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ol doc gully

DIY shift indicator light with distributor/coil?

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With my seating position, my small-diameter fat-ring steering wheel is blocking my tach view right in the key areas. OK for DD but already at the first autox I had to keep hunching over and was undesireably encumbered.

 

So I'm looking to make a shift light/indicator for my 72 240Z. Still in the concept stages and any suggestions/resources/corrections are greatly appreciated; I've found a surprising lack of any information of this kind. 

 

Im aiming at passive electronics, but may consider Arduino. I'm comfortable with either but mostly just would like to figure out how such a shift light indicator would work when fitted to distributor ignitions - ie what that signal looks like coming off the coil, or similarly even just how a tachometer works in that sense.

 

So really the key is confirming that it is a 12V square wave with frequency reflecting that of all spark plugs firing. I have a multimeter but no data collection hardware at my house. 

 

I presume its 12V at the positive terminal where the tach connects. I have a 3 ohm coil, no ballast resistor. I have an electronic dizzy but thinking about it as points seems viable and simpler, so my thinking that I'd like to verify (please correct if wrong): voltage builds in coil when the contact is closed then releases into plug when it opens. so i would think there would be 12V at the input terminal when the contactor is closed and goes to zero when it fires - thus spark signal could be considered a falling edge triggered event. 

 

Although necessary to make sure I'm thinking correctly, that level of detail isn't really functionally necessary as all we need is frequency of this signal. I could just determine the RPM I would like the light to go off at, then correct to 'revolutions per second' then multiply by 3 (inline 6 = 3 ignitions per revolution) and we have target cutoff frequency to design circuit for. Right?

 

I havent thought through the circuitry too much yet but generally seems like it would just be:

high pass filter (tweaked to frequency) --> inverter --> light (?)

 

Arduino may have the advantage of multiple RPM set points without having to reassemble the circuit.

 

Then just use whatever kind of light - maybe even under the windshield vents as a HUD  :lol:

 

As I said any recommendation / correction / resouce is welcomed. Thanks in advance.

 

Ben

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Hmm I don't think there is any real need to get complicated.

 

A cheap eBay tach will have capacity for installation of a shift light. In fact they usually come with one. I believe the 5 inch models will fit directly in the spot where the binnacle is for the factory gauge with a bit of thickening (I used electrical tape wrapped a few times) or with a bracket. The shift light is held on by a ring clamp and can be easily removed and wired remotely. They come with a gaudy light which you can sub out to a small led somewhere discrete if you felt you needed to.

 

Price wise it would also be cheaper then an arduino board as well.

 

Alternatively you could just take the circuity out of the 5 inch tach and have it interface with the stock gauge. My assumption would be that it works as you are describing as it has a switch on the back to work with 4, 6, and 8 cylinder engines. So the circuitry must just have a switch that divides the signal to get the correct actual engine speed based off of the coil input, then as you describe it has a way of triggering a signal once a certain rpm is met. I want to say it was done with a dial so with a potentiometer of some sort maybe to trigger once it passes a certain threshold?

 

My 0.02 hope it helps a bit. Keep us informed. I have an arduino board I'm playing with it really is amazing what those little things can do.

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Oh wow... I had looked all over Jegs and Summit and Amazon and not seen anything for less than $50 (and most of those required separate sending units). The tach+shift lights are on ebay for $40, and separate shift lights are on there for only $12. What a world we live in. Hope it lasts...

 

I was actually hoping for a bit more 'subtle' installed look but this is still certainly the way to go, maybe I'll tear it apart and just mount the light somewhere. Thanks Jester.

Edited by ol doc gully

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3654C9C0-D38F-4465-BD79-01978913FE19-342

 

The gauge doesn't look too out of place, and the light is just a simple power and ground so you can definitely change it to something a bit less conspicuous, I was thinking a shrouded LED or maybe even a tinted drilled into the dashboard.

 

Warning needs to be said, that is eBay so quality might not be great so beware, it worked ok for my purposes. If you are taking it apart to just use components I would be less worried, keep us informed with how it turns out.

