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posifour11

Homemade tachometer build

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The Professor has kindly agreed to lead me by the hand in my efforts to make a tach. Hopefully I can make it pretty too.

My vision is for a robust piece of test equipment that can be pulled out to measure RPMs on my kohler singles and possibly my onan twin.

As far as the case, I have a small metal toolbox(I'll get measurements this weekend) I got at a flea market. I also have some kydex sheets that i use for making sheaths and holsters. I think this combo will turn out a strong and usable product.

For the dislay, either digital or analog is fine with me. Digital may be more accurate though?

If I can make it look nice, I may even see if I can get Vinlyguy to do a one-off decal for the tach-a-matic 5000!

I'm thinking it may be neat to copy this into an onboard unit for a rat build I have in my head.

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Keep us updated, I'd love to have one that works right, I have a few here that are junk

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Measurements for the box. 11 inches long, 4.5 inches wide, 3 inches deep.

The kydex can be cut and formed to any size.

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Honestly, I don't really have a preference. I'm guessing digital may be more accurate?

Lets go with whichever is more KISS. That way I may not mess it up.

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Digital is more accurate since digits can be displayed down to the single RPM - BUT - these engines do waver in RPM over a short period of time and the digital readout may be "noisy" (jumpy) if the readout is updated several times a second. The readout can be made less "noisy" by reading the RPM over several seconds and then displaying an "average" RPM, BUT then the readout only updates few seconds.  The readout will lag behind a bit if you juice the throttle. This may require you let the reading settle out for 5 seconds or so before a valid reading is obtained.

 

On an analog readout, the ballistics of the pointer tend to average out any noise in the display. The weight of the pointer keeps the pointer from vibrating back and forth too rapidly and lends a degree of stability to the reading. There are also degrees of "liveliness" in mechanical pointer meters. Rugged meters tend to be a little more sluggish due to their robust construction. "Lively" pointers tend to be more delicate in overall construction.

 

For setting the governor of a single cylinder lawn / garden engine, either is fine. If you have a motorcycle engine that tachs up rapidly, a live mechanical pointer is usually the way to go.  Digital meters would either lag behind the real RPM or be very jumpy to read.

 

 

If you choose digital, we just slap together some microcontroller chips and display panels off ebay, I do a little programming for the RPM function and you have a meter. I'm not sure how much YOU have learned, but you have a meter.

 

If your object is to learn a little about electronics and exercise your soldering / wiring talents, an analog setup is the way to go. A small breadboard, 2 transistors and a dozen resistors and transistors wired to an analog meter and you have a useable meter the equivalent of any modern day meter based tachometer or "auto analyzer".  The issue with the analog meter is the meter movement. There aren't too many nice sized meter movements on the market which already have an RPM scale ticked off on the meter face. The best we could do is purchase a 0 - 5 volt DC meter and calibrate it for 0 to 5000 RPM. So if you where to read 3600 RPM off an engine, the electronics would put our 3.600 volts to the meter. Visually, this would give you a resolution of about 100 +/- RPM. Larger meters would give you more accuracy. Larger mechanical meters cost more too. If you wanted better resolution, a 5 volt meter could be calibrated to read 3600 RPM full scale, but you would need to customer design a meter face or have a decal printer to apply on the meter face. It can be done.

 

You can start to see why I ask so many questions. There are a lot of possibilities. I just want to start narrowing them down to your skill set, pocket book, and desire for digital or analog.

 

I'm going to try to slap together a 2 transistor pc board this weekend and see how that turns out with a small mechanical meter. In the meanwhile, let me know what additional thoughts you have on the digital / analog choice.

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Wow, there are a lot of choices.

I think i like the idea of actually learning. I'll probably learn more than my college level electronics class i faked my way through.

Firstly, i would like to get most, if not all, parts at radio shack. I have one in decent driving distance and I'd like to see what I'm buying in person.

I think I want to go with the 0-5 voltmeter idea. I'm willing to put up with slower response for a more "shock resistant " meter.

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Posi,

 

I have a schematic of an older Heathkit engine analyzer kit I built back in the 1980's. The circuit is a relatively simple 2 transistor setup and is triggered off the ignition points. I'm assuming you want triggering off ignition points and not inductive coupling like a timing light?

