This circuit can be used to test up to three LEDs at once, connected in series. You can easily increase that number by using a higher supply voltage. If you do so, you should allow 2.7 V for each ...

Is is easy to test the polarity of a circuit’s point whether it is positive or negative by using the tester circuit featured here. The tester circuit has a high input impedance (around 1 megaohms) to ...