How to use transistor as switch..

colin55 said:

The circuit I posted is ALL YOU NEED . It is a delay circuit and the electrolytic can be 47u to 100u.
Just push the switch momentarily and the circuit will work. You may need 100k between base and emitter.
Try it and see if it works. The unknown factor is the impedance of the pads on the PC board, making up the switch.
They should be 50k to 100k or more.

Click to expand...

This is a much better choice since you will have single circuit and forget about common ground.

I have attached the circuit..
I have got an alternative idea as I am designing auto answer mobile..
because I dont have auto-answer feature in my mobile..
The main purpose of this post is to learn new concept or thing by creating it manually..

I have thought that we can use vibrator as a pulse detector..
We can set our mobile in vibration mode and as the call comes the vibrator will start to run..


So the transistor will get the pulse from the vibrator and short the Answer key to answer the call..
The positive voltage on vibrator is : 1.6v.

I have tried using bc547 transistor and 4.7k resistor..
As I was touching the base of the transistor the answer key begins to work.. without vibrator pulse..

So, I changed the resistor values to 1k , 2.2k and at last I've used 100k.
But the result was same.. the answer key was running as I was touching the base of the transistor....

I answer key should run when the vibrator runs..




I just want this way..

Put a 47k from base to emitter

colin55 said:

Put a 47k from base to emitter

Click to expand...

what does the 47k resistor does..

what is the main purpose of adding 47k resistor from base to emitter...

and what will be the value of resistor which will be at base... because the vibrator voltage is : 1.6v..

and hence vibrator will turn on/off the transistor as it run....

Your transistor is connected as an emitter-follower. Then +1.6V on the base results in only +0.8V or +0.9V on the emitter. Maybe not enough voltage.
Touching the base injects 50Hz or 60Hz mains hum that is all around you since your body picks it up. Its voltage is probably a lot higher.

audioguru said:

Your transistor is connected as an emitter-follower. Then +1.6V on the base results in only +0.8V or +0.9V on the emitter. Maybe not enough voltage.
Touching the base injects 50Hz or 60Hz mains hum that is all around you since your body picks it up. Its voltage is probably a lot higher.

Click to expand...

So , the vibrator +1.6 will be connected to the base of the transistor and 47k resistor will be in between base to emitter.

As, the volatge develops on vibrator , it will run the transistor to auto answer the call..

But what will be the the resistor value which will be going to add at the base !

And the ground of vibrator and transistor will be connected together or not !

What is the voltage on the inner ring of the pads?

What is the voltage on the outer ring of the pads?


Does the inner ring connect to the 3.7v of the battery

Does the outer ring connect to the 0v of the battery

Since the transistor is an emitter-follower with a very low base signal voltage then it does not need a series base resistor.
The transistor probably will not answer the call because its output voltage is much too small.
Of course the grounds (0V) of the circuits must be connected together. The transistor does not have a ground.