This is a circuit for a telephone call meter designed with a calculator and chip on board. The calculator enables conversion of STD/ISD calls to local call equivalents and always displays current local call-meter reading. -- K.UDHAYA KUMARAN, VU3GTH


Useful hints. Solder the ‘=’ button terminals by drilling small holes in its vicinity on PCB pattern using thin copper wire and solder it neatly, such that the ‘=’ button could get activated electronically as well as manually. Take the copper wire through a hole to the backside of the PCB, from where it is taken out of the calculator as terminals G and H.

At calculator’s battery terminals, solder two wires to ‘+’ and ‘–’ terminals. These wires are also taken out from calculator as terminals E and F. Affix COB on a general-purpose PCB and solder the remaining components neatly. For giving the unit an elegant look, purchase a jewellery plastic box with flip-type cover (size 15cm x 15cm). Now fix the board, calculator, and batteries, along wit holder inside the jewellery box. Then mount the box on the wall and paste the look-up table inside the box cover in such a way that on opening the box, it is visible on left side of the box.

Caution. The negative terminals of battery A and battery B are to be kept isolated from each other for proper operation of this circuit.


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