555multivibratorpulse
Im trying to do research for something I can't even name. Basically it goes like this:
It's kind of like a multi-vibrator, but I need it to give a short pulse on the output, while input is a constant voltage. Maybe some of you know some circuits, that would be capable of such task.
You can have one 555 timer be the enable for a second 555 timer. One in mono-stable mode and the other oscillating at whatever frequency your pulses need to be at. Then just make the first 555's single pulse long enough to contain how ever many pulses of the second you want.
simulate this circuit – Schematic created using CircuitLab
Note the pin numbers are not in the same spots for the two packages. This was drawn pretty quickly, so also check for errors :)
If you truly want to filter the pulses via hardware, a low pass filter (LPF) is definitely what you want. It will PASS the low frequencies and attenuate the higher frequencies. You are looking for DC values which are of inherently lower frequency than any pulse train. The simplest way is to implement a first order LPF using a resistor and capacitor. The cutoff frequency is then
\$f_c = \frac{1}{2\pi RC}\$
simulate this circuit – Schematic created using CircuitLab
However, the response is going to be pretty slow. It will take a noticeable amount of time for the capacitor to charge up. Also there could be quite a bit of ripple depending on what size components you choose. To alleviate some ripple, you could use a second-order filter via the Sallen-Key topology.
If each resistor and each capacitor is the same value, the cutoff frequency will be the same as previously.
Whichever you choose, the output voltage will be dependent upon the duty cycle of the pulse. Therefore, if the pulse is low and high for equal amounts of time, and the amplitude is 5V, the output of the filters will be 2.5V. If you aren't using a comparator or ADC input on the microcontroller, you will need to implement a comparator as well. Since there will be some ripple on the output of the filter, a comparator with hysteresis will be necessary. From an Analog Devices article:
Now you can probably see why using an input capture interrupt is probably the best bet. It is likely less work and definitely cheaper. Had your pulse frequency been higher, an analog solution may have been more useful.
Best Answer
You are looking for a one-shot or monostable. Your diagrams should look like this.
simulate this circuit – Schematic created using CircuitLab