Mechanical Television & Illusion Generators by James T. Hawes, AA9DT
Ohm's Law is Easy, Part 2

Schematic: LED driver

  • LED: Use red, orange or yellow.
  • Battery: 9-volt transistor battery.
  • Resistor: 390-ohm, half-watt.

You can now understand your multimeter better. Please check the actual circuit voltages with your meter. Real-world resistors and LEDs differ from one another as to how much voltage they drop.

Let's try something more useful and fun. Let's build an LED driver. This circuit will drive an LED from the output of your Radio Shack amplifier, radio or CD player. See the circuit at right...

You will use what you know about Ohm's Law to design the circuit based on my schematic. To turn on the LED, you need 0.02 amps at 1.5 volts. Resistor R3 drops the rest of the nine volts from the battery. Find the R3 resistor value by dividing...

R3 ohms = (9 volts - 1.5 volts) / 0.02 amps
It's Ohm's Law again!

Choose transistor base bias current. Use 10 percent of the collector current, or 0.002 amps (2 milliamps). The 10 percent rule promotes circuit stability.

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