Tone-alert decoder

Posted on Oct 9, 2012

PLL (Ul) is set with R2 to desired tone frequency. LED lights to indicate lock-up of PLL. Reduce signal level (Rl) and readjust R2 to assure lock-up. Delay is selected from counter U3 output. Circuits latches (turns on Q1 to allow audio to speaker) when proper frequency/duration signal is received

Tone-alert decoder
Click here to download the full size of the above Circuit.

To reset latch, a positive voltage must be applied briefly to the R input of U4.

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