Simple continuity tester for telephone - has LED controlled by transistor with internal flash light and high impedance probes between transistor base and collector

摘要

The tester consist of a flashlamp battery (3) lighting an electroluminescent diode (9) through a transistor (1) and a collector load resistor (4). The conduction of the transistor is controlled by the impedance of the circuit under test. The test leads are connected to the collector of the transistor and to the base through a resistor (7). By incorporating a switch (11) and a filament lamp (12) the device can serve also as a flashlight to illuminate the circuit undertest. By choosing the input impedance of the transistor and the base resistor (7) the device can be used to test very high internal impedance circuits for continuity. Because the brightness of the lamp depends on the impedance it can be used as a rough indication of impedance. The tester can be used on electronic circuits without causing damage, e.g. to examine transistors and diodes and consumes little power.