# Help me with this tachometer

Discussion in 'General Electronics Discussion' started by majdi, Jul 10, 2010.

1. ### majdi

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0
Jul 10, 2010
Hi all, i confusing about connecting motorcycle signal to PIC16f628, if signal connecte to
pin 4 pic lower then 5v it can be accomplish, but how do can it connect signal from motocycle coil to PIC16f628 without damage thi PIC?

here i attch the picture, This PIC circuit work as Tachmeter, calculate RPM any analog signal
please help me

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2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,412
2,779
Jan 21, 2010
A connection like that is likely to destroy most of the components in your tachometer.

You need to pick up the spark inductively (I'd still recommend some sort of input protection)

3. ### majdi

33
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Jul 10, 2010

Thank (*steve*) for your recommend, can i have any circuit or any suggestion website to pick up the spark inductively and input protection?

I found this : http://www.sportdevices.com/rpm_readings/index.htm
are this you mean to pick up the spark indectively?

4. ### majdi

33
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Jul 10, 2010
Hm i to noob about this, sory if i asking to much, after Google up, i found inductive pickup
circuit and DIY inductive pickup... cant this protect my tach and make it work?

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5. ### Laplace

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Apr 4, 2010
Obviously the circuit will work since it comes from a working commercial product. But there are a few things to be considered that are not spelled out. There are 3 series-connected capacitors involved but only the 22 pF is specified. There is the capacitive pickup which could be just some wire or foil wrapped around the high voltage ignition cable where the total surface area covered determines the value of the pickup capacitor. Haul out your physics text if you need to calculate the actual value of a tubular capacitor from its dimensions. Then there is the board capacitor formed by the copper foil on each side of the circuit board. Again, if you know the dimensions of the foil then you can calculate the value of that capacitor. Failing that, some experimentation will be necessary.

Just realize that you are trying to tap off a portion of what is probably a 20 kilovolt fast rise time pulse and feed it into a transistor without destroying the base junction. Too much or too little capacitance will be troublesome. In the motorcycle, is the pulse positive or negative voltage? This circuit assumes a positive pulse.

FYI, the DIY "inductive" pickup above is actually a capacitive pickup.

6. ### Resqueline

2,848
2
Jul 31, 2009
That DIY pickup sure is confusing..
Does the bike have "points"? If so then that's a safer (& more common) place to pick up the timing signal (since it has a peak of "only" 300V).

7. ### majdi

33
0
Jul 10, 2010
Yup the pulse is positive voltage, i try some experiment, but the tacho display not acurate value, if bike throttle pull up, the value go crazy, if the throttle normal position with engine start, tacho show nice value.. what are you thing about this situation?

8. ### majdi

33
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Jul 10, 2010
Sorry i dont get it, what mean "points"? are you mean grounding point? whaa cannot sleep thingking about this..my head came with thousen question finding this solution..huhu

9. ### Resqueline

2,848
2
Jul 31, 2009
Points = the Platinum breaker contacts riding on lobes on the crankshaft, opening the circuit to the coil at the exact times a spark is required. There's a capacitor there too.
Your's could also be electronic (optical/magnetic/inductive pickup) and in that case you'd just hook up to the coil primary (if you have access to it).
Crazy tacho behaviour upon revving is caused by double-trigging of the tacho. It'll most likely be resolved with a lower level signal.

10. ### majdi

33
0
Jul 10, 2010

Hi all, this problem was resolve, i just buy manufactured pickup + join to TTL555 signal pin the tacho show perfect value. DIY pickup cannot been used for acurate value since it just DIY with no RnD..hehe thank all for helping

11. ### CarRayser

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Aug 30, 2013
my 2 cents

I have used a homemade inductive pickup on a ucontroller.
I wrapped a wire around the outside of the high tension lead(sparkplug wire) and used a circuit pretty much the same as what you have.
The wrapped wire covers about a one inch space on the plug wires outside of the insulation.
like they show for tiny tachs http://tinytach.com/installation-gas.php
You can use an opto isolator to make it safe for the Uc(i didn't) just put it between the input pin and your signal conditioning circuit. But aftermarket inductive pickups are just coils of wire nothing more. Pretty easy/cheap to make.

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