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Author Topic: Digital noise on my audio circuit  (Read 1274 times)
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I've built a talking clock using the Adruino, the LoL Shield for a matrix of LEDs and a SpeakJet chip to generate the voice.

Everything works superbly, but I get a ton of hum and hiss on the audio circuit due to the LED display. The switching on and off of the 124 LEDs hundreds of times a second is clearly piling buckets of noise into the amplifier. I know the source is the digital circuit, because the frequency of the noise is closely correlated to changes on my LED display.

What's the best way of removing the noise?

My best idea so far is to somehow switch off the audio circuit using a HIGH/LOW from the digital circuit when audio is not needed. The SpeakJet has a pin that goes HIGH when it is busy speaking, which would be ideal to use. My knowledge of analog circuits is pathetic though, so I don't know how to use that pin to turn the audio circuit off.  smiley-red

Any ideas?
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You might need separate voltage regulators for the digital & analog circuits...  Depending on what kind of power supply you have, that might require a new/different power supply. smiley-sad

First, I'd just try a capacitor (1000uF or more) across the audio power supply.   A diode in series before the capacitor usually helps too.  The diode prevents the digital circuitry from discharging the capacitor...  The capacitor can only discharge into the audio circuit.  (The diode will lower the supply voltage by about 0.7V, so hopefully this isn't a problem for the SpeakJet chip.)     Another diode & capacitor on the digital/LED side may help too. 
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