audioguru

Acoustical feedback howling occurs when the microphone (in the handset) can hear its amplified output from the speaker and the sound goes around and around making a feedback howling sound. It is stopped in a cheap speakerphone by using half-duplex which is allowing the sound to be transmitted or received but not both at the same time. Half-duplex in a speakerphone mutes my speaker when I speak then my voice does not come from my speaker but the other end's speech also does not come from my speaker until I shutup. If the other end has a speakerphone that uses half-duplex then their speaker is muted when they speak and at the same time they do not hear me until they shutup. If there is noise or one person always speaking on one end then it forces the direction so that the other end is not heard unless they scream louder than the noise. Polycom makes full-duplex speakerphones that use a digital echo-canceller circuit to prevent acoustical feedback howling and both ends can speak and be heard at the same time. Their speakerphones cost close to $1000 for the cheapest ones.

Of course the transistors will not work when you need them to work if you use resistors to limit their current. Also the current will still be fairly high and the added resistors will get hot. Why not use Mosfets instead? They use NO base current. When the battery is dead or is missing then the circuit will use only the 1mA to 2mA of supply current for the opamp.

If the IC is bad then it might damage the power supply. Then the damaged power supply might burn down your house unless it is fused.

www.radioshack.com does not list an LM339 IC. Maybe some US stores still have some. I hope the one you found is a real one and not a cheap Chinese fake one. Look at the datasheets of the LM339 at www.datasheetarchive.com where they have an entire list of manufacturers who make it. Find the manufacturer's symbol for the one sold at Radio Shack to make sure it is a real one. When Radio Shack was in Canada they sold some cheap Chinese junk and cell phones at very high prices. They left Canada and sold to Circuit City who sold to Bell Canada and now they specialize selling Bell Canada cell phone contracts and some audio products. They are called The Source now.

Many circuits on that website are wrong, especially this one that might NOT be an FM radio. Its transistors are connected with positive feedback that might cause them to oscillate and cause radio interference to a REAL FM radio. Or T2 is saturated and T1 is cutoff and they stay like that. Maybe the transistors are a weird regenerative receiver? If you connect an LED to the output terminals of a power amplifier like the LM386 then its very high output current will blow up the LED in an instant if the supply voltage is high enough for the LM386 to make the LED light up. An LED must have a current-limiting resistor or circuit in series. How are you transmitting data on an FM transmitter? Data is ones and zeros. Are the ones loud sounds and the zeros are no sounds? Or are you using audio-frequency shift keying when ones are a high pitched tone and zeros are a low pitched tone?

The "N" is the type of package. The LM339 in your photo was made by National Semiconductor and has pins so its package is called "N". I do not know what Radio shack sells because they are gone from Canada. Make sure it is in a package that fits in your IC socket.

It is not free. I pay $5.00 then discover that they do not have the service manual I need?

It is a surface mount ceramic capacitor without its value marked on it.

50W RMS into 8 ohms is a signal voltage of 20.0V RMS. 100W RMS into 4 ohms is a signal voltage of 20.0V RMS and the headphones will sound exactly the same as with 8 ohms. Maybe the Trueplay equalization for their speakers boosts high frequencies that the headphones play too loudly.

Your schematic shows a 620 ohms resistor not a 620k ohms high resistance resistor. if it is replaced with 300 ohms then it makes only a little difference.

Of course the input ground pin 7 and the output ground pin 8 both must connect to 0V on the power supply. But if the pins are connected together and a long wire connects them to the power supply then the variable high current from pin 8 in the resistance of the wire affects the 0V on pin 7 which adds distortion and might cause the amplifier to oscillate. Use separate ground wires. How will you prevent acoustical feedback howling? If the handset mic hears its amplified output signal from a speaker then you will have feedback howling. A speakerphone uses a special circuit that cancels the signal from the handset mic from being amplified. If both ends of a telephone call have a simple speaker then you will have long distance feedback howling. That is why most cheap speakerphones use voice-switching (half-duplex): I speak and my speaker is muted. You speak and your speaker is muted. Then we both cannot speak at the same time unless there is a complex full-duplex circuit.

With the inputs in parallel then they work perfectly. Did you connect the outputs of two in parallel? Why? The output of one has a maximum current of 3A. If you need more current then you need a more powerful circuit. You also cannot connect the output of the 5V one in parallel with the output of the 12V one.

No, the opamp and transistors are a follower. 0V input gives 0V output at the 0.5 ohm resistor. +1V input gives +1V output at the 0.5 ohm resistor. +10V input gives +10V output at the 0.5 ohm resistor producing a current of 20A. 220 ohms between the emitter of the BD139 and the bases of the output transistors limits the current in the BD139 and reduces the base current in the output transistors. If the battery is not connected then if the input of the opamp has a positive voltage then the BD139 will more current than when the battery is connected. I cannot remember why you want to kill a battery by discharging it.

