LED Necklace

[Hero999]

The transistors are now connected backwards.

Yes, the 2N3904 or 2N4401 will do, it really isn't critical.

The voltage is too low for two LEDs connected in series, 0.5V isn't enough headroom and if you'd read my previous post, you would have realised that the batteries won't last very long.

I suggest you build the simple circuit, then go for the peak detector if you're not satisfied with the performance. No offence intended but you seem to be having difficulty with getting the simple circuit right which indicates that you need to gain more experience before moving on to something more complicated.

 

[Scott S1]

Wouldn't the 7.5 volt circuit last just as long as a 6 volt one ? The 7.5v draws 200mA, the 6v draws 400mA. 7.5v: 200mA drawn until 6v would last roughly 4 hours. 6v: 400mA drawn until 3v would also last roughly 4 hours. I'm not trying to start an arguement, just trying to point that out.

 

[Hero999]

No because you've now connected the LEDs in series so the minimum operating votlage is doubled.

 

[Scott S1]

Is the circuit now correct.

Thanks

View attachment 41089

 

[Hero999]

No, it's still wrong.

Look at the circuit I posted at the start of the thread, change the component values to those suggested by me and reduce the battery voltage to 6V.

 

[Scott S1]

The voltage is at 6 volts, and the main resistors are 22 ohms. The two capacitors arer also 10 uf, like you suggested. So, the olnly thing I have to change is the resistors in front of the leds ? Also do I have the ground connected correctly ? I wasn't sure if that was where it went or not.

Thanks for your time.

 

[Hero999]

What do you think?

Compare the two schematics and check each connection one by one, you don't have to know anything about electronics to see that the connections on the one you've posted are wrong. I'm not going to go through it and list all the corrections when you're obviously not giving this enough thought.

Why not open my schematic in an image editor, such as MS Paint, and make all the modifications rather than attempting to copy it and potentially introducing more errors?

 

[Scott S1]

Did I correctly fix the circuit ?

Thanks for your help

View attachment 41090

 

[Hero999]

Yes, that will work.

The LEDs might be a bit dim with 150R, don't forget that they're only on for half of the time so will appear dimmer than they would if they were connected to a battery via the usual series resistor. Try it with 150R and reduce to 68R, if they're not bright enough.

 

[Scott S1]

I have a noob question. How does the transistor control the leds, when the leds are connected to the battery and the transistor is connected to the leds ? sorry for the noob question.

 

[Hero999]

You should've asked that before, it should have been your first question.

When there's no signal, the transistor will be off, no current will flow so the LEDs will be off. When the incoming signal is greater than the base threshold (typically 0.6V to 0.7V), the transistor will turn on and connect the cathodes of the LEDs to 0V, forming a complete circuit which will turn them on.

The 22R resistor limits the base current which is needed because the transistor's base looks like a diode which will be destroyed if nothing is there to limit the current. The capacitor blocks DC current and allows AC to pass, some old amplfiers pass a small DC current through the speakers which would cause the LED to light continuously. The diode is required to provide a path to current flowing in the reverse direction, without it, the capacitor will just charge and not discharge, remember the base of the transistor looks like a diode.

Read the Wikipedia article on BJTs.
http://en.wikipedia.org/wiki/BJT

If there's anything you don't understand, please ask.

 

[Scott S1]

I can't get it working. I'm trying the left channel with only one led, trying to start small. Everything seems to be connected correctly. I'll list the components I'm using, maybe one isn't correct. 35v 10uf cap, 2n3904 npn, a 22 ohm resistor, 150 ohm resistor, and a IN4003 diode. Is there any common noob mistakes that I may be making ?

Thank you for your time.

 

[Scott S1]

nevermind, I figured it out, but it is somewhat "blurry". would a peak detector help this ?
Can you reccomend a peak detecting circuit ?
Where would I attach it ?
Also I notice that as I turn the volume up, the leds get brighter.
Is there a way to adjust this so that it would be as bright at listening volume as they are on max volume ?
I plan using a splitter and listening while it blinks.

Thanks for your help

 

[Hero999]

Yes, a peak detector would help to improve the performance. Have you tried Googling for peak detector?

A pot can be added to the input to adjust the sensitivity, try 100R.

 

[Scott S1]

would I put the pot before the first capacitor ?

 

[Scott S1]

Would it be possible to use one pot for the whole thing ? It would be easier to make each color the same brightness that way.
Also, would this peak detector work: http://freecircuitdiagram.com/2008/09/07/single-transistor-half-wave-peak-detector-circuit/
Would it be possible to also have 1 peak detector for the whoe thing ?

Thanks for your help

 

[Hero999]

It doesn't matter whether the pot is before or after the the capacitor.

To alter both channels simultaneously you'll need a dual ganged pot, alternatively you can not bother and mix both channels together by using a 100R resistor for each signal.

That's a clever little circuit but its output impedance is very high and a peak detector is only a small part of the circuit.

I think AG's idea is to use the peak detector to set the threshold for a comparator which switches on the LEDs. The good thing is that is it shouldn't need much adjustment, once set up, it will work over a wide range of volume levels.