This circuit can be used to test electronic components such as diodes, resistors, coils, transformers, lamps, fuses, switches, loudspeakers and many others. The state of the component under test is indicated by an LED.
When the probes are separated orwhen the resistance between them is very
high (more than 1,000,000 ohms) the LED is otf. When the resistance islow
(the transistor is in a conduction state) the LED is on.
The circuit is powered from a 6- or 9-volt supply (AA cells or battery). Current
drain is less than 10 mA with the LED on. R2 value depends on the power
supply voltage. For a 9V supply we recommend 820 or 1,000 ohms for R2.
A schematic diagram of the Continuity Tester is given in Figure 1. Two transistor are wired as a Darlington pair. The configuration reduces the current
flow between probes to microampere values.
Components placement on a terminal strip, used as chassis, is shown in
Figure 2. lt is very important to observe positions of the transistors and LED.
House the device in a small plastic box and use probes or alligator clips to
connect the components under test.
Parts List - Continuity Tester
Q1, Q2 - 8C558 general-purpose PNP silicon transistors
LED - Common red, green or yellow LED
R1 - 100,000 ohm, 114W,5% resistor
R2 - 470 ohm, 114W,5% resistor - see text
S1 - SPST slide or toggle switch
81 - 6V or 9V (four AA cells or battery)
PPl,2 - Red and black probes
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