CONTROL TONE DECODER

  
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Permits monitoring local VHF FM repeater for calls from friends without having to listen to chatter of others or to repeater noise. Operation is similar to that of Motorola paging units in which special tone is transmitted to disable squelch of receiver being called. Each friend has tone encoder for his transmitter, set at correct frequency for co
CONTROL TONE DECODER - schematic

nnecting loudspeaker so desired call can be heard. Red LED comes on to confirm that loud-speaker is connected. Audio from receiver loud-speaker is fed into pin 3 of NE567V PLL U1. When correct tone frequency is received, pin 8 drops from 4V to near 0V, turning off Q1 and turning on Q2. Q2 closes relay K1, to connect loudspeaker, and holds it on until RESET switch is operated. Q2 is Radio Shack 276-1059 or other small SCR. CR1 is 1N4735, and CR2 is red LED. -K. Wyatt, Private Call System for VHF FM, Ham Radio, Sept. 1977, p 2-64.



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