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I am just trying my darndest to keep the dash OEM looking with only subtle hints of race car-ness, so would definitly like to keep the OEM gauge; but youre right, that doesnt look out of place at all.

 

Was trying to find some useful reviews of those dirt cheap shift lights ($12 ones that pop up as "1000-11000 RPM adjustable aluminum shift light") This guy appears to have done pretty much exactly what im talking about as far as taking it apart and using a separate light bulb, and did it with the same light im talking about so i may just try it... http://www.dsmtuners.com/threads/small-shift-light.338723/#post-151888744

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Yeah I never had any problems switching to electronic dizzy but have definitely heard of others'.

Ordered the dirt cheap shift light so I'll let y'all know 'how' crappy it is ..eventually, apparently it's coming over from China via canoe.

Edited by ol doc gully

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For a points type ignition the voltage waveform on the negative side of the coil is a crappy looking rectangle wave with underdamped ringing on the trailing edge. Voltage spikes up to 100 votls are not unusual, depending on the condition of the condenser. I would likely make a shift light using TTL circuitry since I am not adpet with microcontrollers. A suitable signal conditoner feeding a Schmitt trigger should yeild a clean signal that can be used to determine the engine rpm. 3 pulses per revolution translates to 18,000 ppm at 6000 rpm. The period at 6000 rpm is 3.33 mS and the pusle width will be about 0.7 mS, depending the the dwell.

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Yeah I'm not too keen on a buzzer but I am going to toy with different light placements - I'm kind of thinking if I place one near the windshield defrost vent it may work as a makeshift heads up display... as long as its bright enough... Otherwise the clam shell is at least the least worrisome part to drill into so i may just put a more subtle light there.

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I have to shout to talk on the phone while cruising, at full song I doubt I could pickup on any beeping or buzzing telling me to shift.

Do you really need to worry about shift points while cruising and chatting on the phone?

Presumably, the stereo and phone are both off at track events, so a buzzer would work.

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You've misunderstood so much. I'm not sure where to start...

 

Phone example is indicating that even with the phone at max volume I can barely hear the person. It turns into a shouting match both ways, I can't hear them and I have to shout for them to hear me, the reason is the car is too loud.

 

Full song implies the car at full throttle, not the actual stereo.

 

The comma reads as a break/pause so the statement reads:

 

Even at cruising it is very loud. When I am no longer at idle (accelerating vigorously to a point where I would hit the shift light) I would not be able to hear anything at all.

 

This is extrapolated to our cars being fairly loud, so a buzzer would probably not grab as much attention as a well placed light.

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I might be completely wrong here but if you are willing to play with the wires behind the dash you might be able to make your own with a led and a potentiomer (a "volume button", which is a variable resistance). Someone who knows what they are talking about would need to confirm all this, though...

But from what i understand, tach needles are activated by electromagnets, which ar driven by voltage that increases with the rpm. If you manage to get to that wire, you could split it, leave one end on the tach, and run the other side to some subbtle yet highly visible spot. At that point, you can put that into the pot. and the pot into the led. Since leds only turn on when a certain voltage goes through them, it would only light when the voltage (which follows the RPM) gets high enough... And you could adjust the voltage with the pot...

A few things would need to get figured out first though:

- can you actually tap whatever gives the current to the tach.

-does splitting it and running something in parrallel affect its output ( which would defeaut the purpose AND make the tack useless)

-what kind of led would you need ( i.e. how much current/voltage the source gives out when it's around the time to shift... If the voltage is too low maybe it could be run to some kind of relay, too)

Its a lot of ifs but if anyone can confirm if that wouod work, this is a project that could only take as little as an hour or two and cost less than 5$...

Edited by supernova_6969

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The tachomerer is a current meter driven by a circuit that converts the ignition pulse rate to a current that is proportional to the engine speed. Adding an additional load (an LED in our case) in parallel with the current meter will starve the meter of some of the current and make it read low.

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