Telephone to Pa made easy. Schematic example to show you the testversion of this new adapter.
Analog "Telephone to PA adapter ", w/automatic On / Off lineconnection , gives outgoing audio and a priority command signal to control the PA system . ( Easy to connect & easy to use ) .
Invented & composed by Joar Falstad at Jo Tech Service , NORWAY year 2002 . Published on the web january 01, 2003.
Composed to make it easy to send long distance messages to public areas, like unmanned railway stations and similar.
Some specifications :
Safe construction : Transformer and optoinsulated line connection .
The construction is made quite simple, using standard C-Mos circuits .
High electrical noice immunity, the electrical noice from the circuit itself is very low .
Boards availability: We already have important users , therefore we'll always have some service-boards in our stock .
Parts availability is good : Only vell known, videly used standard components are used in this construction.
Easy connection : Analog telephone line, 12V DC Power Supply, audio output to power amplifier , priority relay output , gong output .
Easy to use : One ring in the callers phone before the adapter autoconnects . ( silent connect or an attention signal in the speakers . ). 1-way audio. Output audio level adjustable, max +3db . A prioritysignal is active during the call. The adapter automatic disconnects when the caller hangs up his phone .
How does this simple unit work :
1. Call the adapter from another phone. The low frequency ring signal from the analog teleline make pulses in an opto-insulator, a counter make the offhook-signal after one ring . It's possible the trigg an attention signal from the "ringindicator" output .
The offhook-relay disconnects the ring indicator , connects a line dc load , and connect the la/lb line to a signal transformer. From the transformer the signal goes to the amplifier. The offhook-relay also gives the priority switch output.
2. Talk to the adapter, the conversion is only outgoing .
3. Hang up your phone. In the adapter the cancelling tone is amplified in the bandpass amplifier . If the signal has both the correct frequency and a proper length the following comparator will make the onhook-signal to turn off the adapter . .
More info to come
Latest shematic, ver. 1.2 , mar 2008 : teletoau.png
A standard chime circuit.
Veroboard layout: Standard Europa format .
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