Another Electronic Det Can Example
 

It's finally time to hit the dyno to tune spark. Speaking with the dyno operator (MER in Cresson, TX -- big time Miata race shop), I asked about knock detection while tuning. His answer . . . "we'll just use the existing knock sensor on the car." Uh oh.

My car is a 1990. It didn't come with a knock sensor. We can add one, of course, but you don't have to search much around here before you realize that det cans and human hearing (evolved over millions of years to dynamically detect minor sound variations) is a much better way to go. So, time to build a det can.

Now the question is, which type. There seem to be three main variations:
1. "Sound-Powered Phone" type. These are purely mechanical devices using a crushed piece of copper tube, flexible (but not too soft) air hose, and hearing protectors. If you're not handy with a soldering iron, these are a good way to go.
2. Electronic Stand-Alone type. These are electronic and use an electret microphone. Usually constructed by purchasing an off-the-shelf personal listener and then modifying it by adding wire to remote-mount the microphone in a battery clip. You listen with an ordinary headset.
3. Electronic PC type. Similar to #2 with the variation that you use your tuning laptop's built-in sound card to amplify the microphone signal. Again, an ordinary headset is used for listening.

I decided on option #3. These are the advandages as I see them:
1. As I'm tuning MS, I'm going to have my laptop on hand anyway.
2. The sound card built-in to my laptop is (I think) more high fidelity than an inexpensive personal listener.
3. If needed, I can use my laptop to record and analyze the audio.
4. I can use robust, shielded manufactured cabling.
5. This will actually cost less and be less work to build.

So, here we go:

Parts (PNs and prices are from Radio Shack -- use the vendor of your choice):
1. 1/8" (3.5mm) Stereo Plug -- PN 274-1547 -- $4.19:
https://www.miataturbo.net/attachmen...ine=1321038673

2. Electret Microphone Element with Pigtail -- PN 270-0092 -- $3.99:
https://www.miataturbo.net/attachmen...ine=1321038673

3. Battery Clip (2 to a package) -- PN 270-0343 -- $4.29:
https://www.miataturbo.net/attachmen...ine=1321038673

Total . . . $12.37.

First, solder the microphone to the stereo plug. The white microphone wire goes to the plug tip. The red microphone wire goes to the plug's middle ring. The bare shielded microphone wire goes to the plug's base and shield. See http://www.hobby-hour.com/electronic...microphone.php for more information on wiring a microphone plug for a PC sound card.
https://www.miataturbo.net/attachmen...ine=1321038673

Next, place the microphone in the battery clip and crimp the clip handle's ears around the plug's shield pigtail. This provides electronic shielding (grounded to your engine block) to reduce unwanted noise from things like your ignition system. It also provides strain relief.
https://www.miataturbo.net/attachmen...ine=1321038673

Finally, pot the heck out of it with hot glue. You're doing a couple of things here: (a) strain relief/robustness/waterproofness/etc. and (b) helping the microphone to only pickup vibrations from the clip rather than the external environment. Note that because of the hot glue and the vinyl covers on the clip handles, this assembly should only be used on the cold side of the engine.
https://www.miataturbo.net/attachmen...ine=1321038673

Finished microphone/plug/clip assembly:
https://www.miataturbo.net/attachmen...ine=1321038673

Assembly plugged into a high-quality 20' 1/8" shielded stereo extension cable (PN 420-2562 -- $12.99):
https://www.miataturbo.net/attachmen...ine=1321038673

Test run. Clipped to the engine mount bolt that secures the starter bracket. Note starter (left), oil filter (upper right) and intake manifold brace (far right).
https://www.miataturbo.net/attachmen...ine=1321038673

The sound quality from this assembly seemed good on the initial test (engine started and idled with some revving). I could clearly hear the hydraulic lifters operating the valves. There was little, if any, hiss or external noise. Volume was good and not distorted. This should work well. I'll let you know after the dyno session on Monday.

You should try the same with an eyebolt, under the head of that same bolt. Securing the mic might be a little harder.

Or, a ring terminal could be secured under the bolt and be adapted to secure the pigtail. Or, you could hack up the battery clip shown, flatten one end and drill a hole for the bolt. If you wanted to build this and leave it in place, that would be cool and would probably improve the sound quality.

Hmmmm . . . .

i used a 2Ga copper lug so I could bolt mine in the factory knock sensor port, so far the hightemp hot glue I used has held but I dont drive around with it on there for very long

Sub'd for results! Perfect timing too! I just decided today to look into making a det can!

Went over this AM and the dyno was broken. Two weeks out.

WTF happened to it? Regardless that sucks! I was really looking forward to hearing results of these... :vash:

Something was wrong with the serial data from the electronic control box. They're sending it off for repairs.

I've been driving around with the det cans. They work great.

Awesome, I think im gonna take the plunge on these.

Sub'd for info. Really want to hard mount a mic to the block using a plate/eye bolt/whetever and run the wires to the stereo plug in the back of my head unit for in car engine noise and the option to plug my headphones into the deck at will. Seems like a good idea for those "did the engine just make a funny noise?" moments.

Nice project Esquire Hornetball.

TorqueZombie, Sean did exactly what you did- plug Jeff's knockbox into his stereo. That way he could listen to his engine while he drove his engine.

Just keep in mind that a 600Ω line-level stereo audio input and an electret microphone jack are two different things -- even though they take the same plug. The electret microphone signal is (obviously) mono. The reason that you use a stereo plug is the microphone jack in a PC provides ~5VDC on the second channel (the "ring") to power the microphone. I doubt that your head unit has a microphone jack, but if it does, you're golden.

Thank you Sir Fae.

Fae's info is spot on. While you can't directly plug the electret microphone into the line-level audio jack of the head unit, you can plug in the output of a microphone power/amp circuit (Jeff's knockbox). There are also fairly inexpensive devices called "Personal Listeners" that can be hacked up for use. They provide microphone power and amplification and have headset outputs that can be plugged into the head unit. I think there are some really cheap ones on E-bay. Radio Shack has one that costs about $30 IIRC.

I just reread this thread. didnt see any mention of what software you used if any to listen to the microphone input live through your headphones. Lemme know what you got! meanwhile im gonna search for something like this.

EDIT: I found an informal way to test the mic, sounds pretty good! Still wondering if you have a better solution than to go into a mic test mode.

In Windoze, I double click on the speaker icon in the taskbar, unmute the microphone input and turn it up all the way. Done.

Thas what I was dooin, just making sure you hadnt found a better solution.

Audacity: http://audacity.sourceforge.net/

Remote mounting the mic in a "personal listener", into the big alligator jack in the pic, is exactly what I did.

I use in-ear headphones to keep ambient noise out. High quality ear-phones help a lot.

Great post. Hornetball thanks for all your help. When ever I post up you alway add something of real value and I appreciate it.

You're welcome.