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Sunday, June 23, 2013

Signals Logic Tester Circuit Using Display 7 Segment

Here’s a design of  signals logic tester that is using indicate on a common cathode seven-segment display, if entry is at logic level "1" (one H on the display) or logic level "0" (one L on the display). If an undefined level is detected will display an "n". When the input is "0", T1 locks and T2 and T3 drive. A level of output X IC1a and "0" on that of IC1b therefore segments d, e and f will be lit. Here’s the figure of the circuit;

When entry is logical "1" is saturated T1 and T2 and T3 are blocked. IC1a's output becomes "0" and that of IC1b becomes "1". Besides segments e and f, already lit will light b, c and g, which results in displaying an "H". When entry is in an undefined state, or is not connected, all transistors are conduction (due to R1, R2 and R3) and the icon displayed is a "n" (undefined). Switching thresholds of the tester are 1 V and 3 V (3 V threshold may be reduced slightly by increasing the resistance R4). Input impedance tester is 5 k, so it does not affect the circuit being tested.

Mains Slave Switcher Circuit

There are many situations where two or more pieces of equipment are used together and to avoid having to switch each item on separately or risk the possibility of leaving one of them on when switching the rest off, a slave switch is often used. This circuit is design to used for minding that are a computer/printer/scanner etc or audio amplifier/record deck/tuner combinations or perhaps closest to every electronics enthusiast’s heart, the work bench where a bench power supply/oscilloscope/soldering iron etc are often required simultaneously. This circuit is called main slave switcher circuit. Here’s the figure of the circuit;

This circuit, which is intended for switching power to a work bench when the bench light is switched on, avoids resistors or any modifications to the lamp or slave appliances by sensing the electric field around the lamp cable when this is switched on. The lamp then also functions as a ‘power on’ indicator (albeit a very large one that cannot be ignored) that shows when all of the equipment on the bench is switched on. The field, which appears around the lamp cable when the mains is connected, can be sensed by a short piece of insulated wire simply wrapped around it and this is amplified by the three stage amplifier which can be regarded as a single super-transistor with a very high gain. The extremely small a.c. base current results in an appreciable collector current which after smoothing (by C3) is used to switch on a relay to power the other sockets. Power for the relay is obtained from a capacitor ‘mains dropper’ that generates no heat and provides a d.c. supply of around 15 volts when the relay is off.

The output current of this supply is limited so that the voltage drops substantially when the relay pulls in but since relays require more current to operate them than they do to remain energized, this is not a problem. Since the transistor emitter is referenced to mains Neutral, it is the field around the mains Live which will be detected. Consequently, for correct operation the Live wire to the lamp must be switched and this will no doubt be the case in all lamps where the switch is factory fitted. In case of uncertainty, a double-pole switch to interrupt both the Live and Neutral should be used.