LED driver - improper crossover filter?

[Xerocool]

Hi,

I've been working on this little circuit for a while. Everything seemed to work out in my head, but on a breadboard not so much. I unfortunately don't have an o-scope which has made troubleshooting a little difficult, but I think I've narrowed the problem down, I just don't know how to fix it.

Essentially what we have here is is a signal being produced from an electret microphone, running through a high pass filter to attenuate any dc signal, then being amplified through an op-amp (capable of 200mA output). The signal then passes through a crossover filter, where a high and low frequency signal are split. The individual signals are then rectified, smoothed, and passed through a bank of 4 LEDs. The idea here was to get separate colors for separate frequencies, and brightness based on signal amplitude.

I believe my follie is in my crossover filter. As I did a little more research I don't believe I have designed it correctly, but I need a little help seeing what I have done wrong, and how I can correct it. As of now LED bank 1 works perfectly, but Bank 2 will stay illuminated while there is no microphone input, and dim when Bank 1 illuminates. I have triple checked my breadboard connections and I'm convinced the corssover filter must be the problem.

Thanks for the help!

 

[duke37]

The lower bank is fed by a low pass filter which goes down to zero frequency, thus the DC output of the op-amp will turn on the leds.
To stop this, put an electrolytic capacitor in series with the LP filter with a cut off frequency of say 20Hz.

 

[Xerocool]

Thanks for the reply! I made a mistake in my description though. Bank 2 (the lower LED bank in the diagram, and the one using the low pass filter) is the one working correctly.It is Bank 1 that is not working correctly (HPF side). It is doing what I described earlier - staying on with no microphone input, and dimming when there is input.

Could there be a DC output from the op-amp even though I used a HPF on its input line to prevent any DC signal from reaching it? Both filters downstream of the op-amp are using non-polar capacitors despite the diagram - they are all I had on hand. The potentiometers are only there so I can empirically adjust the cutoff frequencies.

 

[duke37]

The top bank can only be driven with an alternating fequency coming from the amplifier. This could occur due to parasitic oscillation which can only be due to feed back.
Three areas to be investigated are,
1. The microphone feed is taken from the + line with no extra smoothing, add an extra resistor and bypass capacitor. Make sure that C1 is close to the amp.
2. C2 and R2 are scaled to only pass very high frequencies. I would try R2 = 100k and C2 = 100n.
3. A capacitor across R6 would limit the gain of the amp at high frequencies.

A scope will show if oscillations are present but I think they must be.

 

[Xerocool]

I spent some time fidgeting around on my breadboard last night and I managed to get things working fairly well. After your comment i checked the resistors and capacitors in my filters and found a few miscalculations that I corrected. I also found that I wasn't getting any noticeable benefit from the smoothing capacitor associated with the low pass filter so I removed it from my design. Using a tone generator I downloaded to my phone I could see the lights responding to their respective frequencies

The only issue I am experiencing now is that my LEDs are much dimmer than they should be. I checked the DC voltage across them and they max out at about 2 volts for the blues (rated for 3.6V) and 1V for the reds (rated for 2.3V). I get a max output of 7.4V AC from my op amp.

In an attempt to reduce voltage losses I scavenged some schottky diodes to replace the germanium ones I am using in the rectifier on the low pass/blue LED side. Testing the diodes on a separate circuit I found their forward voltage drop to be about 0.7V at 150mA DC, and the germanium ones to lose about 1.4V at 150 mA DC. changing them out didn't have any effect on the voltage across the LEDs or their brightness though.

Am I missing something here? Surely I cant be losing 3 volts across my filter...
Could this be a current supply issue? My Op amp (LM7372MR-ND) is rated for 250 mA output and the circuit should never need to draw more than 160 mA with all 8 LEDs at full brightness. Could it be that my 9V batteries are not capable of supplying the current required?

For the time being I do not have current limiting resistors in series with my diodes, though I know that is bad form.

Do you think replacing the rectifier with a Greinarcher voltage double would help, or is this not a voltage supply issue?

 

[Xerocool]

This is my updated circuit btw...

 

[duke37]

1. I see that you have changed the input components. According to my calculations, the reactance of the capacitor will equal the resistor at a frequency of 1.6kHz so will act as a high pass filter.
2. Heinrich had only one R in his surname.
3. The forward voltage across the leds is a characteristic of the leds and does not vary much with current.
4. If you are driving the amp hard you will get cross modulation between all frequencies, so not giving a clean output.
5. You do not need a rectifier bridge so no voltage loss. Use a resistor to control the current and feed two leds, connected in inverse parallel.
6. If the batteries cannot maintain voltage, then all sorts of funny effects can occur.

 

[Xerocool]

1. I see that you have changed the input components. According to my calculations, the reactance of the capacitor will equal the resistor at a frequency of 1.6kHz so will act as a high pass filter.
2. Heinrich had only one R in his surname.
3. The forward voltage across the leds is a characteristic of the leds and does not vary much with current.
4. If you are driving the amp hard you will get cross modulation between all frequencies, so not giving a clean output.
5. You do not need a rectifier bridge so no voltage loss. Use a resistor to control the current and feed two leds, connected in inverse parallel.
6. If the batteries cannot maintain voltage, then all sorts of funny effects can occur.

1. I mistyped my resistor value of the input components. Mornings are not always easy. The resistance should be 10 kohms. It is meant to act as a high pass filter with a cutoff of about 16Hz. the intent was to eliminate any DC input to the op-amp due to biasing the mic.
2. I am not one to argue this point, but are you sure I misspelled Greinacher?
3. That is what I thought... but my knowledge of diodes is not that expansive.
4.The op amp package I am using contains two op-amps. I am only using one. If this does turn out to be an issue could I run them in parallel to alleviate some load? Would I need precise resistors for gain matching or could they afford to be off a little.
5.I will try removing the rectifier and putting the LEDs in an anti-parallel configuration with a current limiting resistor. This idea had not occurred to me, but should help out greatly with voltage losses.
6. I haven't seen anything to indicate this, I was just concerned that the chemical reaction in the battery may be a limiting factor on current creation.

Thanks for the help! I'll let you know how it turns out.

 

[duke37]

1. I had never heard of Greinacher so I had a Wiki and found the correct spelling.
2. If you have two op-amps, I would not run them in parallel, they may fight against each other. A better solution would be to put your filters in front of the amps and use one for the high frequencies and the other for the low.
3. All batteries have an internal resistance, depending on the chemistry and size. You are asking for a significant current for a diddy battery.

PS With two amps, you can make active filters, one high pass and the other low pass.