how to calculate the resistor value

[simonwai999]

hi
i would like to know how to calculate the resistor value for the base of the transistor
is there a formula?
thanks
as i know i have to limit current to go into the base
not too much enough to make the transistor saturate
in the circuit
it uses a 47k ohms for bc547b
how to calculator this?
thanks

 

[jony130]

Well, the calculation is quite simple.
We suppose the LED current is 10mA.
The minimum base current is equal
Ibmin=Icmax/hFEmin for BC547B
HFEmin=200
Ibmin=50uA
For ensure that BJT is in saturation we multiply Ibmin* 2..10,
IB=5*50uA=250uA
And now the base current
Ib=(Voh-Ube)/Rb
Voh=(2/3)*Vcc=6V
Rb=(6V-0.7V)/250uA=21K=22K
Or we can uses another approach.
Bjt will be in saturation if:
Rb≤hFEmin*Rc
So now we can calculate the RBmax
Rbmax=200*680Ω=130K and again for ensure saturation we uses 2..5 times smaller value. Rb=22K...68K

PS. In this kind of circuit you should uses the Schmitt trigger gate, because the switch "bounce" and RC low-pass filter to smooths the bounce waveform. The time constant 10ms to 25ms is long enough

R2 and C1 is RC low-pass filter that smooths the bounce of reed switch

 

[FvM]

Are you able to perceive the contact bounce in LED output? If the LED light is to be received by a photo detector (or insect eyes), you're right.