Sound to LED

I want to convert audio at normal volume to LED output using only transistors (NPN or PNP), resistors, and/or capacitors. I have looked at the "Stereo VU meter" circuit from talkingelectronics.com and the problem is that the response from the LED is dim. It seems to pick up the signal, but the response is so dim, I could hardly see it with a 4000 MCD LED. a 4000 MCD LED lights brightly and almost blinds you when 6V is applied to it.
I want to see the LED go to maximum brightness in response with the signal.

Is there a simple circuit I can use to resolve this?

I am using TIP31 and TIP32 transistors. Are there more powerful transistors I can use or does that not solve the problem?

try using an LED that is not so bright. your stereo vu meter probably isnt designed to light a 4000 MCD LED.

if you want the 4000 MCD LED @ full brightness, you'll have to amplify the signal

could u post the stereo vu meter schematic?

The led brightness depends on the amount of current that you are running through it. The VU meter IC's (lm3914,15,16) have built in current regulation so that current limiting resistors arent needed. If you use a transistor, you should be able to get the current that you want. (usually around 20mA) Also, a schematic of what you are working with would be helpful.

Hi Guys,
This do-it-yourself discrete "LM3914" circuit should be able to drive any kind of LED. It must have a terrific threshold voltage problem where input signals below about 0.6V peak will not light any LED, and voltages a little higher will light all LEDs.
The 330 ohm current-limiting resistors were probably chosen for old-fashioned 1.7V red LEDs to provide them with 22mA, but can still provide modern white, green or blue 3.5V LEDs with 16mA. Maybe he wants to burn the LEDs and his eyes with 50mA or more.
I bet the circuit is powered by a worn-out 9V battery whose voltage sags when all 9 LEDs light up. A 9VDC/500mA wall-wart power supply should make those 4000mcd LEDs blinding.
Here is the Talkingelectronics circuit:

I forgot,
He is using low gain power transistors instead of the small very high gain transistors that are specified. That will make a big difference in brightness.

:shock: Wow Thats alot of parts, I would much rather use the LM3914 IC

He is using low gain power transistors instead of the small very high gain transistors that are specified. That will make a big difference in brightness.

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then just slap in some high gain small transistors!! thatll solve the problem im sure

I find that weird. If I use small transistors instead of power transistors, wouldn't they blow up?

My current limiting resistor is a 1 watt 60 ohm resistor.

I thought that small transistors would be worse, just because it won't feed much current through.

I have 2N3563 which is rated at 50ma, 12V, and I have a 2N3905 which is rated at 100ma, 40V. My power supply is 9V, and the LED current consumption at maximum brightness with the 60 ohm resistance is at 150ma which I believe will screw up both the 2N3563 and the 2N3905 transistors.

Can I get an explanation on how you guys think that smaller transistors are better?

P.S. Thanks for the large amount of responses.

Hi Ms,
First you were using TIP31 and TIP32 low gain power transistors that are rated at 3A.
Now you are using an RF transistor (2N3563) that I couldn't find a datasheet for, that probably also has a low current gain.
Transistors with a high current gain are required for this circuit because most of the base resistors have a high value. The value is high for the base resistors because they are all in parallel and lower values would overload the PNP transistor.

The original circuit fed the LEDs about 22mA. Any small but high gain transistor will work well at that low current. They certainly won't blow-up. But the current gain of a power transistor will be very low, they are designed to feed Amps of current.

Since you measure a current (with all LEDs lighted?) of 150mA, each of the 8 LEDs is drawing 19mA, which is very close to their 20mA rating for 4000mcd. They should be very bright. Why not?

If your transistors had a high gain, the LED current would be about 88mA through your very-low-value current-limiting resistors and cause the LEDs and your eyes to burn-out.

I only want to measure audio with one LED.
The louder the sound, the brighter the LED.

The only other thing I could be thinking is maybe my radio circuit isn't supplying enough power to the light detector part of the circuit.

Is there any way I can somehow include a circuit that boosts the sensitivity, and make the LED brighter, so that if I was tuned to a station where everyone was almost whispering, the LED would light brightly?

mstechca said:

I only want to measure audio with one LED.
The louder the sound, the brighter the LED.

Is there any way I can somehow include a circuit that boosts the sensitivity, and make the LED brighter, so that if I was tuned to a station where everyone was almost whispering, the LED would light brightly?

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Hi MS,
You contradict yourself. The LED can't light brightly with both loud sound and whispering sound, unless the circuit has a lot of gain and is driven to clipping with any input at and above a whispering sound. But then it would be bright with all sounds.

