thejonesyboy
New Member
I'd like to build a system that displays the current gear, based on the ratio of engine speed to wheel speed.
I've decided that the final solution will run on an Android IOIO board with a connected Android phone used as the display, but for now I'm just tackling the tacho / engine speed measurement problem.
Connecting an oscilloscope to a wire wrapped around one of the two ignition leads (capacitive pickup) shows a spike wave, spaced at ~75ms (13.3Hz) with a peak of about 100V. Given a wasted spark condition (spark also fires during exhaust phase), this correlates to 400rpm.
View attachment 66228
It doesn't matter that there is a secondary interference pulse, I plan to correct this if it is detected by the microcontroller.
Ideally, I'd like to convert this signal into a square wave with 50% duty cycle, and the same frequency. A microcontroller might not have a high enough sampling frequency to detect the rise and fall of the short spike (Android IOIO definitely does not), so delaying the fall time would make it detectable as a digital signal. I know I won't be able to produce a perfect square wave from this, but I need some ideas as to how to approach it - for example, if with a parallel or series capacitor then what capacitance and why?
I know I might have to make considerations for over voltage protection with a zener diode or equivalent.
There a many other threads here with similar issues, but many seem to approach it using zero crossing detection which wouldn't really help here.
Hoping that someone can give me an idea, and some constructive criticism would be great too
I've decided that the final solution will run on an Android IOIO board with a connected Android phone used as the display, but for now I'm just tackling the tacho / engine speed measurement problem.
Connecting an oscilloscope to a wire wrapped around one of the two ignition leads (capacitive pickup) shows a spike wave, spaced at ~75ms (13.3Hz) with a peak of about 100V. Given a wasted spark condition (spark also fires during exhaust phase), this correlates to 400rpm.
View attachment 66228
It doesn't matter that there is a secondary interference pulse, I plan to correct this if it is detected by the microcontroller.
Ideally, I'd like to convert this signal into a square wave with 50% duty cycle, and the same frequency. A microcontroller might not have a high enough sampling frequency to detect the rise and fall of the short spike (Android IOIO definitely does not), so delaying the fall time would make it detectable as a digital signal. I know I won't be able to produce a perfect square wave from this, but I need some ideas as to how to approach it - for example, if with a parallel or series capacitor then what capacitance and why?
I know I might have to make considerations for over voltage protection with a zener diode or equivalent.
There a many other threads here with similar issues, but many seem to approach it using zero crossing detection which wouldn't really help here.
Hoping that someone can give me an idea, and some constructive criticism would be great too