Motorola/ON Semiconductor and Texas Instruments opamps are available everywhere. I buy them from Digikey and Newark. Newark has recently been bought by Farnell who have warehouses all over the world. Go to their website and click on the flag of your country or a neighbouring country. The very old LM741 is the same as a very old uA741 and they will not work with the updated parts list. if your meter needs a completely separate power supply then your idea using a center tap will not work.

Your meter might need a completely separate supply with its own power transformer and filtering. A 741 opamp has a design that is 47 years old and it will not work in the improved circuit. It might work in the original Greek circuit or the Chinese copy of the circuit if the transformer voltage is lower than about 20V-0-20V. If the transformer voltage is as low as 12v-0-12v then many resistor values might need to be changed. MC34071 or TL2141 opamps can be used in the improved circuit or in the original circuit. The MC34071 is no longer made in the through holes DIP case, it is in a surface-mount case now.

ebay cannot even spell the word "meter" correctly, they say it is a motor. Their very cheap meter might not be able to measure its own supply, instead it might need a separate power supply or battery like most meters. The Greek kit and the Chinese one cannot produce 30VDC at 3A and its main filter capacitor is much too small. Its TL081 opamps will have a total supply higher than they are allowed to have when the load current is low. Many parts are overloaded.

The kit from Banggood is a copy of the original kit from Greece with a couple of parts changed. It will not produce 30VDC at 3A and the TL081 opamps will have a total supply voltage higher than they are allowed when the load current is low. The single output transistor will get much too hot when the output voltage is low but the current is high. If you calculate the power in many resistors then you will see that they are overloaded. The schematic is from the Greek kit and does not show the 7824 regulator that would drive a fan. The regulator is also not on Banggood's parts list. If you like to gamble then you can build it and maybe you will be lucky that it does not become destroyed.

I buy food at a market. I buy the newest electronic ICs from an electronics parts distributor like Digikey. Or you can go to the website of Farnell and click on the flag of a country nearest you. If I order online or by phone in an order before 8:00PM then it is delivered to me the next morning even if I order from another country.

An LED sets its own voltage. A red one is about 1.8V to about 2.0V. A blue or white one is about 3.2V to about 3.6V and they are all different unless they were all measured and grouped. The brightness of an LED is controlled by adjusting its current, not its voltage. To avoid burning out an LED it must have a series current-limiting resistor. The series resistor and LED can be fed a variable voltage that adjusts the current which controls the brightness. You said you have a transformer. A transformer has an AC output which might burn out an LED that needs a DC current. +15V at 5A will heat an LED with 15V x 5A= 75W which will instantly fry an LED designed for 2V x 20mA= 0.04W.

You need the part number and detailed datasheet from the manufacturer of the LED to determine its continuous and pulsed absolute maximum allowed currents. The datasheet tells you the range of forward voltage or you can measure it. Then the power (Watts) is the current times the voltage. Or you can try one at higher and higher currents until it fails or smokes then you can guess that the other LEDs are the same or better. Most 5mm diameter LEDs have their voltage and brightness rated at 20mA. Their maximum continuous current might be 30mA or 40mA.

I think the results will frequently be wrong. It might say that a nearby candle is sunlight.

Is it odd that millions of people and animals have lost limbs but never had them regenerated by using your crazy ideas? 200W+ at 5GHz+ will burn flesh and nerves very badly.

In addition to your frequencies being completely wrong. Look at the correct chart I posted where infrared has a frequency of 1THz which is thousands and millions of times higher than you say. A microwave oven does not produce infrared, instead it produces about 2.4GHz to heat mainly water molecules with dielectric heating. My cordless phones and remote controls are cheap and work at 2.4GHz. Newer cheap ones at 5.8GHz are available. Heat on a sore muscle feels soothing and might speed healing. The heat can be from something warm or infrared or microwaves. The latest studies of rejuvenating older people is by injecting stem cells and hormones with good results.

m is milli which is a number divided by 1000. Then 75mHz is a frequency of 0.075Hz which is extremely low (one cycle every 13.3 seconds). M is Mega which is a number multiplied by 1 million. Then 75MHz is 75 million Hz and is a frequency a little below the 100MHz FM broadcast band. IR is infrared and is a frequency above microwaves very high (1 THz) and a little below visible light frequencies. What do radio waves have to do with anti-aging? Some simply make heat ands others might cause cancer. If you want to be a doctor then go to university to learn about medicine, and not become a Quack. (Look up quack "doctor" in Google). Hello Mixos, I can attach my file on Windows 10 now and I do not know what changed to allow it.

Anything that is hot produces infrared. The sun produces infrared, visible light and ultra-violet. A candle is hot so it also produces infrared but at a level much lower than the sun. I doubt a candle produces ultra-violet. Measure what an artificial light produces.