 

I'll take a look at the Radio Shack website in the next day or two and see if all the components are available thru the local stores. Hopefully this will work out as there are no exotic parts in the tach circuit. In recent years, Radio Shack has really lost touch with the do it yourself experimenter. If you are not going into the store to buy a cellphone or a Christmas toy, you are usually out of luck.

 

Let's see what we can come up with.

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SOI, triggering off the points sounds fine to me. You're the expert. I'm just along for the ride.

I totally agree about radio shack. They seem to have forgotten what got them where they are. Hopefully, this different store I've found (in a good size city) will have more of the selection. The store i used to go to was a franchise in a small town.

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Have you guys made any progress on this build? I was interested in seeing what you came up with on this.

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We are both dealing with things. But, we'll get after it soon.

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Oh, I know eBay would be easy. But, this will be fun. When we can get together on it.

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Posi,

 

all the weddings are planned,

 

travel is finished for a while

 

and most of the appliance revolt has been beaten back.

 

Let's take another step on this project.

 

I assume an analog readout will be OK ? say a 2 inch meter able to read between 0 and 5000 RPMS?

 

50uarpmmeter_zpsc35a4ac6.png

 

this image shows 0, 1900 and 3600 RPMS.  You should be able to easily interpret down to +/- 100 RPM's.

 

I started prototyping am NE555 timer circuit that would use common, inexpensive devices readily available at your local Radio Shack store.

 

The device will be powered off the 12 volt tractor battery and should remain useable down to around 9 volts - cranking RPMS should be readable even while the battery voltage may be lowered to 10 volts.

 

555tachproto_zps0d4d1af5.jpg

 

555tach_zpsb737b022.jpg

 

I should have some news this weekend if this design is going to function and be useable. If you have the time to do a little reading on the theory behind the circuit, look on the internet for "555 tachometer". There should be plenty of references to this basic circuit operation.

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I'll do my homework this weekend.

The 2 inch analog would be fine with me.

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Russ,

 

I had a chance to finish off the circuit last night and do some initial testing. I have to say I'm very pleasantly surprised at how accurate this circuit is. I'm also surprised that nearly every component is off the shelf @ Radio Shack.

 

The next thing I need to know is how familiar you are with circuit construction - actually mounting components to a hobby board like the one below.

 

http://www.ebay.com/itm/One-Prototyping-Perf-Board-550-hole-matches-830-Breadboard-/111096349877?pt=LH_DefaultDomain_0&hash=item19dddbc4b5

 

Are you good with soldering and the mechanics of mounting circuit board in your chosen enclosure? I just need to get some perspective on your skill set so we can proceed together.

 

I'll put together a shopping list for you and will will need to make a decision on the analog meter for displaying the RPM's. I'm thinking the total parts outlay may not be much more than $20.

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Russ,

 

I had a chance to finish off the circuit last night and do some initial testing. I have to say I'm very pleasantly surprised at how accurate this circuit performs. I'm also surprised that nearly every component is off the shelf @ Radio Shack.

 

The next thing I need to know is how familiar you are with circuit construction - actually mounting components to a hobby board like the one below.

 

http://www.ebay.com/itm/One-Prototyping-Perf-Board-550-hole-matches-830-Breadboard-/111096349877?pt=LH_DefaultDomain_0&hash=item19dddbc4b5

 

Are you good with soldering and the mechanics of mounting circuit board in your chosen enclosure? I just need to get some perspective on your skill set so we can proceed together.

 

I'll put together a shopping list for you and will will need to make a decision on the analog meter for displaying the RPM's. I'm thinking the total parts outlay may not be much more than $20.

 

Chuck,  Is there any chance you will be selling these units on Red Square ?   :dunno:  I would be very interested in one.

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Chuck, I'm pretty good with a soldering iron. I grew up working on my parent's CB equipment.

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Chuck,  Is there any chance you will be selling these units on Red Square ?   :dunno:  I would be very interested in one.

 

 

PM sent

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More good news - the circuit appears to be linear up to and beyond 5000 RPM.

