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Adjustable RPM activated switch/light

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I was searching for the most practical circuit that would work on my Honda Civic car to activate a relay/light/switch at a preset (adjustable by calibration) rpm. I found this circuit among others and hooked it up to my car.

https://web.archive.org/web/20020605035113/http://www.hondacrx.org/shift.htm

The multiturn potentiometers were working perfectly and i could adjust each one to turn on the each LED at their own preset rpm. However, i encountered issues when th engine was at idling and up to 1500 rpm. I have set the lights to work at 5,000rpm and 6000rpm respectively but when the engine is at idle speed up to 1500 rpm, both lights were flickering on and off quite quickly.

The engine is using an OBD 2 ecu (Honda P2J-003 computer box). When i tried this same circuit on a newer model honda (2009 Honda FIT engine), the circuit works perfectly without the flickering.

How do i go about fixing the circuit? do i have to add some sort of filter to make the input signal cleaner? what components do i need to add or change? I am only using a bench multitester (Volt-Ohm-Amp) for all of my projects.
 
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I suggest a capacitor between the +ve inputs of the LM358 and ground. I would say 100 uF or so. You could also add a 1k resistor between point A and the inputs of the LM358.

Are you getting that voltage varying from 0.5 V to 4 V as the revs increase?
 
Thanks Diver300 for the reply. The answer is a yes that i am getting varrying voltage from 0.5 to 4V but during idling till about 1,500rpm, the voltage is fluctuating upwards then drops to below 1V after revving more than 1500 and increases till 4V as the revs increase. Would there be any changes in your recommendations?
 
That version of the LM2917 should not be run with a capacitor coupled input, making for a signal going above and below ground at the input pin, as stated in the datasheet for the device. You can do it with the LM2917-8 version of the device though. Simply replacing the LM2917 with a LM2917-8 might do the trick.

Alternatively, you can use the LM2917 version with the following modifications to the circuit:

1. Replace the capacitor, C1, with a 20K resistor.
2. Disconnect the ground connection to pin 11 of the LM2917.
3. Connect a 10K resistor from pin 11 to ground and connect a 20K resistor from pin 11 to +12V.

I am assuming that the tach output swings between +12V and 0V. If that is not the case then the resistor values in the modification may have to be adjusted.

The modifications given by Diver300 are prudent, especially the capacitor across the +12V power supply input, but may not be necessary.

Edit: The -8 version is not pin compatible with the straight LM2917 and other differences prevent the option of direct replacement between the two versions.
 
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