Somewhere near topic - Electronics project

[dogshome]

Hi all, I swapped the gauges on my R65 because although functional, they were looking very old internally and the horrid round white connector was broken. I fitted an Acewell speedo, which looks original, has all the idiot lights, and after a little wiring rationalisation works well.

The only issue I had was the high powered Silent Hektic ignition caused the odd 'reboot' of the speedo at higher speeds. Rewiring the ignition power and signal leads in star configuration solved that. Looks the same on the circuit diagram, but currents go where they should instead.

I wanted an RPM counter to match, so the key stayed in the middle and the gauges looked stock-ish. The only one I found in the right diameter, RPM and style is a really cheap one for cars and boats. It's very well made, uses a stepper motor and a micro-controller, with adjustment for cylinder count and lost spark like our bikes have. It does NOT like the Silent Hektic ignition 

[dogshome]

So, armed with $20 oscilloscope kit, I can see the 'triple spark' that SH humerously say gives more power in "the basement" 

All modern ignitions do this to one extent or another, and it is noticeable in better idling and transition.

The pic with the spikes is the engine running and you can see the RPM from the timebase. 40mS is 1500RPM. 4 stroke twin with lost sparks.

The second pic shows the triple spark which appears on a much faster time base,  0.4mS or 150,000RPM.


The ringing and spikes you can see is typical of ignition coil discharge. 

[dogshome]

The RPM gauge can see some of these fast pulses, not just the slow ones we want it to. So it doesn't know whether to display zero, something or full scale.

Enter the trusty 555 timer 

https://en.wikipedia.org/wiki/555_timer_IC

And from the depths of my memory, a monostable (timer) is required. I set the time to something like 4mS and that should fix it 

https://www.allaboutcircuits.com/tools/555-timer-monostable-circuit/

I did have a bag of old 2N3055s, 4001s, 74LS00s, BC108s etc, but can't find a single 555  So now waiting for postman to bring half a dozen at 30p each.

[dogshome]

Something like this:

[dogshome]

Grr. This ignition pulse is causing grief. Difficult to filter. Pikes, ringing, noise. The gauge is very forgiving, taking 1 to 99% mark space at 12V. Also happy at 6V. but it's its the spurious pulses coming out of the SH that are confusing it. Perfectly normal.The 555 has some foibles also. Latest design is detecting the high peaks 18V, but the odd wrong detection is slipping through.

So its off the bike for now.

If I can find an 85mm analogue tacho (integrator circuit), that might be less fussy.

[Justin B.]

Could you use some diodes to "clip" the pulses to around the same size?  I think this is how some of the simple ANL circuits in radios work.  Then you could rig a circuit to basically divide by 3, maybe...

[dogshome]

The problem is the cobination of high voltage pulses (30V), small pulses at low revs (multispark), variable width of the real pulses and limitations of the 555.

I have a cunning plan (involving zener diodes) and a work around of the 555 not being a true monostable though.