Different Colored LEDs

You can use LEDs other than red with the DRO-350.  Pin compatible LED displays are available in green and orange as well.  The green two-digit display part number is MAN6440 and the corresponding indicator LEDs are part number 606-4303F5 from Mouser.  For orange, use part number MAN6640 for the two-digit display and 606-4303F3 for the indicator LEDs.  No modifications are necessary other than using the different parts.

DRO-350 with Green LEDs

Super Bright Display

The DRO-350 updates the LEDs in a multiplexed fashion.  What this means is that each LED is scanned many times per second so that your eye sees one continuous output.  This is very similar to how a TV or computer monitor works by relying on the physiological phenomenon know as persistence of vision.  The main benefit is that a dedicated line to each LED is not needed so that pins can be shared and devices with smaller pin counts can be used.

The drawback to this is that the display is a bit dimmer than if the display was driven continuously without multiplexing.  To compensate for this, the DRO-350 uses what are known as high efficiency LEDs that are much brighter than regular LEDs.  The result gives the DRO-350 very good brightness, but with a tweak and small risk, you can get a super bright display.

The DRO-350 is designed to be able to overdrive the LEDs with more current than ordinarily could be used to drive the LEDs in continuous operation.  This ability is quite safe for the LEDs and condoned by the LED manufacturers.  The only risk is that if the software somehow locked up or if there was a problem with the circuit, the LEDs could be driven with enough continuous current to damage them.  This danger is mitigated by a watchdog process that runs on the PIC that resets the circuit before any damage is done if the software locks up.  I decided against using this feature in the final kits since the display already had very good brightness.

The standard kit and bill of materials shows a value of 150 Ohms for resistor packs R12, R13, R14.  This sets the LED current at about 30mA - the maximum operating level for continuous operation.  To overdrive the LEDs, you can use resistor pack values less than 150 Ohms.  DO NOT go any lower than 68 Ohm since this is the current limit without stressing the drive buffers.  You must also use a 9V wall transformer with a higher current rating than the 1.0A unit included with the kit.  A 1.5A supply will be sufficient for the maximum limit previously described.

If you want to try this modification, do so at your own risk.

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