TIP31 feeding about 25 leds (audiorhytm circuit)

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The extremely simple circuit was An Instructable that was designed by a kid 10 years old who knows NOTHING about electronics:
IF the signal level from the preamp is high enough (0.7V) to turn on the transistor then the base-emitter diode of the transistor will rectify the audio which produces severe distortion. The circuit will work from the 1W from a little clock radio, if it has a series resistor to the base that will limit the current. It also needs a resistor in series with the LEDs to limit their current. Without the resistors then the transistor, the radio or both and the LEDs will blow up.

One resistor from the preamp left channel output and another resistor from the preamp right channel output are joined at the free ends. The joined connection is mono left and right added together that can feed a circuit to drive the LEDs.
 

Sorry guys, that first schematic was not made by me and its just an example, in fact it needs resistors at the leds, and it seems also it need a resistor at the base of transistor, other wise the transistor will get more than needed and will make distortion? if we are talking about a pre amp that gives 0.7V, then whats the correct resistor value in order to feed the transistor and not disturb the line in of the amp? is it really noticeable if I only do this in one channel? or should I do it in both channel because for example adding that transistor with resistor low a little the signal? if that's the case, I think its not a good idea taking something from the pre amp output, and I should look at other sources, for example at the speaker terminals, or is there any other safe source? or I will be okay taking signal for transistor from preamp output? please give me ideas, I need this to be done soon, and still nothing clear in my mind, I am starting thinking dumb things like a vumeter that excites with electrect microhone (which is bad idea, because leds will blink with the noise of a truck in the street), another ridiculous idea is to paste a very thin wire in the subwoofer cone that will make contact with another fix terminal in order to act as switch with the speaker movement.
 

You have made many observations:

1. Yes, you need to add resistors in series with the LEDs because the current will be limited by the total voltage applied. Depending on the LED types, you can select 10mA or 20mA current level. You can mix different color LEDs but when you put them in series, you should select LEDs with similar current ratings.

2. Most pre-amp output is standard line level - that is not affected by the volume control in the amplifier. However, TIP31 is a power transistor and it can handle lots of current at the collector and may need considerable current at the base. You may use a low power NPN transistor that can deliver 100 to 200mA with a few mA of base drive. The output of the pre-amplifier cannot source lot of current.

3. You have not added any emitter bias and the base will take some voltage to turn on. You need to think about that. Perhaps you should plan that the LEDs start to light up around 100mV and glow in full glory at around 900mV.

4. You can think about a two transistor configuration to reduce the base drive and add some resistors here and there to control the turn on level...

5. I agree that taking the signal from the speakers is not a good idea because the level will change with the setting of the volume control...
 

A "consumer line level" audio signal is about 0.5V peak. But a transistor does nothing until its input voltage is about 0.7V. So the first transistor must be biased so that it is already conducting and is an amplifier. Then this transistor can drive a second transistor that lights the LEDs. If the first transistor circuit is designed properly then it will have no effect on the audio signal.
 

Hummmm!!!!? All my leds are 3.2V 20ma rated., they are blue, just 1 color, please dont talk about them like their resistors or voltage, its not a problem to feeding directly from the power supply, the problem is think about the switch that will turn them on/off making them audio rhythmic.

I just thought in another solution, what if I use an amplifier which is about 2W, and I use it as a main preamp? that will feed the amplifier and let two lines ready to also feed the drivers of the leds.

Too much garbage tough doing that, I think that I rater want to do taking some signal from speaker wires just to trigger something that switches my leds, for example the relay, but for that I need some componentes that limit the maximum voltage income for that relay, which is 3V, then are you sure I cannot use a 7805 al the amplifier output? that IC would be able to feed the relay.

Now thinking about giving chance to using electrec microhone, I can paste it in the basket of the subwoofer, and add some filters to the microphone, so it lose extra sensitivity, then this microphone after a circuit, will feed my miracle solid state relay, and this one will switch everything I want, I think that after all I am assuming being a good idea, if I add a sponge filter to the electrect micrphone, or increase its series resistor, I will down its gain, so the leds probably will only turn on with a certain amount the bass coming from the enclosure, and not either from a truck making noise in the street.
 

Choose a supply voltage
Divide Vmin/3.2V to determine series string of LEDs
Duplicate n strings in parallel, so total current is now n x 20mA until you use up 25 LEDs or so.

