MPH (Frequency) Activated switch

[joedirtmcd3]

Hey guys, I'm posting this here because I don't know who else to turn to and I am tired of endlessly searching the internet. I've prepared a Speed Switch circuit using an LM2917 IC which is basically a frequency to voltage converter. It will be used in my Buick 4.1L v6 powered chevette to control nitrous activation.

The car does not have enough traction to launch from a stop with the nitrous turned on. I designed this circuit to be adjustable so I can fine tune it to a specific MPH speed that I will eventually be reading with an Autometer or equivalent Hall Effect sensor in the tailhousing of my 700R4. It is reported that the sensor outputs 16 pulses per revolution. This doesn't tell me much because I do not know what frequency will correspond to what speed. I will most likely need to put a scope in my car and take it for a drive to figure out what the equivalents frequencies are. The other option is if someone already has done this I can estimate the frequencies I will be dealing with and put in a potentiometer with a range that will get me fairly close.

Any body have any idea what the frequency would be close to at 20 or 30mph? I realize it will be different from trans to trans because of the speedo gear in the tailhousing itself. Thanks guys.

 

[Sleeper-6]

Pretty basic math, What rear gear and tire size?

 

[joedirtmcd3]

well the s10 rear is supposed to be 3.42 and the rear tires are about 25.5 in diameter. still, i dont know if the speedo output is a 1 to 1 deal. the trans is from a 92 astro with a 4.3 v6

 

[craggar]

Why not use a timer?

 

[salvageV6]

Why not install the sensor and also the LM2917 and get the voltage output part working.

Then if you have a speedometer in the car and someone reading a voltmeter as you drive each 5 mph write down in a chart each voltage.

If you don't have a working speedo. in the car use another along side on a deserted mall lot or roadway and do the same for each mph up to 40-50 or so.

If the resulting voltage is repeatable/consistent and of course different for each mph you should be able to use a simple comparator circuit to activate the nitrous off a reference voltage trigger so when you exceed that voltage which equals 30mph it triggers a relay etc.

I'd skip the scope part if the sensor is a proven product and you can hook it up to the f. to v. chip on the bench and then put the whole thing in the car.

What's the range you can use with the chip for frequency?

If you can get it to work at low speed first say 0-20 mph range then adjust it from there to go higher if it peaks out voltage wise too early.

There's lots of chips to do frequency dividing if there's too many pulses per mph.

 

[Sleeper-6]

OK, here's what I got.

1 Mile = 63361"
25.5" tall tire circumference = 80.11"
Tire revolutions per mile = 790.912
Driveshaft revolutions per mile = 2704.92
16 pulses per revolution = 43278.70 pulses per mile
At 1 Mph = 43278.70 pulses per hour
3600 seconds per hour
1 Mph = 12.022 pulses per second
1 Hz = 1 cycle per second
So 12.02 Hz for each Mph is my estimate.

If that is true,

60.11 Hz = 5 Mph
120.22 Hz = 10 Mph
180.33 Hz = 15 Mph
240.44 Hz = 20 Mph
300.55 Hz = 25 Mph
360.66 Hz = 30 Mph
And so on..........

My math does not take into account tire growth or deformation from the contact patch, but it should get you in the ballpark. :biggrin:

 

[joedirtmcd3]

Yes yes, I've been through all that. Thanks for taking the time, but this still isn't quite all the information. The sensor/sender that I will be using is the same one Autometer uses with thier electronic speedometers. You can see an image of the product and its specifications if you search "hall effect autometer" on ebay. Its packaging reports 16 pulses per revolution. The way the sender looks like it work is this: there is a small piece of speedometer shaped metal (square key) that slides halfway into the space where the speedometer cable originally went, the other half remains in the sender. This key then rotates along with the tailshaft of the transmission at a given reduction ratio (unkown) and this key is what is monitored/creates 16 pulses per revolution.
In my research, I was able to find that the speedo drive gears on 700r4 tailshafts can range from 15-20 teeth and the drivens can range from about 35-45 teeth. this results in a maxmum possible ratio of 3:1 to a minimum of 1.75:1. The conversion factor I came up with to get driveshaft rpm from MPH was about 45 (takes into account time, circumference, etc.) so 20mph*45=900rpm at the driveshaft. The minimum this could possibly be is 900/3=300rpm at the speed sensor. Then that number multiplied by 16 pulses per revolution is 4800Hz. Thats a pretty damn big frequency when I was originally expecting about 16 or 17Hz to equal 60mph.
I guess I really won't know unless I put a scope on it. I also suppose I could try to measure the voltages seen at the comparator after the charge pump while driving the car. The problem there is that I don't know if I have to change my capacitor and resistor combination on the charge pump of the IC to fit a certain range of frequencies. I think the circuit is only accurate within a certain range for each combination. I'll have to do some more reading.

Anybody have one of these aftermarket hall effect senders already?

 

[Sleeper-6]

I could be off, but in your math you show 300 rpm multiplied by 16 to give you 4800Hz. It reality, it would give you 4800 cycles per minute. Divide that by 60 to get 80 cycles per second, or 80Hz.

I did not account for the speedo gears in my math. I was thinking it was directly reading off of the tail shaft. Oops.

 

[joedirtmcd3]

Dang I missed that, you're right. That might be just the info I was looking for. Now all I need to do is figure out what the range of possibilities the frequency can be for any setup of speedo gears and use a potentiometer and capacitor combo that will allow me to adjust accurately. Thanks man

 

[TurboBob]

you will also want to have a timer in the circuit. If the track/street is a little slippery you will get some wheelspin and the nitrous will activate...... which is not what you want...

you can always look at the front wheel ABS sensor, feed it thru a 4 pin HEI module to generate logic level pulses, or design your LM2917 circuit for the abs pulses....

Bob