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https://www.circuitlab.com/circuit/ra49ze/led-headphone-jack-tester/

The above schematic is one I came up with while doing a little research online. I'm not an electronic expert so don't know what the exact parameters of the TIP31 should be set to and all of that.

What this is pretty much is a audio and visual way to test headphone jacks. I repair electronics like iPads for my job and have to test everything to make sure it is 100% back into working order. So when testing the headphone jack we all use just basic earbuds to test, which works just fine but its a hassle to mess with the cords and i'm a techy geek so decided to make this. I made a basic one with just the earbuds and 3.5mm male jack in a pen style unit but wanting to add the leds to it.

Heres a link to the instructable post I have that has a few pics of the one I already made.

http://www.instructables.com/id/LED-Headphone-Jack-Tester/

What I would like to know is will this circuit work well? Is the battery voltage too high or not enough, I know leds need 2v to work each but I don't want the unit to be real bulky so instead of putting in 3 AA's figured a 9v battery would work.

Is there a way to put a rechargeable battery like a cell phone battery in here with a charging port or would that be too complicated to put together for someone with little electronic experience.

Any help/improvements/etc would be highly appreciated. I'm not making this for mass production or anything mainly for me and a few techs at my job. Thanks for reading this. :)

Tribbs,

I think I can help you figure some of this out.

First, let's start with the basic idea. From the circuit it looks like you would like to have an LED blink or turn on when there is audio coming out of the headphone jack.

Okay, what you've got right now seems like a good start. However, some things haven't been addressed. Let's consider just one side of the circuit at a time since it's symmetrical. When an audio signal is present on the jack, some current will flow out of the jack and through the speaker, with some of it being shunted into the base leg of the transistor. If enough current goes into the base, the transistor will turn on and allow current to flow through the resistor and LED, turning the LED on.

You are trying to use the TIP31 as a switch, but most NPN switch applications would have the transistor on the other side of the battery. What I mean is, you should have the current flow go from the + (Plus) side of the battery, through the resistor, through the LED, and then through the transistor to ground, like this:

Now you have a switch that will turn the LED on. Now let's address the LED and resistor. You said the LED needs 2 V to work. This is not strictly true, unless you are using an LED that is specified to have a voltage drop of 2 V. However, if you keep the current low enough that voltage likely won't matter too much as far as keeping from smoking the LED. You need to pick a value for the resistor that will limit the current through the LED to the value you want.

To figure that out, it works like this. Let's say you have an LED that has a forward voltage drop of 2 V and a max current of 20 mA. To be safe, let's specify we want 16 mA to pass through the LED. So when the switch is on, there will be 2 V across the LED, a small voltage from the collector to the emitter of the transistor, say 0.3 V, and 9 V across the battery, obviously. So we have:

V_bat = V_LED + V_R + V_CE

9 = 2 + V_R + 0.3

Solve for V_R and you get V_R = 6.7 V. So, the voltage drop across the resistor needs to be 6.7 V, and we decided the current should be 16 mA. From Ohm's law, V = I*R, R = V/I, R = 6.7/0.016 = 418 ohms. Use a resistor somewhere around there.

Next let's talk about the transistor. The TIP31 transistor you've specified is a very high power transistor compared to your application, which is not only overkill but will probably not work in this application. I suggest you use a much smaller part, like the 3904 in my schematic. A 2222 would work as well. Both will be a lot cheaper than the TIP31 as well.

Alright, now with a 3904, in order to turn on fully we need to supply somewhere between 1 to 5 mA to the base leg. I don't know if there will be enough current left after powering the headphone speaker to siphon off that much current. Here you will have to experiment. Either way, you will definitely want to put a current limiting resistor between the audio output and the base of the transistor, like this:

The resistor protects the transistor from getting too much current and burning out, just like the other resistor protects the LED.

You will probably have to turn the volume fully up on your device under test because of the low voltage swings of line audio. Also, be sure to connect the ground or neutral of the devices audio jack to the negative terminal of the battery. If you don't, as you haven't in your original schematic, there is no way that your circuit could do anything in response to the audio jack.

If after experimenting with different values for the current limiting resistor on R2 and it still does not work, there is a very cheap, easy to use, and commonly available IC out there called the LM3915 which is designed to measure audio levels and power LEDs to create a visual sound meter. We made some portable VU meters based on them in one of my classes. If your design doesn't work, we could talk about why it doesn't and maybe figure something else out using that chip.

Wow talk about a good comment. Thanks. Yeha the original schematic I just found a led music box, one that the leds react to the audio input and tried to include the speakers as well. But that all makes sense. Do you think it would be to much for a simple headphone jack tester or do you think it would be a good idea, especially for someone or a company that wants to make sure the headphone jack is 100% in working condition? Once I get the parts and do a little experimenting Ill let you know what happens. Thank you so much for your input.

No problem, I'm happy to help.

I don't know if there's much of a need for a device like this as I don't do repairs, but I definitely think you should try out your idea and see if it works for you. Even if you find it doesn't solve your problem completely, you will have learned a lot. You can put that towards the next version that better meets your needs.

@Tribbs,

@wkevina has offered an excellent analysis of your idea and made some very good suggestions.

Below is a slightly different take on what you are trying to do and one which takes into account the problems that @wkevina has highlighted of driving current into the transistor base and the fact that until the positive signal swing reaches around 0.5V to 0.6V then you may see very little light from the LED.

Again it shows only a single channel.

It is a simulation and not just a circuit diagram so you can do:

Simulate > DC Sweep > Run DC Sweep

Simulate > Time Domain > Run Time-Domain Simulation

and see exactly how the circuit works. If you edit the values of R2 and R3 as suggested in the notes you can see the effect of linear vs. log level conversions.

https://www.circuitlab.com/circuit/zfx2ec/headphone-jack-tester-one-chan-02/

It's simple enough to try it in the two options and see what you think.

:)

@wkevina @signality

I talked with a few people and I think im going to drop this idea but I believe I have a better one. I am going to develope a iPad work station equipped with a charging port, speakers and a vu meter. once I have it down Ill show it to yall if you want but I would like some input.

I found: http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?langId=-1&storeId=10001&catalogId=10001&pa=2186769&productId=2186769&keyCode=WSF&CID=GOOG&gclid=CJ7D5pmDx7gCFSho7AodqxEAsg

It seemes to be the kind of vu meter I need both for left and right input, cant really tell if it comes put together or not but what Im curious about is can the leds be extended from the circuit board. Im wanting the circuitry at the bas of the station and the leds for each side going to the sides of the station so Im thinking they would be extended from the circuit board by about 9-12". would that cause any problems with the leds? and what kind of wire would be best?

@Tribbs,

It's a kit.

I looked under the misleading Texas Instruments link:

http://www.jameco.com/Jameco/Products/ProdDS/2186769KitInsturctions.pdf

It uses this:

http://www.ti.com/product/lm3916

which is a variant of this:

http://www.ti.com/product/lm3915&lpos=See_Also_Container&lid=Alternative_Devices

already identified by @wkevina.

The Jameco kit looks over specified for what you want but OTOH it's readily available.

You should be able to extend the LED wires to a metre or more without too much trouble.

Something like this:

http://uk.farnell.com/lapp-kabel/0034316/cable-liycy-16core-0-14mm-50m/dp/1204277

http://www.farnell.com/datasheets/40380.pdf

would allow you to connect 15 LEDs + 1 common wire + an overall screen.

The LED current is quite low so the wire is adequately rated.

Screening might be worthwhile because although the LED currents are a few mA and are only switching at relatvely low speeds, over a length of cable they may cause RFI to nearby kit.