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Ok, im a computer science major at college and my senior project is coming up. I need to know how the rachometer gets its signal in a classic SE-R to know how to move the needle for the rpm at that particular time. I assume it may be through just some simple voltage fluctuations, but im not really sure. Also, for the hell of it, how do the speedometer, fuel gauge, and temperature sensor get their signals for their required output.
Thank you very much,
Shane
P.S. This will allow me to make some SICK in-dash features that i think alot of you guys will like....
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16x7.5 Kosei Racing K1 Rims
Nitto NT555 Tires
Ground Control Coilovers
KYB AGX Struts
Caster/Camber plates
Custom Rear Strut Tower Bar
Grount Control Caster/Camber Plates
JWT Pop-Charger Intake
Custom 2.25 " mandrel Bent Exhaust w/ Apexi N1 Can
Suspension Tecniques Sway Bars
Nope, its simpler than that (replying to the original post, 3bip is right). Think about it, how do you know what rpm the engine is at? Well, you have a distributor driving your ignition system with a pulse each time a sparkpulg fires. So right there you've got a signal that consists of a pulse train (one pulse per spark) and the frequency of this signal is proportional to the rpm. So how do you display that on a gauge? Well, servo motors rotate to fixed positions based on the frequency of their drive signal. That gets you gauges with very little electronics = cheap!
If you want to interface this with a microcontroller or somesuch it helps just to use a frequency to voltage IC rather than having the micro figure out what frequency the signal is.
Hey, thanks for your inputs guys...this is def. gonna be a sick senior project *** im working with a digital design class as well as a copmuter graphics class...I'll keep ya updated on the research once it starts next semester
Yes, the output of the rpm-to-voltage dude will be linear with rpm, so use that to drive one input of a comparator (a comparator is just a "switch" that flips if one input voltage is higher than another) and the other input apply some fixed voltage that you can adjust (thus set the switch point, think volume knob on your radio). Total cost would be under under $10, plus a nice box and some LEDs to make it fancy call it $20. If you use multiple comparators you can set multiple rpm switching (maybe a low point for nitrous on and a high rpm for nitrous off?). This is part of how the old apexi afcs work (the kind with a row of knobs on the front, not the new s-afc with the flashy screen).
Yes, the output of the rpm-to-voltage dude will be linear with rpm, so use that to drive one input of a comparator (a comparator is just a "switch" that flips if one input voltage is higher than another) and the other input apply some fixed voltage that you can adjust (thus set the switch point, think volume knob on your radio). Total cost would be under under $10, plus a nice box and some LEDs to make it fancy call it $20. If you use multiple comparators you can set multiple rpm switching (maybe a low point for nitrous on and a high rpm for nitrous off?). This is part of how the old apexi afcs work (the kind with a row of knobs on the front, not the new s-afc with the flashy screen).
Nitrous window switches is exactly what I was thinking of. I found the schematic below on an RX7 forum and was wondering if it would work for the SR20 i.e. where do you get the tach signal from to make this one work? http://www.rx7club.com/forum/attachm...chmentid=65799
The thread where this was posted is here: DIY RPM Switch
Last edited by T4 Primera; 09-17-2004 at 05:21 PM.
Link no worky, but it wouldn't be hard to get it to work. I'd just splice the tach signal wire going to the ecu. Just look up the instructions for a s-afc or e-manage and see where they're getting the tach signal.
Link no worky, but it wouldn't be hard to get it to work. I'd just splice the tach signal wire going to the ecu. Just look up the instructions for a s-afc or e-manage and see where they're getting the tach signal.
Fixed link - tried to post as an image originally.
Yea, thats exactly what I described earlier, it should work. Its using the built-in op-amp (the little triangle thing with the + and - signs in it) of the LM2907 as the comparator. Pin 3 is the output corresponding linearly to the rpm which is tied to pin 4, the positive side of the comparator (the +). The comparator switches when the positive input is greater (rpm) then the voltage on the negative input (pin 10). The negative input is tied to a voltage divider set by a pot (its the little funny thing that looks like a resistor with an arrow pointing in the center, marked set rev limit in that schematic). So when the rpm is too high the comparator turns out, turning on the internal output transistor (the thing connected to the comparator in the middle with an arrow pointing down). That in turn turns on an external output transistor which then turns on the relay (whew!).
That'd be one half of your nitrous control circuit, make another circuit exactly like that for the other half. If you were reall slick you could set your resistor sizes on the divider circuit (the pot controlling the rpm to switch at) so low and high sides were range limited (so you couldn't accidently set your on and off points to high or low).
I think it'd be worth limiting the range. Note 4 says to change C2 for rpm range whereas you refer to the resistors....I'll have to take some time to digest it all.
Thanks again for your input - sure beats paying MSD or Mallory prices.
I think it'd be worth limiting the range. Note 4 says to change C2 for rpm range whereas you refer to the resistors....I'll have to take some time to digest it all.
