Phone Bill Calculator

Posted on Mar 7, 2013

This circuit is used to give `1` signal when the phone is ringing. As you can see the ct. is directly connected to the phone line. The 4.7 uF blocks the DC loading on the phone line. This means that there is voltage on the 10K resistance only when the phone is ringing. This voltage is of course alternating, and it is half wave rectified by the diode, so that, only the positive spikes charge the capacitor. So the net result is that the capacitor is charged only when the phone line is ringing. The variable resistor is used to absorb the `silence` period during the ringing so that the voltage stays at logic high. By adjusting this variable resistance, you can prolong or shorten the time period after which the capacitor voltage will turn to logic low. This period should be adjusted to be about 4 seconds.

Phone Bill Calculator
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Phone Bill Calculator - image 1
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In fact, is not important and can be entirely omitted from the circuit, it is simply a 4 bit counter that its clear is used to give "1" signal from the anding of the " 4 "o/p bits, but it's importance rises when the ck used is very slow that's when ring 1/0 gives "1" signal, this latch "1" gives instantaneous "1" signal which exhibits "1" signal at o/p , that's we donut have to wait for the ck to get "1" signal from latch 2. 1-Prevent "1" signal at o/p when there is a coming call. 2-Making o/p signal when I make a call. This is done using a delay flip-flop ,and a logic ct. which controls two things, the o/p signal and the ck of the delay flip-flop, as in the design , when phone rings ,ring1/0 gives "1" signal, so "D" pin now have "1" signal on it before raising the hang, ring1/0 gives "1" signal, and also hang 1/0 gives 0 SIGNAL. After raising the hang to answer the phone ,we have : "1" signal from ring1/0 as the capacitor has not yet discharged, and "1" signal from hang1/0 after we raised the hang, so this means that is the ck is enabled on the delay f.f , but when the capacitor discharges, ring1/0 gives now "0" signal ,this means that the expected thing here is to have o/p signal as the hang is still raised , but the design exhibits the ck at this instance on the delay f.f , making its "q" pin high as long as the hang is raised ,when the call is finished and the hang is off, this enables the ck and we go back to...

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