PAGE 140 OF 150 · PROJECTS
Build a Polarity Tester
Two LEDs show which way a supply is connected.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
What it does
A polarity tester shows which way round a DC supply is connected. Two LEDs of different colours are wired in opposite directions, so only one lights at a time.
Circuit
- Red LED and green LED in anti-parallel.
- One resistor in series: 680 ohms for up to 12 V, or 1.5 kilohms for 24 V.
- Probe leads at each end.
Current stays under 20 mA, so it is safe for the LEDs. A single bi-colour LED does the same job. Label which colour means positive on which lead.
REMEMBER
Anti-parallel LEDs show which way the supply is connected.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
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