C2 changes the frequency limit of the freq-to-volt conversion. If you change that you adjust the maximum frequency it can convert to a voltage. Think about it like this, the freq-to-volt changes a low frequency (idle) to a very small voltage and a high frequency to a large voltage. Lets say it changes 2000rpm to 1.5 volts and 7000rpm to 5.5 volts and 9000rpm to 7 volts and thats as high as it goes. Changing C2 adjusts the total curve, so now maybe 7 volts is 12000 rpm, etc. What I had in mind was a little different. The comparator takes the voltage(rpm) and compares it to some other voltage you give it (your switch point). So using the above f-to-v function, if you want to set one switch at 2000rpm and the second at 7000rpm you'd have one comparator compare against 1.5V and the second against 5.5V. Then when the voltage(rpm) goes over 1.5V(2000rpm) the first switch fires and when it goes over 5.5V(7000rpm) the second switch fires. What I suggested was to limit the comparison range. What you don't want is the lower switch value to be above the upper, or the upper above redline. So in other words you would limit the range of comparison voltage for the first comparator to like .5V to 3V and the upper from 4V to 6V or whatever. You do that by changing the resistive divider network feeding the comparator (pin 10). In that schematic this is the 50k pot and 15k resistor above it dividing down the 7.56V from the output of the internal zener diode on pin 9 to some voltage you set on the input of the comparator (pin 10). Note in that schematic the divider network is set so the comparison voltage can go all the way to 0V, but is limited by the 15k resistor from reaching up to the full 7.56V (its limited to approx 5.8V) setting the upper limit for his rev limiter.
It would be trivial to add in switching based on throttle position to automatically shut off if you took your foot off the gas (think another comparator but against the tp sensor and some logic at the output). Also if you wanted to be really slick, set multiple trigger points along the rpm to ramp your nitrous as a function of rpm is as easy as adding more comparators.
C2 changes the frequency limit of the freq-to-volt conversion. If you change that you adjust the maximum frequency it can convert to a voltage. Think about it like this, the freq-to-volt changes a low frequency (idle) to a very small voltage and a high frequency to a large voltage. Lets say it changes 2000rpm to 1.5 volts and 7000rpm to 5.5 volts and 9000rpm to 7 volts and thats as high as it goes. Changing C2 adjusts the total curve, so now maybe 7 volts is 12000 rpm, etc. What I had in mind was a little different. The comparator takes the voltage(rpm) and compares it to some other voltage you give it (your switch point). So using the above f-to-v function, if you want to set one switch at 2000rpm and the second at 7000rpm you'd have one comparator compare against 1.5V and the second against 5.5V. Then when the voltage(rpm) goes over 1.5V(2000rpm) the first switch fires and when it goes over 5.5V(7000rpm) the second switch fires. What I suggested was to limit the comparison range. What you don't want is the lower switch value to be above the upper, or the upper above redline. So in other words you would limit the range of comparison voltage for the first comparator to like .5V to 3V and the upper from 4V to 6V or whatever. You do that by changing the resistive divider network feeding the comparator (pin 10). In that schematic this is the 50k pot and 15k resistor above it dividing down the 7.56V from the output of the internal zener diode on pin 9 to some voltage you set on the input of the comparator (pin 10). Note in that schematic the divider network is set so the comparison voltage can go all the way to 0V, but is limited by the 15k resistor from reaching up to the full 7.56V (its limited to approx 5.8V) setting the upper limit for his rev limiter.
It would be trivial to add in switching based on throttle position to automatically shut off if you took your foot off the gas (think another comparator but against the tp sensor and some logic at the output). Also if you wanted to be really slick, set multiple trigger points along the rpm to ramp your nitrous as a function of rpm is as easy as adding more comparators.
Fun stuff.
Great explanation. You have increased my understanding immensely. If I understand correctly, narrowing the comparison voltage range would also give much finer control over the switching voltage using the same pot.
Now I just need to find a similar DIY solution for timing retard.
Last edited by T4 Primera; 09-18-2004 at 05:00 PM.
I simply spliced the lead behind my stock tach - Worked perfect and has been for over 2 years....
If its a B13 it is easier if you locate the white connector comming out of the ecu that goes to the driver side (to the gauge cluster) and search for the blue/black wire. Thats the wire to connect the tach.
Pito
I realize this is very old and im a noob. but, i have a problem in my gti-r. the tach died on me a while ago. The accessory belt was really loose and the car died on me and i heard a bunch of clicking and lights flashing and all that. so i tighted the belt and it works fine now. But i have no rpm reading, its completely dead. I bought a new distributor a while ago before this happened, but should i start by buying a new distributor, or just the cap and rotor? Thanks guys.
**correction, i had bought a new cap and rotor before this happened, not the distributor. Could it just be something fried in the wiring or a plug? Thanks guys.
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