transistors
I am trying to Optoisolate a signal from microcontroller. I am driving the signal to base of MMBT2369, and opto isolator LED is connected to Collector of the transistor.
When transistor is not in place, uC output is as per logic i.e toggles every 1 second. When Output from uC is connected to base of MMBT2369, uC stops toggling and output is always constant low.
What can be the reason for this behavior?
Circuit is as shown. SOL_ENB is signal from Microcontroller.
Okay, looking at the picture I think you may have the transistor the wrong way round.
Try turning it round.
See this picture for reference:
As you can see the collector is on the right with the flat part facing you, so you have the collector connected to the LED in your circuit (if the 2N2222A part you are using has the same pinout)
I got the picture from here.
EDIT - It's actually a 2N222A, but the above advice still goes as the pinout appears to be the same from the picture posted.
As Russell mentions the more standard way is to connect the LED to the collector, but your circuit should work if set up correctly.
You really should post a schematic instead of all the hand waving. However, there is one obvious flaw in your description. One side of the relay could should be connected to the collector of the transistor as you say, but the other side must be to the power supply, not ground.
When the transistor is off, the collector is effectively not connected or left floating. When the transistor is turned on, it will pull the collector low. Since that end of the coil will be low, the other end must be high for there to be voltage accross the coil so that the relay can turn on.
Here is how you should hook things up:
Don't leave off D1. It is there to catch the inductive kickback from the coil when it is suddenly shut off. Without it, the coil will generate high voltage and fry the transistor.
Best Answer
You need a current limiting resistor between the base and the microcontroller. Otherwise you are driving the micro controller pin to a low impedance input which drives it low, your micro controller pin is currently switching between 0V and 0.7V and wasting a lot of current. I recommend trying a 1k resistor between the micro and the base.