- Connect battery "+" to 9VDC Bus
- Connect battery "-" to Ground Bus
- Connect phone plug tip to point "A"
- Connect phone plug ring to point "B"
Find R2. The transistor base must have 0.7 volts more than the transistor emitter has on it. Our emitter connects to an LED, which turns on at 1.5 volts. Then the emitter has 1.5 volts on it. The base must have 1.5 + 0.7, or 2.2 volts on it. Find the base resistor R2 value by dividing...
R2 ohms = Base voltage / Base current
R2 ohms = (2.2 volts / 0.002 amps) (Ohm's Law, right?)
Find R1. Resistor R2 drops 2.2 volts. Resistor R1 drops the rest of our battery's 9 volts. Find the R1 value by dividing...
R1 ohms = (9 - 2.2 volts) / 0.002 amps (Ohm's Law, one last time!)
Check your answers with the button below.The button also provides real-world part values to substitute for calculated values. You can buy the real-world parts at Radio Shack or Mouser.
Better driver for modulated LEDs
If you want to modulate the LED, you need different R2 and R3 values. The new values produce a better grayscale output than you'll get with the above circuit. The reason is that the above circuit starts with the LED all the way on. For this reason, the above circuit might clip when the input signal goes positive. When the input swings negative, it might only dim the LED. The LED might never shut down. The result is a reduced gray scale. The circuit functions as a limiter. The answer to this problem is to bias the LED about halfway on. To improve the LED's modulation response, let's rerun the last two calculations...
Find R2. The transistor base must have 0.7 volts more than the average emitter voltage. The average emitter voltage is half the LED turn-on voltage of 1.5 volts. We give the emitter half of the 1.5 volts, or 0.75 volt. The base must get 0.75 + 0.7, or 1.45 volts. Find the base resistor R2 value by dividing...
R2 ohms = Base voltage / Base current
R2 ohms = (1.45 volts / 0.002 amps)
Find R1. Resistor R2 drops 1.45 volts. Resistor R1 drops the rest of our battery's 9 volts. Find the R1 value by dividing...
R1 ohms = (9 - 1.45 volts) / 0.002 amps
Check your answers with the button below.The button also provides real-world part values to substitute for calculated values. You can buy the real-world parts at Radio Shack or Mouser.
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