Now how many strings did you figure? and what is your supply?
 

SunnySkyguy said:
Choose a supply voltage
Divide Vmin/3.2V to determine series string of LEDs
Duplicate n strings in parallel, so total current is now n x 20mA until you use up 25 LEDs or so.

Now how many strings did you figure? and what is your supply?
Click to expand...

Dude... that's not the problem, please read that I need certain quantity of leds to being audio rhythmic, all On/OFF at the same time, same brightness level all the time when they are ON. This is unusual and very basic, because that nobody here know what exactly I want to know or what is the best way to do it.

The problem is not how to feed all the leds, but how to switch it without to either stealing power after preamp or amplifier output in order to trigger the thing that will switch the led strings, as I mentioned, I think that surely I will go for a vumeter with single led that is ON by the activity of an electrec microphone, I will down its gain so it will only trigger when subwoofer is giving about 30%, damn! forget what I said, I actually don't like this idea, because leds won't turn on if the volumen of the amp is very low, maybe this is a very good idea is the subwoofer enclosure was sealed, so no noise from outside can come in, but my design is ported, then I still have problems and not solution for me... maybe I should go with a pre amp which is not to weak, so it will feed with good quality signal the main amplifier and also trigger the transistor that will trigger my leds. Please please please! I although I can think many ideas, I am still a noob in electronics, can you suggest something? thanks.
 

My audio systems have an output for a recorder that is not affected by the volume control. Does yours? It is line level so it needs one little transistor to amplify it without affecting the audio, and this transistor drives the TIP31 power transistor that blinks the LEDs with the loudest parts of the music (the bass beats).

You are talking about connecting incandescent light bulbs, not LEDs. They can make light bulbs to be exactly the same. But a blue LED has a range of voltage maybe from 3.0V to 3.4V. Your LEDs will probably all be a little different so it you connect 25 of them in parallel like light bulbs then the 3.0V ones will hog all the current, be extremely bright and maybe soon burn out and the 3.4V ones will not light up. So the LEDs must be in series and you need extra voltage in case all your LEDs are 3.4V and some more voltage for the current-limiting resistor.

25 x 3.4V= 85V plus maybe 10V for the current-limiting resistor= 95V that is crazy. Use an 18V power supply to light strings of 4 LEDs or 12V to light strings of 3 LEDs and each string with its own resistor.
 


Hey friend! As I mentioned, connecting the leds its not a problem for me, I will make groups of leds in series and parallel, so they will get the "proper" current/voltage.

You just gave me a nice idea, hanging instead a little transistor in the pre amp line, and with it excite a low voltage solid stay relay that will switch the led groups, and so I can forget about tips31. What do you think about this? Or are you sure I can not play with the amplifier output? where there are 100w, cannot I connect a IC7805 there and with it trigger the solid stay relay?
 
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Avoid using the amplifier output to trigger the LEDs because its level changes with the volume control. A VU meter has many levels so it shows LEDs flashing at many volume control settings.The output from the signal source (MP3 player?) or preamp has a fixed level.

Yea, a little transistor can amplify the line level signal enough to feed the input of a solid state relay. A solid state relay is usually used to turn on AC incandescent light bulbs, not LEDs that use DC only.
You can use a power transistor like a TIP31 instead.

Why do you talk about a 7805 voltage regulator? It is not needed.
 

Johanx2, you are not understanding several things so I will try to clarify for you:

1. LEDs are NOT driven by a voltage. You obviously need a voltage supply before they will light up but if you connect a 3.2V supply across them they will burn out, probably in a spectacular way! A LED is a constant voltage device, that means your circuit has to provide the current you want (10mA, 20mA etc) and the LED drops 3.2V across it by itself. If you just apply a voltage, the LED will try to draw unlimited current and fry instantly. The simplest way to limit the current is to wire a resistor in series with the LED and calculate it's value so the current is set at the level you want. The resistor value should be "(voltage you have available - LED forward voltage)/LED Current" so for example if you have 24V and you want 10mA to flow, use (24-3.2)/0.01 = 2,020 Ohms. That's an odd value and not readily available but you could use 1.8K or 2.2K without noticing significant brightness change.

