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i am making a circuit that controls electrical applinaces in my home using the telephone where the tones are the control circuit.
i need a circuit that accepts 1st a secret code to enable controlling the appliances.
I know what your looking for, an off the shelf version of this can cost as much as $400!
The following link is a good starting point
**broken link removed**
The circuit is fairly simple and simple or short code sequence to allow the legitimate user control can be implemented using logic chips.
Or if you are a bit more advanced this is ideal territory for the micro-controllers that are becoming very popular.(Sadly I can't help you with these yet. I'm trying to get up to speed myself!)
i already know this circuit
in fact, i have a better one but i am adding a new feature in it which is the secret code. i want to add to this circuit a secret code so that no one can be able to use this circuit unless he enters the right secret code from the other phone.
thanx 4 ur response
the secret code is about 4 digits
this means that i will be able only to control 5 appliances and the digit zero is used to hookup the phone line
but may i ask u a question ?
after reading the documentation file and seeing the schematic of this teleremote control, from ur opinion does the IC KT 3170 ( or HT 8870 as an alternative ) decode DTMF before the phone is picked up while ringing ?
Apologies for the delay,been busy this end and for the life of me I cannot figure out how to get an image onto this board
(perhaps some kind soul will read this and drop me a line)
And no I dont mind answering questions we all have to start somewhere, there are some topics that are a complete mystery to me.
In answer to your last question - Yes
The dtmf is constanly listening for those specific tones and ignores everything else.
One is for a 4 digit lock the other is a rough sketch of your circuit with some suggested modifications.
A rough explination of the changes is this;
the output of the DTMF is inverted through a 4049 before being passed to the decimal decoders, both recieve the same input but only one is selected as active, this device provides the input to the lock circuit,use wire links to choose four numbers 0 - 9 (can only use each number once) and connect in the desired sequence to lock circuit ABCD the unused numbers get connected to the reset pin.
When you call the phone line the phone will continue to ring until your code is entered and the lock circuit changes state,this change triggers a 555 timer in monostable mode (how long a delay is up to you), it is this output that does two things.
It activates the transistor driving relay 1 to silence the phone and switches over to the second decoder that controls the latches that drive the control relays. ( you now have ten
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I've considered this problem (decoding while phone ringing with DTMF decoder) I never seen this solution, and i'm sure this cannot work in my country.( maybe some phone company allow this) One thing work with this method: the caller identification (CLIP) signal (if existiert) coming between first and second ring
I recommend to count the number of rings upto 15 and "pick-up" the phone with 200ohm. After pick-up the decoder wait for code sequence (DTMF). Successfull code reciving give a pacifier-tone, and wait for control-codes. With asterisk or other number make on-hook.
about ur last idea about the hock up circuit using the counter
i worked on it but the was a problem
i took the input from the telephone line and put as an input to the 741 OPAMP as a comparator to convert the superimposed sine wave of the telephone line when ringing into square wave as a clock to the counter
but i think it's a very high frequency so i think i will use a frequency divider 1st. could u plz tell me the frequency of this signal when the telephone starts ringing ?