the exhaust and listen to it slap the pipe...

It can only take 25" vacuum to about 10psi of Air only. NEVER connect to any Coolant, Oil or Fuel lines.

I used a regular 1/4 NPT tap to make the threads in the end cap. I never seem to model the taper correctly....

I used a little E-6000 glue to attach the piezo to the end cap. There is a ridge in the base. I was going to use an O-ring to seal the piezo in but why bother. E-6000 is great for this.

To help the screws go in to the housing a little dip in some grease will do. Reminds me of the old carpenters who drilled out the handles of their hammers so they could will the void with beeswax. That way they could dip nails in the wax easily....

Step 7: Alternate Capture/Display Device

I tried using a USB guitar capture device to capture probe sounds. It works well with some Sound Card Oscilloscope programs too....

You just need a BNC tee connector and adaptor cable to tap off the probe signal...