Need Help with Passive High Pass and Low Pass Filter

Discussion in 'The Projects Forum' started by SwanseaStephen, Apr 22, 2017.

  1. SwanseaStephen

    Thread Starter New Member

    Apr 22, 2017
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    Hey everyone I'm a total noob in need of some help here. I am just trying to make a very basic passive high pass filter, and a low pass as well (don't know how to set up a pass band filter but any tips would be helpful). I don't have a function generator, or oscilloscope, etc that I see in all the youtube videos for this. I have built a simple oscillation circuit (from Make: Electronics by Charles Platt which I am going through); I can change the resistors and capacitors to provide varying output frequencies. Now I want to feed that output through very simple high pass and low pass filters to an LED as a simple indicator. This has maddeningly frustrating, I must be overlooking something obvious. After several unsuccessful attempts I looked into the math for frequency cutoffs. I can use a 1 microF capacitor and a 47K ohm resistor for a cutoff frequency of 3.39 Hz (I have also tried a 1,000 microF and 47 ohm). I can configure my oscillator circuit to have an output of 4.83 Hz or 2.8 Hz. So when I set up my high pass filter I expect the 4.83 to pass but not the 2.8, and vice versa for the low pass. I have tried much slower or faster set ups too but still no luck. Attached is an image of my circuit on the breadboard. Can you tell me what I'm doing wrong, and in SIMPLE language (recall I am a total and utter NOOB). Thanks!
     
  2. SLK001

    Senior Member

    Nov 29, 2011
    1,437
    591
    You are probably not going to have much success without some way to view the signal besides an LED. The LED will light with a large range of voltages.

    Get yourself a cheap oscilloscope.
     
  3. absf

    AAC Fanatic!

    Dec 29, 2010
    1,929
    547
    It is really hard to figure out what is going on on the breadboard. Dont you have a schematic or circuit of the breadboard?

    You can also try getting a circuit simulator like LTSpice (free) or MultiSim (not free) to simulate your circuit on them.

    On MultiSim and proteus, test gears like signal generator and scope are included.

    Allen
     
    atferrari likes this.
  4. crutschow

    Expert

    Mar 14, 2008
    23,092
    6,850
    Simple RC one-pole filters have a poor selectivity so it would be hard to see with LEDs.
    For example, after the filter corner frequency, the voltage only changes by a factor of 2 for an octave (1/2 or double) change in the frequency.

    Also note that if you are feeding the LED directly from the filter, the LED impedance will affect the filter frequency response.
     
  5. SwanseaStephen

    Thread Starter New Member

    Apr 22, 2017
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    Let me explain further with 2 better pictures. Here I am trying to make a low pass filter. The top half of this image is just the oscillator circuit to create an alternating signal. In image 1 I have the oscillator using 68 microF capacitors so it has a very slow frequency (so slow I got bored counting, well under 1 Hz). In image 2 I am using .1 microF capacitors in the oscillator, which makes it go so fast I can't see it alternate with the naked eye (though it most certainly is). For both I have my low pass filter set up with a 47 kilo ohm resistor and a 1 microF capacitor, which should have an ideal frequency cutoff of 3.39 Hz. In each image I've put 3 LEDs. The top one is connected to the oscillator, indicating yep its working. The second LED is also just a marker, in this case placed after the 1 microF capacitor of the low pass filter to see if anything goes through the path "set to ground". The 3rd LED is the target, connected in series to the 47 kilo ohm resistor and parallel to the capacitor, as I am told a low pass filter should be. As you can see in the image, the filter is passing the oscillator's signal in image 1 with the slow/low frequency signal (yay!) and also in image 2 with the fast/high frequency signal (boo!).

    The two ends of this oscillator are extremes as I don't want to be caught in the area in the middle, I feel like my filter should be able to make the distinction but its clearly not happening and I do not know why. Idk how big LED impedance will play in this circuit because I am the noob, donyou think it would really be that disruptive in this case?
     
  6. SwanseaStephen

    Thread Starter New Member

    Apr 22, 2017
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    I just realized I was using a 470 ohm resistor in these pics instead of the 47 kilo ohm AND...no new effect, the 3rd LED is just dimmer now since it has 100 x more resistance in front of it :)
     
  7. SwanseaStephen

    Thread Starter New Member

    Apr 22, 2017
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    My circuit is currently set up like in the two new images I posted below, yes LED is directly from the filter. How should I set it up to avoid this impedance effect?
     
  8. crutschow

    Expert

    Mar 14, 2008
    23,092
    6,850
    You add a buffer amp between the filer output and the LED.
    This can be an op amp or a transistor.

    For better filtering you can use op amps as 2-pole active filters.
    Here's some help with that.
     
  9. atferrari

    AAC Fanatic!

    Jan 6, 2004
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    Please post the circuit you are actually trying to implement..
     
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