Your radio doesn't need to supply any power to this high input impedance circuit (except the supply power for the LED), the audio just needs to supply enough voltage (0.6V peak) to turn-on the PNP transistor as I explained before. If you used an opamp as an input preamp/recifier instead of the PNP transistor, there wouldn't be that threshold problem and you could easily make it as sensitive as you want.

How do you feed 150mA through a single LED without burning it out?
How do you feed 150mA through 60 ohms in series with a 3.5V or more LED with your supply of only 9V?

audioguru said:

You contradict yourself. The LED can't light brightly with both loud sound and whispering sound, unless the circuit has a lot of gain and is driven to clipping with any input at and above a whispering sound. But then it would be bright with all sounds.

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what I am saying is that I want the light detector to have great sensitivity. so that if someone whispers, I see bright light. If no signal is present, the light is off.

This is my circuit.

**broken link removed**

I have placed a 7pF trimmer in parallel with the 0.1uF inductor closest to the 27K resistor.

I believe that the values of my components are incorrect.

What I need to know are some equations I can apply to this circuit so that I can choose the right parts correctly the first time.

My objective as well as obtaining a good LED response is to obtain a good, loud, and undistorted signal from both a local station, and a distant station.

Hi Ms,
You have a super-regen tuner whose bandwidth is too narrow for broadcast FM and is built differently than the plans, an audio amp without an input coupling capacitor, without proper biasing and without any negative feedback. Your LED driver transistors have a threshold problem.

What's wrong with using IC's? They work fine with batteries.
The LM386 1/2W audio amp IC that is on your original plans works with a supply voltage from 4V to 18V. It has an input that can be directly connected to a grounded volume control without capacitor coupling (but the input to the volume control needs a coupling cap), built-in biasing and built-in negative feedback for low distortion.
The LM3914 or LM3915 LED driver IC works with a supply voltage from 3V to 25V. It has a built-in rectifier and delivers a constant current to the LEDs so that their brightness doesn't reduce when the battery runs down.

audioguru said:

You have a super-regen tuner whose bandwidth is too narrow for broadcast FM...

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How were you able to determine the bandwidth of my circuit?

...and is built differently than the plans,...

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I built it differently because I want broadcast bands instead of aircraft bands and I wanted to try to avoid oscillation noise.

an audio amp without an input coupling capacitor, without proper biasing and without any negative feedback...

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I replaced the input coupling capacitor with a diode (as shown in the circuit) because I was afraid that the capacitor will cut off a set of frequencies, some which may be broadcast frequencies.

What is the best way to bias the transistor in my case to get the maximum output?

Your LED driver transistors have a threshold problem.

Click to expand...

How do I fix the threshhold problem?

What's wrong with using IC's? They work fine with batteries.
The LM386 1/2W audio amp IC that is on your original plans works with a supply voltage from 4V to 18V. It has an input that can be directly connected to a grounded volume control without capacitor coupling (but the input to the volume control needs a coupling cap), built-in biasing and built-in negative feedback for low distortion.
The LM3914 or LM3915 LED driver IC works with a supply voltage from 3V to 25V. It has a built-in rectifier and delivers a constant current to the LEDs so that their brightness doesn't reduce when the battery runs down.

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I feel that by using transistors, I will learn more about electronics. Also, I want to use the least amount of space possible for my designs. Some IC's are also more difficult to find than others.

mstechca said:

How were you able to determine the bandwidth of my circuit?

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Its positive feedback increases the Q of its tuned circuit, especially at high gain required for low signal levels. Isn't it very distorted with distant stations? The super-regen was intended to track a beeper, not to listen to the radio.

I built it differently because I want broadcast bands instead of aircraft bands and I wanted to try to avoid oscillation noise.

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For example, you replaced the low resistance coil that feeds power to it with a 27K resistor, seriously reducing its supply voltage.

I replaced the input coupling capacitor with a diode (as shown in the circuit) because I was afraid that the capacitor will cut off a set of frequencies, some which may be broadcast frequencies.

What is the best way to bias the transistor in my case to get the maximum output?

Click to expand...

The diode and base-emitter junction of the amp's 1st transistor are seriously loading-down the DC output voltage of the super-regen. The amp's input should have a coupling cap and its 1st transistor should be biased with a voltage-divider and have DC and AC negative feedback from the output. If you bias the amp's input so that its output voltage is at half-supply, and add another resistor and capacitor to the 1st transistor's collector for bootstrapping, the output will be much more.
If you add another diode between the bases of the output transistors and add small-value emitter resistors to them, you will eliminate its crossover distortion.

