Homemade 555 Tachometer

[Kris01]

Hi everyone, I found this schematic online and I'm curious how I can adapt it to my motorcycle.

My bike is a 1 cylinder, 4-stroke engine that fires the plug on every stroke (not every other like normal). As I understand it the circuit outputs 0.01 ua for every 1 RPM. 50ua = 5000 RPM. In my case I would need a 100 ua meter to read 5000 RPM since my plug fires twice as much as most engines. The circuit is designed to read a plug that fires every other stroke. My bike has a "wasted spark" ignition system. There also are no points. It is a CDI ignition system. Can I wrap a few turns of wire around the spark plug as input instead of connecting to the non-existent points? Does any of this look out of place or could it be done better?

 

[alec_t]

I can see that circuit posing durability problems, even for its original intended use.
Personally I would not use a sensitive moving-coil type of meter on any vehicle. It's likely to get shaken to bits unless very well protected from vibration.
If your bike has CDI ignition then presumably the magneto/alternator has a dedicated coil for driving that? That would be the best place IMO to pick off a voltage for driving a tacho.

 

[Kris01]

If your bike has CDI ignition then presumably the magneto/alternator has a dedicated coil for driving that? That would be the best place IMO to pick off a voltage for driving a tacho.

Do you mean in place of using the inductive pickup on the plug?

I was looking at some cheap Chinese made meters on Ebay. They're likely to get shaken to death so I don't expect a long lifetime. However, I'm only experimenting so I may just use this project to tune my idle RPMs. Thanks Alec!

 

[alec_t]

Do you mean in place of using the inductive pickup on the plug?

Yes. But if the inductive pickup works ok then go for it. I was thinking a voltage pick-off would avoid the need for one.

 

[Kris01]

Ok, so the circuit isn't picky. I need to find some part of my ignition system that turns on and off when the plug fires. That would take the place of the points opening and closing in the diagram. I guess I need to find a wiring schematic for my bike.

Thanks.

 

[Kris01]

Here's an ignition diagram for my bike. I deleted all the unimportant bits. Any clue as to what I should use as an input?

 

[Les Jones]

The coil shown on the left side of the alternator is the ignition trigger. (Wires G and O) You next need to see if either one is connected to ground. Bear in mind that the coil resistance will be quite low so you will need to unplug it from the igniter unit. It is the pins on the ignitor unit you will be testing. (All the other plugs must still be connected to the igniter unit for this test)

Les.

 

[Kris01]

So either G or O will provide the pulse for the tach?

Thanks Les.

 

[alec_t]

So either G or O will provide the pulse for the tach?

Only if one of those is non-grounded and the other is grounded. You need to check that, as Les said.

 

[crutschow]

For a more more rugged meter, you might just use a cheap digital panel meter from ebay.
Just make sure it's one that doesn't require an isolated supply voltage.

 

[Kris01]

Not really wanting to go digital. I like the analog movement. I'll check for ground when I get some free time (and it warms up above freezing). Thanks.

 

[alec_t]

I like the analog movement

If you can find one with a basic sensitivity of 1mA, say, it would probably survive longer than a 50uA meter. The series resistor (trimmer) can be chosen accordingly.

 

[hyedenny]

This would be a terribly inaccurate tachometer! There are other WAY more accurate (and easier) ways of making an analog tach. Maybe an LM 2907/LM2017 frequency to voltage converter?

 

[spec]

POST ISSUE DRAFT 8 of 2017_01_26

Hi K01,

I can't resist anything involving motor bikes so I had a look at the original circuit in post #1 and worked out a universal trigger input that would just connect to the 'CB' terminal on the coil, regardless of what type of ignition: traditional Kettering, solid state assisted, CDI. I also improved the accuracy and tolerances by changing a few components here and there and specifying component types that would improve the accuracy, the 100nF timing capacitor being the most significant.

But I also looked at other circuits, and on balance, the circuit shown below may be worth considering.

I assume that the engine on your bike produces a spark every revolution of the crankshaft and that the maximum engine speed is 10,000 RPM.

Do you have any idea, at this stage, what meter you are going for. I have assumed a moving coil meter with a maximum full scale current of 1mA.

spec

SCHEMATIC TO BE POSTED
​

NOTES
(1) The timing capacitor is a polypropylene type for high stability and zero leakage
(2) All resistors are 1/4 W (250mW) or more, 5% or better, metal film, Except the input resistor which is 1/2 W (500mW) or more, metal film, 10% or better with a 500V minimum rating.
(3) The capacitors are critical to the design and must be the type shown.
(4) The physical layout must be compact, using short wires and component self leads.
(5) The deign is suitable for both 6V and 12V systems.

DATASHEETS & SUPPLIER
(1) CD74HC4538: https://www.ti.com/lit/ds/symlink/cd74hc4538.pdf
**broken link removed**
(2) LP2950-5: https://www.ti.com/lit/ds/symlink/lp2951-n.pdf
https://www.digikey.com/product-det...r/LP2950ACDT-5.0G/LP2950ACDT-5.0GOS-ND/918530
(3) IRLML6302TRPBF: https://www.infineon.com/dgdl/irlml6302pbf.pdf?fileId=5546d462533600a4015356688c702627
https://www.digikey.com/product-det...ogies/IRLML6302TRPBF/IRLML6302PBFCT-ND/812510
(4) 47nF polypropylene capacitor: https://www.vishay.com/docs/28128/mkp416to420.pdf
https://www.digikey.co.uk/products/...=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25
(5) 1uF ceramic capacitor: https://www.vishay.com/docs/49363/49363_vmn-pt9161.pdf
https://www.digikey.co.uk/product-detail/en/vishay-bc-components/K104K10X7RF5UH5/BC2665CT-ND/2356879
(6) 10uF, tantalum capacitor: https://www.vishay.com/docs/40080/tr3.pdf
https://www.digikey.co.uk/product-detail/en/vishay-sprague/TR3C106K025C0500/718-1769-1-ND/2260014