I design DTMF generator with HT9200B. But how can I interface the output of the HT9200 device to the telephone line? there is a schematic in HOLTEK web site.(**broken link removed** ) But there are 3 undefined transistor. What kind of transistors must be used in this circuits. thanks.
One transistor is MPSA92, the other one is MPSA42 and both of them MUST use high voltage type transistors. Otherwise the NPN transistor will switches on when ring signal applies to it, as a result the isolation fails.