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 1021 Using Arduino / Audio / Re: reading audio files from SD card waveshield on: March 07, 2011, 04:39:16 pm I don't know Mike , they mention the N suffix in many datasheet titles ( like the one you linked ) but do not actually mention it in the sheet, and everything in the sheet states the 4.7v minimum supply.I recall seeing a post from the guy who wrote the wavehc library, about using the buffer instead of a resistor voltage divider ( as used in the adafruit first waveshield version )I don't seem to be able to find that post now, but I might try the resistors tomorrow anyway..
 1022 Using Arduino / General Electronics / Re: Transistor as a switch. Having a hard time with the equasions/ data sheet on: March 07, 2011, 04:20:00 pm This is a great example of where a simple constant current generator comes into it's own,  a transistor, led, and two resistors.  See my scribbled sketch :-  ( I still haven't mastered the inserting pics here  ! )http://www.flickr.com/photos/johnandlyn/5506763085/The bottom part is the actual circuit, the top part shows the calculations, the only critical component is the emitter resistor, which sets the current through the LEDs , and can be trimmed to suit.The advantage of this circuit is that the current through the LEDs will be almost the same as through the emitter resistor ( minus the tiny base/emitter  current )As long as the LED on the base is lit, it will have 2.1v on the base for a red LED, so the emitter will always have 0.7v less due to the base emitter forward bias, =  1.4 volts  ( in our example with a 2v1 LED and a 0v7 base emitter bias voltage )This means that the transistor will always try to draw enough current through the LEDs in the collector circuit to maintain that emitter voltage, so if you pick the value of the emitter resistor to drop this voltage ( by Ohms Law ) for whatever collector current you require, it doesn't matter how many LEDs there are in circuit, or what the supply voltage is, you will get the same chosen current through every LED.  ( as long as there is enough supply voltage, and the transistor can handle the heat if the voltage is much higher than required )I have scribbled some first choice resistor values for the three LED strip currents that you mention, but start with a 22 ohm say, and using a meter, put resistors across it in parallel until you get the current you want.Using a red LED for the base reference voltage, rather than 2 diodes in series, is that the voltage/temperature drift of the LED cancels out the transistors drift  - ( I read somewhere back in the sixties, and it seems to work.)
 1023 Using Arduino / Audio / Re: reading audio files from SD card waveshield on: March 07, 2011, 09:10:45 am I see that the one on the circuit states 74125N  which I cannot even get the specs of on the net ??
 1024 Using Arduino / Audio / reading audio files from SD card waveshield on: March 07, 2011, 07:59:51 am I dont know if this is the right place to ask this but I have been using the waveshield based circuit on some devices to play sound clips from a SD card.One unit failed, and while faultfinding I see that from the circuit diagram that I sort of copied from http://www.ladyada.net/images/wavshield/v11/wave11schem.png  that the 74ls125 is used as a buffer between the 5v data lines from the micro, to the 3v3 SD card.What I didn't notice was that the 74125 is powered from 3v3 ( the data sheet says min Vcc as 4v75 ? )The inputs to the chip are 5v but the outputs are less than 2 volts.I don't know if this is part of the trouble, I havn't got it working yet, but has anyone had problems with this ?
 1025 Topics / Product Design / Re: How long should a project take? on: February 05, 2011, 04:30:39 pm or I could connect the probe through a 3.9pF cap that I have in the drawer, its got 22pF caps to deck anyway.
 1026 Topics / Product Design / Re: How long should a project take? on: February 05, 2011, 06:06:10 am the problem with the audio micro was a lazy crystal,  I stuck an LED ( and res) on pin 13 and loaded the chip with old faithful "blink.pde" and nothing happened, changing the xtal restored the blink...  You can't see it on the scope as the probe stops the oscillation anyway..