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Telephone handset tone dial encoder

2016-02-14 11:20  
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This encoder uses a single contact per key keyboard and provides all other switching function electronically. The diode between terminals 8 and 15 prevents the output going more than 1 volt negative with respect to the negative supply.The circuit operates over the supply voltage range from 3 volts to 15 volts.
 

Telephone handset tone dial encoder