2. Inside a transistor like the TIP31 there is a diode junction between the base and emitter pins. Like the LED, it is a constant voltage device but this time the forward voltage is much lower at about 0.6V. Just as with the LED, if you connect the amplifier directly to it, the diode will try to draw unlimited current. You really don't want to do that because any voltage you would feed to the speakers, above 0.6V, will be 'shorted out' by the current being diverted through the base-emitter junction. Instead of the speaker voltage swinging positive and negative equally, it would swing no more negative than about 5V or positive by 0.6V before the transistor conducted all the amplifier output into itself. It would result in severe distortion as well as overloading the amplifier and probably cooking the transistor as well.

3. Using the original circuit, if you add a resistor in series with the base pin, you can limit the distortion and risk of damage to some extent but it would take even higher voltage (more volume) before the LEDs lit up at all.

4. A voltage regulator is not appropriate, it needs more input voltage than it produces out and it would be damaged by the reverse voltage your amplifier produces. Remember your loudspeaker signal is AC, not DC, the polarity across the speaker wires reverses during each cycle of audio waveform. For a 7805 for example, even if it wasn't damaged by the reverse voltage, would need about 8V of signal before it could stabilize 5V out.

5. Most SSR need at least 3V DC to operate them and they will only switch low frequency AC power. They are for heavy duty lighting or motor control, not LEDs. Even if you turn the SSR on sucessfully, the nature of their operation is that the stay in the on state until the voltage they are switching (not the control voltage!) is removed. In AC power control that isn't a problem because the AC swings through zero as it changes polarity and that turns them off. If you try to control DC, as the LEDS need, the SSR will switch on and stay on until you manually kill the power. That means the LEDs would light up on the first sound then never go out again!

I hope that explains!

Brian.
 


Yes I finally ended understanding the problematic of taking signal from the speaker terminals, the energy range is so big, so the tip31 (yes, somebody told me I can connect it even there with a big resistor) will almost always be excited, so the leds will be always on, I thought in 7805 to limit the range, but of course it does not make sense and thats wrong. I concluded that I simply will connect a tip31 just after the preamp and see how the audio behaves.

I dont know who told me that in that case, tip31 does not need resistor, because the base is getting very low voltage, do you think can I leave it with no resistor, but just a little potentiometer at the base, so I can control a type of gain?

My other question is that I am planning controlling the brightness of the leds, can I use a lm317, and adjust some resistors to make them not giving more than X volts? so you can control brightness with the potentiometer, and turn off the leds because they are not getting enough voltage when the pot is at max resistance. I think that using lm317 its a good idea to control led brightness by playing with the voltage value modified by the pot.
 

Your original circuit was ok, IMHO, but for a few resistors.

You may add a resistor in series with the LED so that the current in the LED is restricted to a safe value.

You also need a couple of resistors to bias the base so that it is just close to turn on.

You may use a low power transistor because power transistors will be current hungry at the base. A low power NPN will turn on with a couple of mA or less.
 

How can you design a circuit without knowing the details?
1) What is the total peak current from the power transistor to the LEDs?
2) Is the preamp output level "consumer audio" with a 0.32V peak?
3) What is the resistance of the minimum load on the output of the preamp?

I am guessing:
1) 200mA because the LEDs will operate at 20mA and there will be 10 strings of 3 LEDs in series and in series with a current-limiting resistor.
2) The preamp output is probably "consumer audio" level.
3) My idea of using one small transistor as a little amplifier will not overload the output of the preamp.

Yes you can use an LM317 adjustable voltage regulator to power the power transistor which will adjust the maximum LED brightness. Then if the music tries to cause the LEDs to dim and brighten they will look odd with their maximum brightness clamped lower that it should be. You need to turn down the brightness and the contrast at the same time which is difficult to do.
 

I dont know all those questions, I ordered an amplifier Bluetooth board 2.1 50wx2+100w

This is the amplifier for my project:
https://www.ebay.com/itm/361432245179?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT

But for sure anyway I will need to do what you guys said, I need a little transistor before tip31 in order to not to consume too much from there, right? or is it enough just by adding the right resistor at the base, big enough that only gets just the right amount of necessary energy, even doing that, is like will consume too much from the pre amp?

Please tell me what kind of transistor should I use to excite the tip31, something like bc548? I have no big idea designing, I just can build things reading schematics.

