Phone Bill Calculator

Introduction

Since phone bills have become a dreadful nightmare for most of us, causing parents to smack their internet addict children, and husbands to strangle their blabbering wives, we tried to work out a way to help keep track of time and to stop phone bills from running wild. You know what they say; Need is the mother of invention!

General view on the aim of the design

The main idea is to calculate phone bill, so we want to find time taken by each call I make, and by knowing the bill of the minute, we can calculate the whole phone bill for any period.

Technical view

In other words , we want to enable a counter only when we dial a number using the phone , also a good tip for this device is that this device is operated on the phone line, so we don't have to put each device at each phone set.

Construction

The device is composed of 5 sub-cts., these are :

1-Ring 1/0
2-Hang 1/0
3-Latch 1
4-Latch 2
5-Counting stage.

Analysis

Ring 1/0:

This ct. 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.

Hang1/0

This ct. is used to give "1" signal when the hang is raised, as we know the line voltage before raising the hang is 38.5 Vdc, but after raising it, it becomes about 5.8 Vdc, so we use comparators like IC : LM324, to give "1" signal while raising the hang, by entering the line voltage on a divider, then on the inverting terminal of the op-amp, so we get VCC at o/p when hang is raised.

Latch 1:

In fact, this ct. 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.

Latch2:

This is a logic ct. ,that's we can say ,it is the big brain of the whole device. ,it's main goal is to :

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 square one having"0" signals out of both

ring1/0 and hang 1/0,and so waiting for any further signals.

COUNTING STAGE:
THIS STAGE CAN BE DONE USING VARIOUS WAYS:

1-Digital or analog clock: Any digital or analog clock can be used by activating its counting process or deactivating it using the o/p signal.

2-Port programming: A very interesting way, which needs a PC though. It is used by entering the o/p signal in the write bit of the printer port at the back of the PC, and by writing a suitable program using any programming language i.e.: C, C++, Basic,...etc. At that point, the counting process takes place.