How do I fix the threshhold problem?

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Use an opamp instead of a transistor.

I feel that by using transistors, I will learn more about electronics. Also, I want to use the least amount of space possible for my designs. Some IC's are also more difficult to find than others.

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I know, it is best to do things yourself but you should also study the theory. I don't know where you are, but in North America orders are placed on the web or by phone and the parts are delivered the next morning. "Yeah, that's the end of my order, I'm glad you always have everything I need. Here's my credit card number, just a minute someone's at the door" (its the delivery)
Many different manufacturers make the same parts. You get to choose, frequently their prices are slightly different.

For example, you replaced the low resistance coil that feeds power to it with a 27K resistor, seriously reducing its supply voltage.

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I was experimenting with this resistor. I noticed that lower values reduce the volume but provide clearer reception of local stations.

I tried replacing the resistor with a 0.1uH inductor and the diode with a 47nf capacitor. I was able to pick up nothing, not even a hiss. I will try again tho.

Its positive feedback increases the Q of its tuned circuit, especially at high gain required for low signal levels. Isn't it very distorted with distant stations? The super-regen was intended to track a beeper, not to listen to the radio.

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Since the capacitor from emitter to collector is the positive feedback capacitor, you say that contributes to Q. What other component contributes to Q? or what equation can I use with the capacitor value to create Q?

In my circuit, I have 2 pairs of tank circuits. One is 33pf and 0.1uH, and the other is variable cap and 0.1uH. Is it true that these tank circuits define the low and high frequencies, and the bandwidth is the difference between the two frequencies?

Hi Ms,
Where is the original "Beeper Tracer Receiver" project that you posted? Did you delete it?

mstechca said:

I was experimenting with this resistor. I noticed that lower values reduce the volume but provide clearer reception of local stations.

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I think that higher values of that resistor (that was supposed to be a piece of wire) reduced the gain and distortion for two reasons:
1) The gain, Q and distortion were reduced because the supply voltage was reduced.
2) The DC output voltage was reduced enough to allow the amplifier to be close to proper bias.
Didn't the original project have a manual gain control?

I tried replacing the resistor with a 0.1uH inductor

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Then the amp's bias voltage was much too high.

... and the diode with a 47nf capacitor. I was able to pick up nothing, not even a hiss. I will try again tho.

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Then the amp's bias voltage was much too low, requiring a voltage-divider from its output.

Since the capacitor from emitter to collector is the positive feedback capacitor, you say that contributes to Q. What other component contributes to Q? or what equation can I use with the capacitor value to create Q?

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Everything that increases its gain also increases its Q.

In my circuit, I have 2 pairs of tank circuits. One is 33pf and 0.1uH, and the other is variable cap and 0.1uH. Is it true that these tank circuits define the low and high frequencies, and the bandwidth is the difference
between the two frequencies?

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Maybe you were thinking to stagger-tune the tank circuits to get a wider bandwidth. Wasn't the tank circuit you added to the emitter just a low-value resistor in the original project? The lower its value, the higher the gain and Q. Your emitter tank circuit is a high impedance at its tuned frequency which reduces the gain. Since it is tuned to a much lower frequency (30MHz?) it doesn't do much in your circuit except increase the gain and Q at the tuned frequency of the collector tank circuit. The bandwidth is determined by the reciprical of the Q, which is too narrow for low-distortion FM broadcast band reception. Besides, your super-regen tuner doesn't even have an FM demodulator.
I'm sorry, nice try.

Where is the original "Beeper Tracer Receiver" project that you posted? Did you delete it?

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this topic is two pages long. check the last page.

Then the amp's bias voltage was much too high.

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I tried some biasing, and I have a slightly improved result. The base - emitter resistor I am using is 360K and the resistor from base to one end of the 27K resistor is variable from 0 to 500K. It seems that if this value is lower than about 250K, the reception is a little better.

What is the maximum bias voltage I can use without screwing the circuit up? and is there a better bias voltage I can use so that I can replace the tank circuit from emitter to ground with a piece of short wire?

Hi MS,
Nope. I don't see your original author's project anymore, I see only your modification of it. I remember seeing it and my posts are comparing your modifications with it. Since I cannot remember its details about a working super-regen tuner, I can no longer comment about it.

Look at any audio amp or IC on the web and you will see more transistors for enough current gain and biasing involving negative feedback.

go to https://jbgizmo.com/page23.html