Circuit to count pulses and output true while pulses per second >= n?

Hi everyone,

I didn't see a place to introduce myself, so I'll just do it here. Please skip ahead to the next paragraph if you want to go straight to the content. My name is Matt and I'm a computer programmer by trade (C#). My current interest in electronics has mainly to do with my vintage motorcycle, though I did take a digital electronics class about 15 years ago. I'm very much an electronics newbie at the moment but I'm hoping that will change after some hands-on time with a decent project. Hopefully what I've decided to tackle for my first project isn't too ambitious.

My first project is related to my motorcycle. I have an external oil pump that I want to automatically trigger to turn on (and stay on) so long as my engine is running at 6000 RPM or more. My initial thoughts are to use an input from a wire wrapped about a spark plug lead (or even the coil itself) and rely on inductance to generate a pulse which I can then count. As the engine is four stroke, the spark plug will only fire every other revolution and so I need my output condition to be true so long as the input is generating 50 (or more) pulses per second. The output would then trigger a relay to turn on my oil pump. When the RPMs fall back below the 6000 mark, the pump would then turn off.

As I mentioned before, this is pretty much my first project since I took a digital electronics class 15 years ago and I would very much appreciate any assistance you could render in figuring out how to start. Any articles I could/should read or anything like that would be very useful.

Thanks in advance,
Matt

Welcome Matt.
Sonreirás ------> LM2907.
Made the circuit for my automobile, clicks at >45 MPH to engage the transmission torque converter locking; been working fine for years. Do not use a spark plug as a pulse source.

Thanks, Extranet! That looks like something I can make use of.

I do have a few questions though (did I mention I was a bit of a newbie?):

1.) If I don't use a signal from the spark plug wire as a pulse source, what other options do I have without fabricating something like Hall effect sensor? I can get a siginal from my points if the concerns is the high voltage.

2.) Am I correct in thinking that the output from the LM2907 is analogue? Output voltage rises as pulse frequency increases? Does this mean I'll need an analogue to digital converter to trigger my relay for my oil pump?
2a.) If I need an analogue to digital converter anyway, is it a bad idea to just skip the LM2907 and, instead, measure the AC voltage coming from my alternator and derive the RPM based on the VAC ouput?

Thanks yet again,
Matt

Input is from ignition coil where 12v pulses are provided.
Use schmitt trigger via a pot to set the required voltage to trigger.

Hi.
Yes, use the points as pulse input, trough a series 1µF capacitor and 10KΩ? into pin 1; load pin 1 to gnd with 4.3KΩ

The components in MY LM2917N-8 = NTE995M circuit application are:
Pin 8 = 10KΩ
Pin 6 = 10K Ω
Pin 5 = 500 KΩ
Pin 2 = 2200 µF
Pin 3 = 10 uF and 50KΩ trimmer potentiometer in series with a 120KΩ resistor
Pin 6 to +12v = 680Ω
Does not mean your will use same values, but take as reference.

-----> Read all the data sheet.

I used a 280Ω - 12V coil relay directly from pin 5 to +12V. No complexities in the circuit.
That relay can drive another relay if your oil pump needs more than 1 Ampere to work; very likely.

The tachometer-oriented schematic posted by alertlinks is not the configuration you are looking for.

The LM2907 I mentioned first may not be the most convenient IC of that family for you.