o - blocking - circuits

iran is a rather wide country, and, as a rule, the chrage per one call in s. t .d . (subscriber trunk dialing ) is required more compared with a local call , so, just after the opening of the std service widely in national telephone network, unconcerned use of the s. t. d. service has been caused on an incon- ceivable increase in telephone charge of th subscriber. this is a common problem for all subscribers from home telephone to private or govermental offices. then, many subscribers requested to have a device available to control trunk dialing. therefore tci (telecommunication company of iran) asked us to solve this problem specially for governmental offices . we recommended several methods to t. c. i. for controlliug of toll dialing such as rearrangement of wiring in telophone offices, applying the counting method, applying the ac signalling method, and so on. this report is the main part of our recommendation about «0» blocking by impulse counting method. in this article, four circuits arc presented , which have been developed one by one in our ladoratory. so we have improved the device from the first circuit to the next, and in some steps the circuit has been fully modified . there are many interesting points, mentioned hereafter that we may say they are suitable circuits. the first and second circuits have to be installed in telephone offices . the first circuit begins to count 1200 ms since the arrival of first dial pulse in the first digit, but in the succee ding three circuits is applied another method called only first digit counting . the third circuit should be connected to telephone line on telephone set or pbx (private branch exchange), with a high parallel resistance (more than 3meg ohms) . the fourth circuit employs only a very low resistance (1 to 2 ohms) in senes to the line , and provides two considerable features, no restriction for connecting point of the circuit to the subscriber's line, and no using any de current during hook on as previous one . the consideration in those circuits were studied as follows; 1) reasonable price 2) small size 3) using a 5- volt de power supply only 4) wide range for acceptable speed of dial with high reliability to count the pulse 5) using a _very high resistance in parallel or a very low resistance in series, 6) using power off relay circuit and emergency battery .