Hey, im working on a way to read the tachometer output from my honda civic and use this to pwm a led to control the brightness depending on how fast the engine is running. Should i build and analogue circuit or use a microprocessor? The Tachometer output is a square wave with a frequency equal to the rpms.

 

You can use an LM2907 to measure the frequency and output a voltage, and connect that to a 555 timer or op-amp based PWM driver. Post here if you need any help on any of these parts...

 

IF you us a frequency to voltage IC, you can dim a light that way.

Or, you can use a uC to count the frequency and make a lookup table to set the duty cycle of the PWM output according to the frequency.

AMD makes some nice uC's that have ADC inputs and PWM outputs.

A google search for a frequency counter circuit for the uC you decide to use would make this quite easy to attain.

 

If you use the signal from the existing tach to trigger a one-shot, that will, in effect, generate a PWM signal. Just need to make sure the pulse width of the 1 shot is less than the cycle time of the highest anticipated frequency from the existing tach.

 

I considered that billB3857. Thing is, a frequency that slow will cause incessant blinking of the led rather than the dimming effect that I beleive the OP is looking for.

At 6000RPM they would be getting a 100 hz PWM signal. That is far to slow for the LED and 6000RPM is pretty high of an RPM.

I think keeping the frequency set at or above 17kHz to ensure POV (persistence of vision) then changing the duty cycle to obtain dimming.

Unless I was missing something in your idea.

 

17 KHz? In actuality anything much over 10 Hz appears to be steady state to the eyes. If it didn't you couldn't watch movies at the theater, which are 24 frames per second.

I'm all for a simple F to V converter IC.

 

....Ugg... I confused vision and hearing ...

17kHz is at the top of many humans hearing range.

You are completely correct.