# Audio Frequency LED Indicator

Discussion in 'Homework Help' started by stollvj42, Dec 4, 2013.

1. ### stollvj42 Thread Starter New Member

Dec 4, 2013
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So for a class project we have to design a circuit that when a specific frequency is inputted into the circuit a specific LED comes on. Our input signal is a 1 V AC signal which we have to amplify to 5 V with an op-amp, which we have done. But then we have to use high and low filters to have one LED come on for a frequency range of 0-250 Hz, another LED for 250 to 2 KHz, and a final one for above 2 kHz. We can use most components, including op-amps, capacitors, inductors, resistors, two power sources. Can anyone help guide us in the right direction?

2. ### shteii01 AAC Fanatic!

Feb 19, 2010
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It looks like you will need three filters.
Low pass filter for 0-250 Hz.
Band pass filter for 250 Hz to 2 kHz.
High pass filter for 2 kHz and up.

The next design question is:
Passive filters using RLC components or active filters using op-amps.

3. ### MikeML AAC Fanatic!

Oct 2, 2009
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And to light an LED when a certain AC signal is present, you need a "detector" that makes DC for the LED with an AC input...

4. ### stollvj42 Thread Starter New Member

Dec 4, 2013
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Well for the Low Pass filter we were gonna use a resistor in series with a capacitor that hooks up to the (-), non-inverting i think?, op-amp input, with a resistor across between the (-) and the out of the op-amp, does that make sense?

5. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,918
600
The details should be in your textbook. Using op-amp, that would mean you are using active filter. It will need its own power supply. Assuming you are doing it in the lab, do you have enough power supplies?

Last edited: Dec 4, 2013
6. ### WBahn Moderator

Mar 31, 2012
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It would be really helpful if you could post a schematic of your present status or at least of what you are discussing. Verbal descriptions of circuits seldom accurately communicate the circuit details.

Of course, we have to be aware that other members of your class may be watching this thread, so you have to balance what you convey of what you are doing with what you would be allowed to convey to your classmates that are also working on this assignment.

7. ### stollvj42 Thread Starter New Member

Dec 4, 2013
4
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Here is what our general design is(we think is) looks like:

and here is our amplifier:
http://tinypic.com/r/260ba6q/5

basically I think we need to figure out the filters

8. ### stollvj42 Thread Starter New Member

Dec 4, 2013
4
0
dont know if that first one worked

9. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,918
600
Ok. Amplifier is not bad, but there are currently two problems.

Problem 1.
Input frequencies of the amplifier. Make sure you amplifier can actually accept your frequencies.

Problem 2.
Input signal. Your amplifier setup for signals from 0 to 5 volts.
If you input signal that is 1 volt peak to peak and centered on zero, that means that the signal is from -0.5 volts to +0.5 volts. Your amplifier is from 0 to 5 volts, it will block any part of the signal that is from -0.5 to 0 volts, so only half of your signal will be amplified.

10. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,918
600
The block diagram is not bad, but it has 3 problems.

Problem 1.
MikeML mentioned "detector" circuit that will convert AC to dc. You need it to power the LED.

Problem 2.
Your LED are connected backwards.

Problem 3.
I expect your LED to have resistors in series. Why? General use LED have current rating of about 20-24 mA, this is the max. You don't want this max. You want something about 16 mA. So you need a resistor in series with LED to reduce the current to 16 mA. It might be a good idea to measure the current before you hook up LED, to see what it is, just in case.

Last edited: Dec 5, 2013
11. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
197
For the single supply amplifier circuit you posted, the ground connection of R1 should instead be (typically) connected to a voltage equal to 1/2 of the supply voltage.
The input and output should be AC coupled using capacitors.

Have you covered any of this in class?