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frequency to voltage noise filtering


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Hello all,

i want to display the speed of a model train, the optical sensors (link below) are placed around the track, and i am using the lm2907 to pick up a frequency off of them and change it to a voltage. then i was feeding that voltage into some cascaded LM3914 bargraph drivers to display the speed on 20 led's.

I got the freq input going and had the LED's rigged up before, but now when i try to integrate them the resulting display on the row of LED's is really noisy and there seems to be a huge burst of voltage at low speeds, like if the train is going slow, whenever it trips a sensor the voltage leaps and lights almost all 20 led's for a split second and then drops back down. Just overall the response of the LED's seems too sensitive to fluctuations in speed, when i measure the output votage with a DMM and fluctuate the speed, the output voltage seems to increase/decrease appropriately without any wild fluctuations.

So my question is how can i get the LED's to not be so sensitive it seems and read the voltage more like the voltmeter. Or can someone suggest another method to display the speed. Could i use a ICL7107 and a 3.5 digit display with a voltage divider to display the speed in like inches/sec or something. Would the ICL7107 have the same sensitivity problem? I guess i could probably also use an analog voltmeter and "redraw" the tick marks on the meter, but i'd like to have a nifty digital output of sorts. (note: i tried analog meter and similar results slightly better though, but still not good enough)

The max freq i anticipate is about 30Hz maybe 40. so i want to display speed from stop to about 30 Hz. here is a schematic of the circuit.

sensors being used


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:)hi bonanz,

would you care to provide me with the full circuit diagram of your project once you have solve this problem. i really would to help you out but am not very good at digitals, but am good in building circuits. :) i have read articles about those problem and what they suggest is to use pi filter.

hope this can solve your problem.

pls. email me once the problem is solve, thanx ;)

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