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Looking to build an rpm activated switch

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gte

New Member
I have looked around google, and I'd like to build an rpm switch. My experience level is very basic and it's been a while since I built a simple circuit from a 555 timer.

I found this online
https://www.rx7club.com/attachment.php?attachmentid=65799&stc=1&d=1196984006
and I'd like to use something that sets the on/off rpm via and LED readout and small buttons (a small software gui and computer connection) OR rotating dip switches that go from 0-9, and if possible, a circuit that has the ability to have 5 or 6 rpm controlled outputs.

My car is a 4 cylinder coil on plug, and it redlines at almost 9k, so I would like the range of 2k to 9k. Am I best just trying to do 5 lm2917's, if so, how would I then electronically set the potentiometer value?

I just read this

Thanks for reading!
 
Last edited:

Grossel

Well-Known Member
An RPM based switch could consist of a monostable flipplop and some logic. Depends of what type of input you have.

If the input is not a short pulse, you can make it short pulses by using a two-input AND-gate with one input inverted. The signal should then be feeded to the one input of AND-gate and the other ti the input of the inverter.
 

gte

New Member
Here is the factory service manual Oscope diagram, if this helps

What do you think about having 5 or 6 outputs? Any suggestions on a schematic?


8738-cranksignal1.jpg




An RPM based switch could consist of a monostable flipplop and some logic. Depends of what type of input you have.

If the input is not a short pulse, you can make it short pulses by using a two-input AND-gate with one input inverted. The signal should then be feeded to the one input of AND-gate and the other ti the input of the inverter.
 

MikeMl

Well-Known Member
Most Helpful Member
Actually, at higher rpms, the period of the CMP waveform gets shorter. Usual practice is to use a re-triggerable monostable multivibrator whose period is set to the critical rpm. If the CMP period retriggers the multi before it times out, the multi output is always high. If the multi times out, that means the CMP period is below the critical rpm.

A second multi (with a long period) edge-triggered by the first will produce a logic signal which is low if the rpm is too high (i. e. it stops being triggered).

Here is an LTSpice sim consisting of two 555s. R1 controls the critical RPM as shown in the table in the schematic The Led lights if the RPM is above the critical RPM.

I assumed that there are two pulses per crank revolution.
 

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