I have a NPN Common emitter configuration circuit using KSP2222A transistor. The power supply is from a 3Volt battery source. Please see the attached image. The circuit lights up the LED when the phototransistor detects infrared light.

I observed that the LED is bright when the Infrared light source is closer to the detector and not so bright when the IR source is slightly far. I don't want this behavior. I want the LED to have constant brightness even when the IR beam strength goes weak.

How do I make the transistor work like a real ON/OFF switch (instead of merely amplifying the intensity of IR signal)? LED should be switched ON even if the phototransistor senses the weakest IR signal. LED should be completely OFF when phototransistor detects nothing.

 

I have a NPN Common emitter configuration circuit using KSP2222A transistor. The power supply is from a 3Volt battery source. Please see the attached image. The circuit lights up the LED when the phototransistor detects infrared light.

I observed that the LED is bright when the Infrared light source is closer to the detector and not so bright when the IR source is slightly far. I don't want this behavior. I want the LED to have constant brightness even when the IR beam strength goes weak.

How do I make the transistor work like a real ON/OFF switch (instead of merely amplifying the intensity of IR signal)? LED should be switched ON even if the phototransistor senses the weakest IR signal. LED should be completely OFF when phototransistor detects nothing.

Instead of a single transistor you need an amplifier with at least 3 transistors or an opamp or a comparator. The circuit needs to have hysteresis so that it switches on and off abruptly.

 

Thanks. What is hysteresis (I am a newbie) and in my case how do I implement it?

 

http://en.wikipedia.org/wiki/Hysteresis

 

The Wikipedia description of Hysteresis is too complicated.

It is the addition of a small amount of positive feedback to produce a fast "snap-action" to a slowly moving DC voltage, at an almost centered threshold voltage.

 

Am I reading your answers correctly to mean like a darlington pair set-up?? I used a second 2n2222 to improve the switching on an colpitts oscillator to square wave output circuit and it worked great!!!

 

A darlington pair amplifies the current.
But you need a voltage amplifier not a current amplifier.