new(very new) to electronics need help with making a tach for a briggs engine

[cowboy666]

i have absolutley now knowledge of electronics and i'm trying to build a tach for my racing go kart(5 hp b&s methenol motor) i have some parts and if anyone knows a way to build a circuit with what i have please let me know also if you put up a schematic please explain it to me in simple people terms (aka im retarded when it comes to electronics i thought it was great cause after 5mins i finally figured how to get an led to light up)
ok what i have is a
1 ne555 chip
2 lm339 chips
2 7 segment leds will have 2 more after i tear apart my old cb radio
4 zener diodes and a bunch of resistors from a circuit kit my younger brother got and never put together
also have some capacitors
i have a bread board to try circuit out on and a board from radio shack to put working circuit on i need to be able to measure from 100-8500 or 10000 rpm any help is greatly appreciated
also im not sure i posted this in the right forum if not could one of the admins please move it to where it belongs thanks

 

[cr0sh]

The short answer is you're going to need more parts than what you have, plus more knowledge.

The easiest way to tackle this would be to use a microcontroller; unfortunately, you would still need the electronics knowledge to interface it with your engine, then some programming chops to create the code.

The concept of an RPM counter, though, is fairly simple - you just have a trigger (off the magneto, the spark plug, or perhaps with a hall effect sensor) that starts a timer counting upward, which is reset every rotation; for instance, let's say the motor was turning at 1 RPM - so it takes a minute to rotate once; at TDC you start counting seconds, if you count 60 at the next TDC, then you are at 1 RPM. For higher speeds, you would obviously be counting milliseconds, and you would have to do some division in some manner to get your final display, but that's all simple math in the end.

Doing this with discrete components and ICs is certainly possible, but it would be much simpler to implement with a microcontroller in the end.

 

[carbonzit]

Yep; as that reply said, much easier with a microcontroller, meaning probably a PIC (that's what most people use these days). So unfortunately you won't be able to use many of those parts you've collected (maybe for another project, eh?), but then you still get to mess around w/electronics.

You'll also have to learn a little computer (micro) programming, or you can find a 14-year-old kid to do that part of the project ...

 

[bryan1]

going off memory here but a LM2917 frequency to voltage chip and an analogue gauge would be the easiest way to do this. Put a magnet on the output shaft and use a hall effect sensor for the input a few discreet parts and a rpm gauge. I'm quite sure there would be a thread on the forum so a search may be needed but that is the easiest way I can think of to do this.

Regards Bryan

P.S. Welcome to the forum mate

 

[Mr RB]

Tachos have been made with monostables and voltmeters since the early days of electronics.

Rig the 555 as a monostable (with a fixed output period), that triggers once for every input pulse. Then filter the 555 output with a resistor and a cap. The voltage there will be linear and proportional to RPM. Then you can just use a analog mV meter. If you use a 10v meter and 12v supply you can easily calibrate it so 10v = 10000 RPM.

 

[cr0sh]

Tachos have been made with monostables and voltmeters since the early days of electronics.

Rig the 555 as a monostable (with a fixed output period), that triggers once for every input pulse. Then filter the 555 output with a resistor and a cap. The voltage there will be linear and proportional to RPM. Then you can just use a analog mV meter. If you use a 10v meter and 12v supply you can easily calibrate it so 10v = 10000 RPM.

>smacks forehead<

That's actually a great solution; I hardly ever think of analog-type solutions to problems - part of it stems from being a software developer with intimate knowledge of digital computers, and the other part stems from never gaining a real good grasp on certain analog techniques (never mind RF!). Though what you propose really isn't much different from methods used to generate sound output with a microcontroller without using a resistor ladder and a ton of pins. Plus, it gives the OP a means to use and learn about the parts he already owns (and you could probably rig a digital ammeter for the output if you wanted numbers instead of a gauge). Great solution!

 

[debe]

2 circuits ive used for single cylinder engines, one using a hall device salvaged from an old VCR reel sensor . The other using points, or can be used by wraping about 8 turns of wire around the spark plug lead as a pickup point. Daryl

 

[cowboy666]

ok MR RB could you please explain how that would be done i posses no electonics knowledge and i have seen shematics for that but i dont understand them if i can find the one i have seen and post it can you or someone please explain it to me (in stupid people terms) it would be alot of help cause one of these race kart tack goes for any where between$200-400 maybe even a little more so i was tryin to go a cheaper way and this seemed like this was it only i seem to have got in over my head thanks

 

[tcmtech]

How about this or something similar?
No electronics skills required.
**broken link removed**

 

[carbonzit]

2 circuits ive used for single cylinder engines, one using a hall device salvaged from an old VCR reel sensor . The other using points, or can be used by wraping about 8 turns of wire around the spark plug lead as a pickup point. Daryl

Here's a redrawn version of your "tacho" drawing. Not that yours was bad, mind you. But notice the file size difference (~9K vs. ~1,800K):

**broken link removed**

 

[debe]

Hi cowboy. In electronics this is as simple as it gets for a tacho. Do you have a freind who is capable of doing it for you.

 

[carbonzit]

Hi cowboy. In electronics this is as simple as it gets for a tacho. Do you have a freind who is capable of doing it for you.

To explain further, if you decide to use this little circuit, it can be very easily made from parts that are very available, maybe even locally. It can be built on a little piece of perfboard (insulated phenolic board with lots of holes drilled in it), or some other method; no need for a printed circuit board or anything. Probably the hardest parts will be calibrating it; the adjustable resistor needs to be set, and the meter scale needs to be redrawn if you want it to read directly in RPM (or you can just leave it and put a conversion table next to it, like "5 volts = 1000 RPM" or whatever). Requires a little soldering skill, but that's about it.

 

[debe]

To calibrate if your using a Hall device I used a small coil to triger the Hall. 50HZ gives 3000RPM. 60HZ gives 3600RPM. also pic of the completed circuit. You will notice Ive used recycled Tachos, prefer these to digital as they are quicker to respond to RPM changes.

 

[Mr RB]

ok MR RB could you please explain how that would be done i posses no electonics knowledge and i have seen shematics for that but i dont understand them if i can find the one i have seen and post it can you or someone please explain it to me (in stupid people terms) it would be alot of help cause one of these race kart tack goes for any where between$200-400 maybe even a little more so i was tryin to go a cheaper way and this seemed like this was it only i seem to have got in over my head thanks

There have already been some good circuits posted. It's hard to advise when you have "no electronics knowlege" as all circuits with an IC will take some skill to build.

Here's another option, look around the cheapy auto stores, and see if you can get a $10 or $15 "auto tune up meter". I see them quite often. The ones with a analog needle type meter and that show RPM or DWELL with a switch to select cylinders etc.

Provided this works with your motor (which it probably will) you then pull the circuit board out of the device and connect your own (smaller) analog meter. And put it in a small neat box if you like.