In Reply to: Phone alarm!! Help Please!!!! posted by Gerard on 05/25/00 at 1:11 AM:
information of use
1. you can find the information on the LM555 timer on the national.com [and also on the philips website, I believe] website. You can use this timer to generate a pulse at a given time after an initiating event. The timer is triggered by the trigger input being driven to zero. Therefore you may need a reed relay as an interposing relay to achieve this.
The simplest way to achieve an automatic phone hang up is to wire a normally closed relay contact in series with the normal hang up sensor, assuming this CLOSES when the receiver is off hook. Thus all you need do is pulse this relay [100 ms will be sufficient] with a oneshot signal and you will drop the user out to a dial tone.
You may find it simplest to do this by using a monostable LM555 driven by another 555, which gives the time duration.
The orginal alarm can be generated from a piezo-acoustic which is the type with a built in oscillator.
ring voltage on standard telecom systems is 90v and off hook voltage is below around 13V. The on hook voltage is 48-50V.
you will not be able to draw power from the telecom lines or legal and impedance reasons ; the phone will not ring if it presents impedance less than around 200K to the line.