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In & out of circuit LED tester The idea for this circuit came to me when I was having difficulty locating faulty LEDs in a string, as used in an LED light bulb or infrared illuminator. Assuming that you’re prepared to spend the time, repairs can be made or good sections isolated to repair other units. Probing with a multimeter works, but you have to reverse the probes quite often. So I built this simple circuit around a 4093 quad schmitt trigger input NAND gate IC (one of my favourite chips for quick and dirty solutions). IC1d is configured as a simple oscillator. I have tied input pin 13 to ground via a 4.7MW resistor, so the siliconchip.com.au oscillator will not function until this pin goes high, accomplished with a simple touchpad made from a piece of Veroboard. Until this is activated, power consumption is virtually nil. When it does oscillate, it drives two gates in series out-of-phase (IC1c & IC1b), so their outputs continually swap polarities. These outputs are connected by the device under test (DUT) via a 22W resistor and LED2/LED3. These LEDs are connected in parallel with opposite polarity. LED2 and LED3 can only light when the DUT passes current. In the case of a functional device, the LED that turns on must have the same polarity Australia’s electronics magazine as the DUT. If both turn on, you have a short; if neither do, then there is an open circuit. A low resistance across the DUT could cause both LEDs to flash, but that is uncommon. The DUT can be connected by a simple socket or a pair of probes, used to check devices in-circuit. The accompanying breadboard diagram shows how you could build this circuit on a breadboard, or an IC style protoboard. Graham P. Jackman, Melbourne, Vic. ($80) June 2021  63