How to use transistor as switch..

[yusuf]

Hello friends ,

I have transistor BC547.

I have two circuits. Circuit 1 uses 9v and Circuit 2 uses 3.7v.

I want to connect the transistor to circuit 1 ... to act as switch to switch circuit 2.

So as the circuit 1 triggers it will turn on the transistor to supply power supply to circuit 2.

circuit 1 will turn on/off the transistor.
circuit 2 will turn on/off as the transistor conducts.

So how do I base the transistor in saturation mode to act as switch ...
Which values resistor I will have to add in base of transistor...

 

[Pommie]

It's too late for me to even try to work this out but a general rule is a 1k resistor in the base and the load between collector and positive supply. If there is any smoke then a BC547 is probably a PNP rather than an NPN. I'm sure other posters will be more helpful.

Mike.

 

[alec_t]

How much current does circuit 2 draw (if it's very little then Pommie's suggested 1k could be increased to save power, if that's a consideration) ?

 

[yusuf]

How much current does circuit 2 draw (if it's very little then Pommie's suggested 1k could be increased to save power, if that's a consideration) ?

Actually , the second circuit draw few ma current to operate..
The second circuit is the mobile keypad..
I want to connect the transistor to short the "CALL BUTTON" to accept incoming call..

So again , As I trigger circuit one it will short the CALL BUTTON to accept incoming call..

 

[audioguru]

A transistor is not "triggered". Instead it is turned on and is turned off.
A BC547 is NPN so positive base current turns it on.
If the source turn on voltage goes to about +4.4V or more then it can feed the base of the transistor (maybe through a series resistor) and the emitter of the transistor will go to almost +3.7V when the transistor is turned on. The collector of the transistor can connect to +3.7V.

 

[mycircuits9]

Transistor switch working

Go Here to know how <a href="http://mycircuits9.blogspot.com/2012/07/transistor-as-switch-working.html">Transistors act as switch</a>

Hope it Helps You.....

best of luck

 

[yusuf]

Thanks for your replies guys.. but I am not getting any proper way to uses transistor BC547 as switch(saturation) properly..

It is not working...

So can you guys tell me what resistor values should I give to turn on/off the transistor (mobile key pad).

 

[davenn]

Thanks for your replies guys.. but I am not getting any proper way to uses transistor BC547 as switch(saturation) properly..

It is not working...

So can you guys tell me what resistor values should I give to turn on/off the transistor (mobile key pad).

you need to tell us a bit more about your circuit 1, the one running off the 9V, so we can get a correct idea of how to connect that to the BC547 transistor
do you have a circuit diagram of your circuit 1 ?

Dave

 

[yusuf]

Herewith I have attached a circuit which will be used to run mobile key via the BC547 transistor..

circuit 1 : 555 IC , 1sec delay monostable circuit.
circuit 2 : mobile key pad.

circuit 1 will be triggered and it will run associated key on mobile(circuit 2) via BC547 transistor..

All key pad on mobile run when their positive and negative connection are shorted with each other.

The work of the transistor will be to just short both pins(+ , -) of mobile key pad.

It requires a short duration pulse to run key.. so I have used only 1sec delay monostable circuit.

Please have a look on my attachment ....
Zoom for better image..

 

[davenn]

hi yusuf

excellent !!

OK, as pommie suggested in post #2 in this thread, you could use a 1k resistor between the output pin (pin3) of the timer and the base of the transistor
it is there to limit current flow through the base of the transistor
You may also need a small value resistor in the collector lead of the transistor as well, that is really dependant on how much current could be supplied by that 3.7V

Your other choice of method would be to use a very small relay to short the keypad connections

cheers
Dave

 

[yusuf]

hi yusuf

excellent !!

OK, as pommie suggested in post #2 in this thread, you could use a 1k resistor between the output pin (pin3) of the timer and the base of the transistor
it is there to limit current flow through the base of the transistor
You may also need a small value resistor in the collector lead of the transistor as well, that is really dependant on how much current could be supplied by that 3.7V

Your other choice of method would be to use a very small relay to short the keypad connections

cheers
Dave

Thanks for your reply..
Which one will be better !

Transistor or a relay...

And how should I calculate the value of the resistor which I will have to add on the collector lid.
any formula exists...

It is necessary to limit the current or we can directly use the transistor as switch. Because current factor is not very important for me.. I just want to operate as a switch..

 

[alec_t]

Because current factor is not very important for me

But it is for the transistor! Too much current and the magic smoke escapes .

 

[yusuf]

But it is for the transistor! Too much current and the magic smoke escapes .

Alec the current consumption is very less , 20 to 30 ma..

So , please provide the values of resistor which will be used in my circuit..

 

[colin55]

Try this circuit:
View attachment 65920

 

[alec_t]

A 1k to 2.7k base resistor will be fine.

 

[yusuf]

Try this circuit:
View attachment 65920

where do I connect the output pin 3 of 555 monostable circuit... to the above circuit to turn on/off transistor.

 

[colin55]

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.

 

[davenn]

where do I connect the output pin 3 of 555 monostable circuit... to the above circuit to turn on/off transistor.

I dont think Colin55 read that part of the thread

just put the 1k to 2k2 resistor in the base line of the transistor from the output of the 555, as alec, I and others have suggested

cheers
Dave

 

[aljamri]

Hi,

Your two circuits has no common ground in between, so, I think the transistor will not properly function. and of course do not forget to connect collector resistor suggested above, it is to limit the current when your transistor (switch) turned ON which will almost connect your 3.7 positive to negative.

 

[davenn]

Hi,

.......and of course do not forget to connect collector resistor suggested above, it is to limit the current when your transistor (switch) turned ON which will almost connect your 3.7 positive to negative.

yup exactly


D