* important update. I realized that radio shack has pulled another boneheaded stocking maneuver and placed a totally different part in the 2N3904 box. Simply eliminate the 2N3904 and drive the anode of the LED with the emitter of the phototransistor.
Extremely simple design
Can be mounted on a nine-volt battery clip
Only 5 components (not including battery clip or Vero-board)
Schematic:
Parts:
1K ohm
330 Ohm
LED
IR transistor
2N3904 NPN transistor (any NPN will do)
Operation:
The Infrared transistor will supply current to the base of the ‘3904 when it receives infrared light. The ‘3904 will supply current to the LED depending upon the amount light falling on the IR transistor. Depending on you IR transistor and amplifying transistor, you may or may not need the 1k bypass resistor. The purpose of the 1k resistor is to allow the LED to turn fully off, if little to no IR is falling on the IR receiver. This is necessary because there may be some leakage current through the IR receiver, even if no light is falling on it. This small current will be amplified by the DC current gain of the transistor and will cause the LED to light. The 1k bypass transistor allows the current to flow, but keeps the voltage across the LED below its conduction threshold. As the current increases, the voltage across the 1k resistor will increase, causing the LED to reach its forward bias on-voltage. At that point, the LED will take over in the consumption of current as its current is exponential with respect to its voltage drop, and the resistor is linear. Thus, to increase the current 2 fold through the emitter of the ‘3904, a 20 millivolt increase is required for the LED, but twice the voltage is required to force twice the current through the resistor alone.
At the points to the left of the line marked “1” is the current produced by small leakage currents through the IR receiver, as you can see, no current is flowing through the LED, so it is off. As the Emitter current is increased, the LED is turned on, and the majority of the current goes through the LED, and not the 1k resistor.
Test the circuit by holding it close to an IR remote control and pressing any button. If everything is working (including the remote control) then the LED should light up.
P.s. you may notice in the picture that the resistor is NOT 330 ohms as I specified. I built this circuit with a 100 ohm resistor originally, but then I realized that it allowed more than 20 mA to flow through the LED, which is generally not a good thing. I changed that value to limit the maximum current through the LED.
