SUPER-SENSITIVE CURRENT DETECTOR

Everybody knows paper doesn’t conduct electricity. Or does it? Many materials that we consider insulators actually conduct very tiny amounts of current. Here’s a circuit that can detect current flow so small that even expensive laboratory meters can’t see it. An LED blinks if there’s any current flow at all. The faster the blinking, the more current is flowing. The smallest amounts of current most normal meters can detect will blink this circuit’s LED so fast that it will appear to be on solidly. This thing is sensitive! You’ll have fun connecting it to books, bed sheets and just about anything else you can think of that isn’t obviously conductive. The results will surprise you.

The circuit works by storing in the capacitor the tiny amount of current flowing through whatever you’re testing. The electricity gets stored like water in a cup, with the voltage rising as more is put in. When the voltage gets high enough, the chip detects it and turns on the LED, while also quickly draining the charge from the capacitor. As soon as the charge is gone, the light goes out and the process starts over again. The more current that flows through your object, the faster the capacitor charges up and the faster the light flashes.

What You’ll Need

Breadboard, Radio Shack #276WBU301

7 jumper wires made from solid hookup wire, Radio Shack #278-1222 or similar

4 clip leads

TLC555CP timer chip, Radio Shack #276-1718

100 pf (picofarad) capacitor, Radio Shack #272-123

1 MΩ (megohm, or million ohms) resistor, Radio Shack #271-1356

10 MΩ resistor, Radio Shack #271-1365

LED, Radio Shack #276-304 or similar. Any small LED will work.

Battery holder, Radio Shack #270-383

4 AA alkaline batteries

What You’ll Do

1. Turn the chip so that the dimple for pin 1 is in the lower left corner. Position it over the center of the breadboard, and press its pins into the holes until you feel it snap in.

Resistors use bands of color instead of numbers to show their values. The 1 MΩ (megohm) resistor’s bands are brown, black, green, gold. The 10 MΩ resistor’s bands are brown, black, blue, gold. They’re similar, so be careful not to confuse them. If you do, the circuit won’t work right, but you won’t hurt anything.

2. Connect the parts as shown in the drawings. When you plug in the LED, be sure the shorter lead (from the flat edge of the case) goes to pin 3 of the chip. Connect two clip leads as shown to use for connecting to whatever you want to test. It’s best not to use red and black leads for these because we’ll use those colors for the battery connections.

3. Put the batteries in their holder with their flat, negative ends going to the springs, and connect a red clip lead to the smaller terminal on top. Now connect a black clip lead to the larger terminal. Connect the leads’ other ends to the upper- and lower-most holes on the breadboard, the ones that connect from left to right, using jumper wires. The red clip goes to the top row on the board, and the black one goes to the bottom. Be certain not to get this backward, or you will probably ruin the chip.

4. The light might blink a time or two, but then it should stop. Now hook the loose ends of the test probes onto something non-metallic. Try a sheet of paper first. Take your hands off the test probes, because touching them can fool the circuit and make it blink when it shouldn’t.

Does the light blink? With most paper, it will. Even long stretches of paper conduct enough current for this circuit to detect it. Try clipping to two separate sheets of paper, and then lay one on top of the other. Use a drinking straw or the eraser end of a pencil to press one paper onto another. Just a small area of contact will blink the light because a very tiny current is flowing. Now try other household items. Stay away from electrical wiring or anything else that could be dangerous.

Troubleshooting

If the light won’t blink, you probably have an incorrect connection on the breadboard, the LED is in backward or the battery is not hooked up correctly. Disconnect one of the battery’s leads and check the wiring, and also check that you have the resistors in the right places. Then hook the battery back up. Grabbing the free ends of the test probes should turn the light on.

If the light blinks when the probes aren’t connected to anything, be sure you’re not touching them or their wires, and that their free ends aren’t touching something that could be conductive. Even a tabletop may show some current flow.