A 555 timer drives Ql through Q4 with a square wave. LEDl and LED2light when an npn or pnp transistor respectively, are connected to the text terminals. If LED3 and LED4 light equally as LEDl and LED2, the transistor is functional. If LED3 and LED4 are brighter than LEDl or LED2, the transistor is shorted.
(Source: Encyclopedia of electronic circuits volume 4)
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