How to reverse a TIP31 connected to an audio jack and LED's effect?

[Bibbi]

Hi, recently I've found circuits for a TIP31 connected to an audio jack to make an LED change it's brightness to the music. I'm wondering, is it possible to make the LED dim when the music gets louder, instead of getting brighter? And how would I do this?

 

[ericgibbs]

Hi, recently I've found circuits for a TIP31 connected to an audio jack to make an LED change it's brightness to the music. I'm wondering, is it possible to make the LED dim when the music gets louder, instead of getting brighter? And how would I do this?

hi,
Post your circuit or a link to the circuit, so that we can suggest a modification.

 

[audioguru]

The horrible "circuit" is an Instructable that was designed by somebody who knows nothing about electronics:
1) It does not have a base series current-limiting resistor to protect the music source and protect the transistor.
2) It does not have an LED series current-limiting resistor to prevent the LED from burning out.

 

[Bibbi]

Well, if you would be kind enough to show me a proper circuit, I'd really appreciate it.

 

[audioguru]

Well, if you would be kind enough to show me a proper circuit, I'd really appreciate it.

I corrected the horrible schematic:
1) I added a base series resistor.
2) I added an LED series resistor.
3) I limited the max allowed input peak voltage.
4) I stated a recommended supply voltage.

With no music then the LED is turned off. When the music is very loud then the LED has an average current of 12.5mA so it is bright.

It is not simple to change the circuit so the LED dims when the music gets louder.
If I add a simple inverter to drive the LED then its current can be 12.5mA (bright) when there is no music and when the music is very loud then the current in the LED will be 6.25mA which is still fairly bright.

 

[alec_t]

A TIP31 is a bit OTT for driving a little LED. But I suppose if that's all you have to hand ......

 

[Bibbi]

Well thanks for this at least, I was hoping to have one that got bright, and one that got dim, but I guess for now it'll just be the one that gets bright.

 

[drabina]

audioguru: I am building the simple circuit you have modified for Bibbi. I have two questions though:

1. My supply voltage is 12V. I have added limiting resistor for LEDs (I am running 3 red LEDs) but I am not sure if there are other modifications to the circuit needed so it runs on 12V. I just do not want to fry the TIP31 transistor.

2. The audio source would be 15W amplifier based on TA2024 (15W into 4Ohms, 10W into 8Ohms). Would that change the value of the limiting resistor for the TIP31?

Thanks.

 

[audioguru]

audioguru: I am building the simple circuit you have modified for Bibbi. I have two questions though:

1. My supply voltage is 12V. I have added limiting resistor for LEDs (I am running 3 red LEDs) but I am not sure if there are other modifications to the circuit needed so it runs on 12V. I just do not want to fry the TIP31 transistor.

3 red LEDs in series have a total forward voltage of about 6V. Then the current-limiting resistor in series with them is about (12V - 6V)/20mA= 300 ohms. It dissipates a max heating of 0.12W. The transistor will be cold.

2. The audio source would be 15W amplifier based on TA2024 (15W into 4Ohms, 10W into 8Ohms). Would that change the value of the limiting resistor for the TIP31?

The peak input voltage is 12.7V so the base current is 12mA which is high but will work fine. The 1k base resistor can be as high as 20k ohms.
When the signal goes to -12.7v then the emitter-base junction of the transistor will have avalanche breakdown and be damaged (like most silicon transistors its max allowed reverse input voltage is only 5V) so a diode should be added across the emitter-base.

Wait a minute. Where did you get the Tripath amplifier IC? Tripath went bankrupt a few years ago. Its output is 10 Whats (SEVERELY DISTORTED) into 8 ohms or about 6 Watts (AT CLIPPING). The 1k to 20k input resistor will be fine.

 

[drabina]

Thanks a log audioguru. One more question (just out of curiosity): Can the input be pulled from just one channel or having two channels connected plays a difference here?

Where did I get the TA2024? I had few already assembled boards lying around. I bought them way back. Used two already for small projects and had one left. It is going into a small boombox for my kid. The blinking LEDs are going to be mounted underneath the case for visual effect.

 

[audioguru]

Thanks a log audioguru. One more question (just out of curiosity): Can the input be pulled from just one channel or having two channels connected plays a difference here?

Stereo has two separate channels. One channel has only half the sounds.
I forgot to say that the amplifier might be blown up by having its outputs shorted together. Each channel should have a 1k to 20k resistor in series with the base of the transistor and the emitter-base protection diode.

 

[drabina]

Stereo has two separate channels. One channel has only half the sounds.
I forgot to say that the amplifier might be blown up by having its outputs shorted together. Each channel should have a 1k to 20k resistor in series with the base of the transistor and the emitter-base protection diode.

I am total noob. Could you tell me what type of diode should I use? This diode would go between the E (emitter) and the negative line in the circuit, correct?

 

[audioguru]

The emitter of the transistor already connects to the negative power supply voltage.
The protection diode can be a 1N914 or 1N4148. It connects between the base and emitter of the transistor to stop reverse voltages from harming the transistor (its max allowed reverse emitter-base voltage is only 5V). The cathode of the diode (it is marked with a black bar) connects to the base and the anode connects to the emitter.

 

[drabina]

Thanks for the explanation.

I have come up with a new circuit. Hopefully I got it right.

 

[audioguru]

Your sketch of the extremely simple circuit is correct. The LEDs will be bright when the music plays loud and will turn off when it plays soft.
The circuit will also work with a smaller transistor (but its pins might be different).

 

[drabina]

Thanks again for all your help.

 

[drabina]

Need help with circuit (flashing LEDs using TIP31)

I have built a simple circuit that audioguru helped me with. The circuit is pictured in the post #14.

It is a very simple circuit that should make the LEDs brighter with music but for some reason it doesn't work. I have two channel amp based on TA2024. Speaker is connected to one of the outputs and when I connect this circuit to the other speaker out on the amp, the sound dies. Even if I touch the negative lead of the circuit to negative speaker terminal the sound that was coming out of the speakers goes out.

I have rebuilt the circuit again thinking that I have made a mistake soldering wires but the second version does exactly the same thing.

Any ideas?

Thread Merged: Mod.

 

[audioguru]

I have built a simple circuit that audioguru helped me with. The circuit is pictured in the post #14.

It is a very simple circuit that should make the LEDs brighter with music but for some reason it doesn't work. I have two channel amp based on TA2024. Speaker is connected to one of the outputs and when I connect this circuit to the other speaker out on the amp, the sound dies. Even if I touch the negative lead of the circuit to negative speaker terminal the sound that was coming out of the speakers goes out.

The Tripath amplifier has bridged outputs. Each speaker wire is at +6.9 and has signal.
The LED blinker circuit has a signal input that must not have DC and it has a negative lead (ground) that must not have DC and must not have signal.

Add a 0.33uF capacitor in series with the 20k input resistor and connect one speaker wire to it. Connect the ground of the amplifier to the ground of the blinker circuit.

 

[drabina]

Thanks for replying. I will try that once I get back home.

Just to clarify things for a newbie (me), should the capacitor be between the resistor and transistor or before resistor? Also, I guess, non-polarized cap, correct?

 

[audioguru]

Just to clarify things for a newbie (me), should the capacitor be between the resistor and transistor or before resistor? Also, I guess, non-polarized cap, correct?

The capacitor must be in series. It makes no dfifference if it is between the resistor and transistor or before resistor.
One speaker wire of the Tripath amplifier is +6.9V and the base of the transistor is +0.7V so the Trident amp end of the capacitor is the most positive. But simply use a non-polarized film capacitor.