Arduino With LM3915 Dot/Bar Display Driver

Projects General Guidance
1

I would like to interface the output pins of the LM3915 'Dot/Bar Display Driver' chip to the digital input pins of the Arduino Mega.

As many of you know, and I have only recently learned, the LM3915 can be used as the heart of a VU meter, by taking an analog input signal - from a speaker, microphone, or other analog sensor, and driving 10 LEDs in a progressive display. The stronger the analog input, the more LEDs light up (e.g. 1 LED lights with a low-level input, 10 LEDs light with a very strong input, and some other number of LEDs light in between.

Here is a diagram taken from their datasheet.

In a my application, the LM3915 Vcc runs at 7 volts. The output pins, to which the LEDs are attached are normally HIGH, and I measured their voltage at 5.75 volts. Of course, at that voltage the LED doesn't light (potential difference = 1.25 VOLTS). However with sufficiently high analog input signal, the pin goes LOW, and I measured the voltage at 5.25 volts. At the LOW level, the voltage between VCC and the PIN is 1.75 volts, and the LED lights. Somehow, I'd like that .5 volt change to cause a change in the Arduino PIN STATE (LOW to HIGH or vice versa) which I can program against.Of course there are 10 output pins on the LM3915, and each would have a corresponding digital INPUT PIN on the Arduino Mega.

I am hoping my inexperience is causing me to overlook something obvious, but I would very much appreciate any feedback on a suitable method for reading these changes so to change the logic at`the Arduino pin.

2

I would connect the common anodes of the LEDs through a resistor of about 15 ohms to +5v instead of to +7v, and to an analog input pin. The more LEDs are on, the more the voltage on pin that will drop from 5v.

3

It's not clear, if you'd like to keep leds connected to LM3915.
As outputs of chip appears to be open collector, probably it would be easier to connect outputs of LM3915 to arduino digital inputs directly w/o leds, activate internal pull-ups, and then re-assemble led bar graph on others 10 digital outputs (with resistors) if you still need it. Lucky, Mega has plenty pins