Faking an RPM signal for a Mychron 5 Karting lap timer - Page 1
Forum section: electronics / projects

author: Perry
author_rank: Newbie
timestamp: February 27, 2017, 06:38:27 pm
content: I recently bought a Mychron 5 lap timer for use in an endurance karting arrive and drive series.http://www.mychron5.com/I bought this one specifically because it is self-contained compared to its predecessor the Mychron 4 in that the 5 can use GPS (instead of an IR sensor and trackside IR beacon) to calculate lap times, splits, deltas, etc.Unfortunately, one of the assumptions made by the programmers was that the unit would always be getting an RPM signal while in use. In my case, that is not true. I do not have time to hook up an RPM lead and even if I did, it would be against the rules.The unit will also auto power off without receiving an RPM signal, and there is no way to disable the auto power off.Clearly, this could all be fixed with a firmware update to the unit, but I am not holding my breath on that. In the interim, I wanted to overcome these issues by making the unit think it was getting an RPM signal.The unit is designed to read rpm through a wire that is wrapped around the engine spark plug wire. The other end of that wire is looped through a hook on the back of the Mychron that holds the wire close to what I assume is an internal inductive (or capacitive?) pickup.The end if the wire goes through the plastic hook here on the bottom right.https://c1.staticflickr.com/9/8668/16351804111_6b7aed1b27_z.jpgI wrapped a wire around a spark plug wire of a running engine and took a look at it with my oscilloscope. It was a very noisy signal with obvious voltage spikes. The spikes occurring on every other rotation of the crankshaft (for a 4 cycle engine). I don't recall the amplitude, but it was high... at least 100 volts (no load).I tried to think of a small DC circuit that could replicate this, and I immediately thought of: shock pranks, stun guns, cattle prods, etc. I eventually recalled the cheap electronic flyswatters like this one:http://www.harborfreight.com/electronic-fly-swatter-62540.htmlMore info here:http://rimstar.org/science_electronics_projects/electric_fly_swatter_zapper_racket.htmAfter taking the cap, bleed down resistor, and diode off of the output, I ended up with a signal that looked alot like what I was looking for.... periodic spikes of 100's of volts. When fed to a wire looped through the Mychron, this signal actually turns the unit on and will prevent it from powering down - all good news.The problem is that this signal is still not seen as RPM by the Mychron because the frequency is too high. The frequency is right at 5000 Hz. Even if I tell the Mychron to divide the RPM signal by 4 that still equates to 75,000 RPM. That is too high for the unit to read it. Ideally I need the RPM to be down around 6000 (although it will read up to 24,000). Unless the unit sees a valid RPM signal, it will not record data or provide sector times and splits.So, what I need help with is:1) Slowing down the output of this circuit if possible. I have fiddled with the resistor on the transistor side of the transformer and increasing that resistance increases the frequency. Lowering it lowers the frequency, but not enough - and too low risks damaging the transistor.or2) Identifying another circuit that has similar characteristics... a) a 100V periodic spike b) generated using a small number of batteries c) cheap.... and also has a lower frequency.... say 100 - 800 Hz.Any help is greatly appreciated.-Perry

author: tpowell1830
author_rank: Frequent Contributor
timestamp: February 27, 2017, 06:56:26 pm
content: Without knowing your input load characteristics of your device, it is guessing.A simple 555 timer and a voltage multiplier circuit should do it. Will run from 4-6 triple A batteries or a 9 volt battery.Another way, since your signal is not noise sensitive, is to make or buy a small step up transformer and and drive it with the 555 timer and a mosfet driver.!!! CAUTION HIGH VOLTAGE!!!

author: Rerouter
author_rank: Super Contributor
timestamp: February 28, 2017, 08:48:47 am
content: I dont know the exact device, but i regularly work with similar devices, you will likely find it will work off a 5v square wave, and if not, ac couple it through a capacitor 47uf and use a 10k resistor between signal and ground on the unit side

author: CJay
author_rank: Super Contributor
timestamp: February 28, 2017, 11:55:54 am
content: A 555 with very low duty cycle driving a Darlington transistor like the ZTX600 or BD683 to pulse a loop of wire will simulate an ignition pulse quite adequately.The main thing is the drive signal has to have sharp edges and be very low duty cycle or you'll not get good pickup and/or fry the transistor.It'll run on a simple 9V battery.(from experience with developing and marketing a pulse shaper that removed problems with wasted spark ignition systems causing multiple and or spurious triggering on diagnostic engine analysers)

author: Perry
author_rank: Newbie
timestamp: February 28, 2017, 02:59:41 pm
content: Thanks for the help. I will experiment and give these a shot when I get a chance.A few comments...As far as a load, there is not much of one at all... the only load with be whatever coupling occurs between the little hoop of wire that sits near the back of the device and whatever sensor it is using inside to detect the signal. There is no conductive connection between the RPM lead and the Mychron - it is picking up the signal inductively (I think) through the plastic casing of the Mychron. It just has a small holder to trap the wire close to the sensor.Regarding a 5V square wave. That was one of the first things I tried. It did not work. The Mychron wants to see pulses in the 100V range. I would have to go back to check, but I think 75-80 volts was where the cutoff occurred. I tested that by lowering the input voltage on the electronic flyswatter circuit. (Though, that changed the shape of the waveform somewhat, so that could have affected things as well.)

author: Ian.M
author_rank: Super Contributor
timestamp: February 28, 2017, 04:41:24 pm
content: If the sense wire isn't either a continuous loop, or grounded at both ends, it must be a capacitive pickup. Try sticking foil tape in the recess either side of the clip, connected to your fake spark signal to maximise the coupling area, which should reduce the voltage required. It would be worth trying an Xenon flash trigger transformer driven by a HV transistor, pulsed by a 555, with a very low duty cycle. If you are lucky, you'll be able to get enough output running it off a low voltage supply, otherwise you may need a boost circuit to feed the top end of the trigger transformer primary.

author: CJay
author_rank: Super Contributor
timestamp: February 28, 2017, 07:09:58 pm
content: Quote from: Ian.M on February 28, 2017, 04:41:24 pmUmm, sort of, it's been a while since I was in automotive diagnostics and the probes were used for different things, the inductive probes were used for triggering timing, the capacitive were used for measuring actual wave shape, I *think* what he has there is a capacitive probe working as an antenna.However, I digress, having read his original post again in detail, I think a TENS machine would work to provide a suitable input signal and there are plenty of schematics on the 'net for those.

author: mdrost
author_rank: Newbie
timestamp: September 11, 2017, 06:58:21 pm
content: Quote from: Ian.M on February 28, 2017, 04:41:24 pmPerry, did you ever figure out a solution? I am in the same situation now and need a solution.