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The Soundwerx Designs DoodleBug

DoodleBug Tweaks

Being a fairly simple device, there is not much to tweak. One exception is the ouput voltage setting.

Notes on setting the output voltage -
The voltage specification for the USB standard provides for voltages ranging from 4.4 to 5.25 volts (for hubs), with 5 volts being the average of what we should expect. However, if you are unlucky enough to have a USB port that only meets the minimum spec and/or are using a long/poor quality USB cable, your device may be getting less than the required minimum voltage. The Doodlebug allows you to compensate for this voltage drop and assures your USB device of getting the voltage it needs to operate properly. Using the information above, you could conceivably increase the voltage of the DoodleBug to more than 5.25VDC to compensate.

That said and as noted in the Checks and Setup page, we recommend that you set your DoodleBug to 5.25V, the maximum that the USB standard allows. However, some of you may note that you can't achieve this voltage setting - even at the least resistance adjustment on VR1. Note that the Beezar kits use a walwart that will give approximately 5.2VDC in most cases with a fully populated PCB. (See the Checks and Setup page.) That is still plenty of voltage headroom for USB devices if you are not using it with a long or poor cable run. That said, you can still increase the voltage supply output.

Changing the R1 resistor or the LEDs -
The LM317 (IC1) has a voltage of 1.25 volts between the ouput and adjust pins. The LTL-4221N LED has a voltage drop of 2 volts @ 20ma. R1 and VR1 set the current through the two diodes, LED1 and LED2. The combined voltage drop of the two LEDs should then be 4 volts plus 1.25 volts, the sum of which determines the expected output voltage: 5.25VDC. Of course, in reality, things vary.

The resistor R1 sets the maximum current through the LEDs (when VR1 is open at ~zero resistance). The value of 49.9 ohms gives a maximum of 25 ma. VR1 can vary that maximum current down to about 8 ma. If you find you want more output than allowed with the BOM values, you can reduce the value of R1 to around 25 ohms, giving a current through the LEDs of 50 mA. By also choosing LEDs that have a different voltage drop, almost any output voltage can be created.

Use different R1 and LEDs to change output voltage ...

Notes on AC adapter (walwart) selection -
The Doodlebug allows for a wide range of AC adapters that can be used. The LM317 regulator needs an input voltage of about 3 volts higher than the output voltage to operate properly. With a target output of 5 volts, we need our AC adapter to provide a minimum of 8 volts as measured across C5 at the desired load.

Measure with your probe at Gnd and other probe at either the left lead of D3 or the top pin of IC1. This voltage should be more than 8VDC.

AC adapters are unregulated devices and their voltage output varies according to the load. The rating on an adapter might be something like "6VAC, 1A". That does not mean it will always be 6 volts. At loads less than its rated current, it will have a higher voltage and at loads greater than the rated current, the voltage will be less. Moreover, its DC output, once rectified, will always be higher than its rated AC output.

For example, a 9VAC, 500mA adapter gives 12VDC across C5 when powering a PupDAC. There are several versions of calculations that can be made to estimate the rectified DC voltage from an AC supply. Most describe multiplying 1.414 times the AC voltage, then subtracting the drop in voltage across the rectifier diodes (usually 2 x 0.7V). In the example cited above, that calculation would be (1.414 x 9) - 1.4 = 11.326VDC. Why is the actual voltage (12VDC) still higher?

That AC voltage used in the calculation is the RMS (root mean square) value. RMS is pretty much the standard in measuring and specifying AC voltage and is always less than the peak AC voltage. In reality, the difference between that peak AC voltage and the resulting DC rectified voltage is ripple. However, real practice always includes a "smoothing" capacitor after the rectifiers that removes most of the ripple. (One reason why high-ripple value capacitors are preferred in power circuits instead of "audio-quality" capacitors.) This boosts the voltage by "filling in" the spaces between the peaks of the sinusoidal curve tops, resulting in an even higher voltage. Suffice to say that the resulting DC voltage is always higher than the typical calculation (1.414 times AC rms voltage minus a two-diode voltage drop). Add to that an uncertainty in the low-loaded output voltage of the AC power supply. That AC rms output voltage is specified at a given load. Less than that load, the peaks of the AC voltage are even higher.

Notes on DC adapter (walwart) selection -
A DC adapter may also be used with the DoodleBug. When using DC, an adapter with an output at 500mA at 9 to 12 volts would be acceptable. When using a DC adapter, the diode rectifier bridge does not need to be installed and the diodes can be jumpered instead. If the DC adapter uses a center positive plug, jumper D3 and D6. If the DC adapter is center negative, jumper D2 and D5. If you use a DC adapter with the bridge rectifier installed, you will not have to worry about the polarity of the adapter, because the bridge will maintain the polarity correctly. However, the voltage will drop through the diodes (approximately 1.5 to 2 volts depending on load). With the 9VDC, 500ma adapters supplied in the Beezar kits, a voltage of over 10VDC should be attained at the C5 trace with all diodes installed, which is more than enough. The Triad walwarts used in the Beezar kits are very high quality, though. Your mileage may vary if you select a different brand. A 12VDC adapter might be a better choice or jumper the diodes previously noted and shown below:

Delete D3 and D6 and instead install jumpers if your DC walwart has a center-positive plug and you do not have 10VDC or more at the C5 trace.

Delete D2 and D5 and instead install jumpers if your DC walwart has a center-negative plug and you do not have 10VDC or more at the C5 trace.
file last changed:Saturday, November 8, 2014 7:46:13 AM
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