In few hours will make a research taking a look to those schematics that are amplifier based on cascade transistors, some of them ended with a tip31 at the final stage, I think that I can get some configuration from there, I will thank you any help throwing me some info like what little transistor use and how to connect it.

My power source will be 24V, which is the voltage that board needs, I am making an enclosure with 3 speaker drivers, I want to add nice lights at the side of the enclosure, I am going to show my design, which is pretty awesome its led pattern. Its like six hexagons, each one has 6 leds, I will made some art actually, its not just adding some leds, every hexagon has a hole of 1.5", which is cover by the plexyglass I am going to use, I think its ending nice.
 

Audioguru said:
...Then if the music tries to cause the LEDs to dim and brighten they will look odd with their maximum brightness clamped lower that it should be. You need to turn down the brightness and the contrast at the same time which is difficult to do...
Click to expand...

I presume that you mean the range (of brightness) when you say contrast. I do not know if the *contrast* of LEDs can be manipulated at all, because they are fixed during the manufacturing process.

The span or range can be easily adjusted: you set the base bias just near conducting without any signal. If the full signal is not able to turn on the transistor fully, you use another 1 transistor amplifier.
 

@audioguru has said it correctly, but he is referring to the envelope range. WHen filtered to Bass only, it will have much more fluctuations as desired.1) Contrast = gain for TV and monitors but for LED display or VU meters will depend on the peak /attack decay times and musical cadence.Radio levels are compressed to a very constant level so a very fast responding VU meter is required.2) Brightness is average level adjusted by offset.SO much has been written about how to modulate indicators, that it would be pointless unless the OP desires something more accurate or adaptive while still being dynamic and perhaps is just happy to have it flicker at a certain pre-amp fixed gain setting.
 

Why did you buy a Chinese no-name-brand audio amplifier that has no audio spec's??
Does it use ICs from Texas instruments or are they cheap Chinese copies that work a little differently?
It looks like the input is only Bluetooth so it does not have a preamp output to drive your LED circuit.

The power amplifier speaker output can drive 4 ohm speakers and strings of low current LEDs. 50W per channel into 4 ohms is 20V peak but the 50W is very distorted. The amplifier probably delivers 30W before distortion gets bad which is a peak of 15.6V. The outputs of the power amplifier can have strings of LEDs connected back-to-back. Each string needs a series current-limiting resistor. Each string needs 10.2V for three 3.4V LEDs in series and the remaining 5.4V will be across the current-limiting resistor. If the volume is low or is turned down then the LEDs will not light. Turn down the volume to dim the LEDs.
 

I knew that that thing is only Bluetooth, but if I am not wrong, taking a look at the picture, I can see some IC that easily can be pre amps, and considering that theres supposed to be something able to feed those class D amplifier, surely there are pre amps installed, so I am planning soldering some cables at certain points, if not, I will add one preamp, I also have planned adding 3.5mm inputs and RCA, so I will have to hack a little that thing, I dont know how much, but I think its nothing from other world. I am bad economically, so I am buying things like that, and pyle speakers, which I think everything be ok in the threshold that the thing will perform.

I was thinking, is not it better to add a little pre amp just to feed the tip 31? do you know if there is some pre amp that not takes too much, which would be what I want looking for a transistor before the tip31? Maybe all i have to do is to add a dedicated pre amp, and problem solved. Whats better? a little transistor before tip31, or a pre amp ic just to excite the tip31?
 

PLEASE refer back to the advice given earlier. By buying the Bluetooth module you have already spent more than it would cost to do the job properly - and you still haven't solved your problem.

The objectives are:

1. It should work on the audio signal, not the loudspeaker volume. This lets it work without shaking the house down to get enough brightness.
2. It must draw almost no power from the audio signal. Adding it shouldn't load or distort the sound you hear through the loudspeaker.
3. The LEDs should be on or off, not at individually different brightness or dim when the volume is low.
4. It should only respond to bass notes.

As I have already explained, you need a low pass filter to 'block' mid and high notes, a comparator to set the volume threshold that turns the LEDs on and off and a single transistor switch to sink the current from the LEDs. It is very simple to achieve all your needs, I estimate about 15 components and a total cost of just a few $$$.

Brian.
 

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