would like to build an engine rpm controlled switch

[adown]

Hi, I would like to build an rpm (window) switch. That will activate at 3000 rpm and stay closed till about 6000 rpm. Any ideas? Any and all input is appreciated. Thanks, adown

 

[phalanx]

This is a trivial task for a microcontroller, especially one with a CCP module.

First, what is your application and why do you need it?

Second, what are you using to get RPM feedback from the motor?

 

[stevez]

Just some comments/questions -

1. How will you determine/detect RPM?
2. How accurately do you need to determine RPM and the beginning/end of the window?
3. Implied but not stated is that the switch should open below 3000 and above 6000 - otherwise it is to be closed.
4. Do you want to have some differential built in to the points of interest? An example - if RPM is rising from 2000 rpm and gets to 3000 rpm you might have it close - but you might also hold on opening if RPM drops util 2800 rpm. If you don't do something like this by accident or by design the system could toggle or oscillate if the RPM was close to the desired points. A timer might help with this.

While I am not proficient at PICs or similar devices this seems like a good application. You could count pulses and program the desired action. If you were to use a generator or tach with DC output you could still run this to the PIC. Lots of ways to do it. Just something to get you started.

 

[adown]

This will be a nitrous oxide controller for my car. I want the switch to close(activate) at 3000 rpm and open about 5800 rpm. I will be taking the tach signal from my tach signal wire. I know just enough to know this probably wont be easy. Can it be done analogue or will I need an adc and do it digitaly. I can buy one for $180. but would like to build it if I can and incorporate a throttle position sensor so it activates at full throttle only. I think the tps at full throttle is about 5 volts, I haven't checked it yet. I know I will need it. Other than that I dont know where to start. Thanks, adown

 

[stevez]

Some more thoughts-

1. Can NOX damage the engine if it is incorrectly used?
2. Are there conditions that might exist when you would want to lock out or turn off NOX regardless of driver demand?
3. Is your application related to continuous driving or simply a "drag race"?
4. What is the tach signal - pulses, voltage ...?
5. Does vehicle speed matter?

Sounds like something programmable that allows other conditions to be monitored would be best but that could be beyond your capabilities. If you wanted to wire something up you might proceed in this fashion:

A. Get yourself an indication of RPM that is a continuous voltage. The tach signal might be that already - or not.

B. Send the voltage that indicates RPM to two comparators - one to output hi when above 3000 rpm and one to output hi when below 5800 rpm. The set points can be adjustable.

C. Use a comparator to look at the TPS - to output high when wide open.

D. Use a 3 input AND gate - when all three outputs are high it goes hi - when hi the NOX is on.

E. I'd add timers so that if the AND gate goes hi then low momentarily, that the NOX valve wont toggle rapidly.

Personally I would not do it this way unless I really understood that the improper firing of the NOX would not damage the engine or cause surprises while driving. Note that an auto is extremely noisey in an electrical sense. Your circuitry should probably be built with considerable immunity. Again, this is a way but not really a recommendation. The cost of a commercial module ($180) seems small compared the the damage you might otherwise do.

 

[chiba]

I'm wondering why you just don't trigger the NOS at full throttle regardles of how many rpm the motor is doing? You could wire in a preset delay if there's too much wheelspin.


edit: added this info link **broken link removed**

 

[bpi]

Nitrous at low rpm and low load is a big recipe for disaster.

I think a 3 input AND comprised of high/low rpm evaluation and TPS at 100% is satisfactory.

You will also want a standard on/off switch for that circuit so that it doesn't activate when the NOS is disconnected on the street.

 

[Hero999]

A centrifugal switch?

 

[adown]

Thanks for all the input. I have it drawn up. I found an rpm speed switch through national(lm2907) so I'll be using 2 of them fed by a transistor controlled by a TPS sensor comparator. The first will be on until 6000 rpm and will feed the Vcc of a second that will pick a relay at 3000rpm. Above 6000rpm the first will shut of the second and like wise the tps will kill them both. Thanks again, adown