 

Last weekend, I had the opportunity to attach the tach circuit electronics to a physically larger and easier to read multimeter - a Simpson 260. The meter has a 50ua scale that is compatible with the output of the tach circuit.

 

555tachsimpson26050ua_zpsbe3f9e6f.jpg

 

 

 

 

The user must interpret RPM off the 0 to 50 scale on the meter face and perform a mental x100 multiplication. A meter indication of 20 would be 2000 RPM. A meter indication of 35 would be 3500 RPM and so on.

 

 

555tachsimpson260rpmscale_zps318ec3bb.jp

 

 

 

3600 RPM represents the set point for proper governor adjustment on most engines used in lawn and garden applications. This is the point at which I will calibrate the circuit to be most accurate. At this RPM, the ignition points are opening and closing at a rate of 30 times per second. The frequency generator I have hooked to the tach circuit simulates the points opening and closing at 30 times per second.

 

 

555tachsiggen_zps6ef7bd5a.jpg

 

 

The "calibration" control on the tach circuit is used to then set the meter readout for exactly 36 on the 0 to 50 scale on the meter.

 

 

555tachsimpson2603600rpm_zpse8116bf7.jpg

 

 

After the tach circuit is calibrated to a known RPM, the frequency generator is turned to a setting representing 1800 RPM or approximate idle speed of a single cylinder engine.

 

If all goes well, the tach meter indication should display 18 on the 0- 50 scale.

 

 

555tachsimpson2601800rpm_zpsc3a794b4.jpg

 

 

Impressive!

 

Let's check 4800 RPM

 

 

555tachsimpson2604800rpm_zps72ed2677.jpg

 

 

I'll buy that. The resolution on the Simpson meter would make it possible to read down to about +/- 25 RPMs (the pointer could easily be interpreted to be 1/4, 1/2 or 3/4  the distance between adjacent marks on the meter scale).

 

At this point in the project I feel very comfortable with the feasibility of this design and will start to put together a "Radio Shack" shopping list for anyone who wants to follow along with our experiment.

 

I'll try to get a schematic posted in the next few days. I'm still working out a voltage stabilizer to keep varying battery voltage from affecting the meter reading. I have a few small three terminal voltage regulators on order for this week. They should put an end to any meter variances caused by changes in battery condition.

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Nice work,Chuck! :bow-blue: Just noticed your posting times.  I didn't know squirrels were nocturnal . :ychain:

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I try to maintain a predominately crepuscular schedule during the hot summer months. Next time if you are up and awake, send me a instant message and we can chat. Thanks for the kudos.

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Simply amazing! Soldering gun at the ready!

ETA: I've read about the 555. Couldn't really wrap my brain around most of it. But, Wikipedia gave me the best understanding...

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Yeah, it is pretty cool. I really did not expect this circuit to yield the outstanding performance that it has. I expected the meter to be somewhat jittery due to the low pulse rate coming off the a 1 cylinder engine, but the mods I made to the circuit show no jittery meter movement what so ever.

 

The next step to take is search out a suitable meter for your tachometer. Search for "50ua meter" on ebay. This search should result in many meter movements which would be useable for your project. Select a movement that has a scale from 0 to 5 and is within your size and budget restrictions. I have ordered meters from China before and found them to be of good quality. I see meters that range from $5 to over $50 that would be suitable. The only requirement is that the meter is 50ua and has a scale from 0 to 5.

 

I like this one

 

http://www.ebay.com/itm/Panel-Meter-SD-670-60x70mm-DC-50uA-Class-2-0-/110937113234?pt=US_Radio_Comm_Antennas&hash=item19d45e0292

 

Make certain it will fit properly onto your enclosure - also check the depth of the meter versus your case.

Remember, the bigger the meter, the easier it will be to read. The "ua" designation on the meter face can be covered with a sticker printed to read "RPM X100". Most meters have a clear plastic cover which can be temporarily removed to allow modification of the print on the meter face.

 

Let me know if you have any questions.

 

I will put together a Radio Shack shopping list this weekend. I will also do a small writeup on how the circuit works to convert electrical signal from the ignition points to a current to be read off an analog meter.

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