Digital tach cheap!

[rwd4evr]

So i figured out a way to build a cheap digital tach. i ordered this,
DIGITAL RED LED PANEL FREQUENCY SPEED METER TACHOMETER (230920680485) from ebay. i got it for 19 bucks. i have a hall effect sensor(speed sensor) off of a proform life styler tread mill, i got it free but they are like ten bucks. a sensor for a rc gas engine ignition should work too. it will run on a 9 volt battery and counts to 50,000 rpm. the best part is it is lit up, i have a fake tiny tach that worked for one engine test. i put it away and now it doesn't work any more. it was hard to read and was super slow to react to engine speed changes. it will read a pulse when a magnet passes by it. on some engines the flywheel magnet will work, but some have two magnets and that will cause it to read double what the engine is turning. one of the engines i'm working on has two in the flywheel so i will use a small rare earth magnet epoxeyed inside the edge of the flywheel(and a counter weight across from it). i'll post more when i get it.

 

[rwd4evr]

and i found this.
0.56" DIGITAL Red LED Frequency and Tachometer Rotate Speed Meter Two Function also on ebay.it will work too i'm pretty sure its programmable and its cheaper, DOH! make sure you get the 9-12 volt model.

 

[rwd4evr]

It works

so i got the tach read out today. i put a quick test rig together with my ryobi two speed drill with a grinding stone in it. the shaft is steel so i attached a small rare earth magnet to it. the hall effect sensor was mounted in a paper clip close to the magnet. wiring was easy. power from a rc car receiver pack to the supplied power pigtail. ground the tach to the battery pack negative.the sensor is hooked between the positive and the pulse wire on the tach, so when the magnet passes by it pulses. here are some pics of it working. the drill is rated at 1300 rpm in high and 350 in low. it is almost dead on and im sure the drill is off not the tach. it also has a adjustable jumper so you can run two pulses per revolution or even ten. it nice and bright, was messing with my camera. it responds very quick too.

 

[rwd4evr]

It has a built in voltage regulator too, so it will run from 5 to 15 volts. i think a 9 volt will be perfect. it only draws 100mA.

www.dropbox.com/s/n1fth97h249snjh/6150rpfTACHO.pdf?m
instructions

 

[mckutzy]

So how doe it work with larger diameter shafts(flywheel) ect. Does it compensate or need to be adjusted for what ever diameter of rotating wheel or shaft it is used on?

Cool none the less.

 

[Doc Sprocket]

I'm going to run out on a limb and say that diameter does not matter. A revolution is a revolution whether it's 1/4" or 50'.

That's a nice, big display. I like!

 

[mckutzy]

I was just thinking that some tach's need a specific dia to work. I guess they have a computer to work that out as opposed to this one.

 

[Kaptain Krunch]

I'm going to run out on a limb and say that diameter does not matter. A revolution is a revolution whether it's 1/4" or 50'.

That's a nice, big display. I like!

However, the time it takes to make that revolution depends on the diameter of the flywheel, or whatever the pickup is taking its reading from. A motor spinning 1500 rpm, with a 12" flywheel is going to take far fewer readings/min than a motor spinning 1500 rpm with a 2" flywheel.

Also i might add, epoxying a magnet to your flywheel, with a counterweight could be very risky. If you do this i would suggest sending it out to get balanced, especially if your running ungoverned.

 

[Doc Sprocket]

However, the time it takes to make that revolution depends on the diameter of the flywheel, or whatever the pickup is taking its reading from. A motor spinning 1500 rpm, with a 12" flywheel is going to take far fewer readings/min than a motor spinning 1500 rpm with a 2" flywheel.

I need further clarification on this. If a 2" wheel and a 12" wheel are both turning 1500RPM (for arguments sake) then both wheels rotate 1,500 times every minute. Both wheels rotate 25 times per second. 25 times every second, the sensor is triggered. Where is the difference?

 

[HaM551]

The angular velocity is different but rpm is the same.

 

[Kaptain Krunch]

I need further clarification on this. If a 2" wheel and a 12" wheel are both turning 1500RPM (for arguments sake) then both wheels rotate 1,500 times every minute. Both wheels rotate 25 times per second. 25 times every second, the sensor is triggered. Where is the difference?

Hmm, now that im thinking you might be right. Not sure way too early for this haha.

Wouldn't the rpm at the outer edge of the flywheel be different than that at the crank though? maybe im just overthinking this...

 

[Doc Sprocket]

You're overthinking. RPM is RPM. The angular velocity thing is what's got you. The outside edge is moving faster. If these were tires, the 12" wheel would be travelling further in the same amount of time, yet RPM is the same. So, you're travelling faster at the same shaft speed. Imagine increasing rear tire size on your ride without changing any other factors.

A speedometer is sensitive to diameter. A tach is not.

 

[mckutzy]

Ok, I see now what you are saying.

 

[rwd4evr]

i'm gonna try to use the magnet in the flywheel first. if its doubling the reading then i can change the jumper so it reads two pulses per revolution. i'm not sure how that will work though since it will be two pulses very close together and not evenly spaced. i have a lock collar on the inside of my clutch i'm going to mount the magnet on there i think if i have to. the small diameter will make balance less of an issue. it is ungoverned but i'm using the governor arm to hit a kill for a rev limiter set at about 4500. i figure that should be safe. i have a long spring on it i can loop wire through to adjust the stiffness. going to work on it now i'll get some pics.

 

[Doc Sprocket]

Very interesting proposition for a rev limiter. I should like to see the detail on that. Is it as simple as the arm hitting a momentary pushbutton switch tied into the kill circuit? 4500 is probably about as far as you want to push the flyweight/arm assembly.

Awhile back I proposed an electronic rev limiter, IIRC Kibble was working on it. Don't know whatever happened to the concept, guess he got busy. Unfortunately, my electronics knowledge isn't quite strong enough to pull it off myself.

 

[rwd4evr]

yeah, thats the plan. i hadn't thought about the governor parts letting go. that would probably be bad. i'll keep it a little lower for now.

 

[Doc Sprocket]

Still- a good idea, IMO. It would be a great solution for somebody that wants to swap carbs, and yet still have the maximum RPM limited.

 

[Nodroz]

Would this be possible to install on a Zongshen 150cc engine? It's free shipping to Belgium, so I would love to get it. I need to be sure I can use it though. This engine has 5 gears.

 

[gixxerkart]

one of the engines i'm working on has two in the flywheel so i will use a small rare earth magnet epoxeyed inside the edge of the flywheel(and a counter weight across from it). i'll post more when i get it.

God help us all....
When those magnets and counterweight INEVITABLY come flying off I hope no one is around...

 

[builder bob]

just wondering, could u use a tach that's designed for r/c airplanes and just stick a prop on to test the speed?