BC547 transistor

[darren20]

I am doing a flip flop circuit project i found online i have left the link below , It uses 2 LEDS and 4 resistors and 2 BC547 transistor , the two 330 ohms are for the LEDS and the other two resistors are for the 2 BC547 , The resistors for the bc547 are 10k for each , Can anyone explain how i calculate the resister value using the bc547 specs .
Flip Flop LED Flasher Using BC547 Transistors | Lindevs

 

[bertus]

Hello,

Have a look at the following page for more info on the transistor astable multivibrator:
https://www.electronics-tutorials.ws/waveforms/astable.html

Bertus

 

[Alec_t]

Can anyone explain how i calculate the resister value using the bc547 specs

A red LED drops about 2V, so for the linked circuit the collector current will be a ~(5V-2V)/330Ω = 0.01A = 10mA.
For circuit design purposes a bipolar transistor used as a switch is assumed (by a rule of thumb) to have a current gain in the range 10-20. Provided the transistor used has an actual gain considerably greater than that it should make a good switch.
That rule of thumb implies a base current of 5mA-10mA here. The base current is actually about (5V-0.7V)/10kΩ = 4.3mA, which is in the right ball park.
The time constant RC defined by the base resistor and the connected capacitor is what sets the time for which one LED is 'off' while the other is 'on'.

edit for Atec_t : Base current implied is indeed ~ 0.5mA-1mA, and actual is ~0.43mA.

 

[Audioguru]

The decimal point is in the wrong place. (5V - 0.7V)/10k= 0.43mA.

 

[AnalogKid]

A red LED drops about 2V, so for the linked circuit the collector current will be a ~(5V-2V)/330Ω = 0.01A = 10mA.
For circuit design purposes a bipolar transistor used as a switch is assumed (by a rule of thumb) to have a current gain in the range 10-20. Provided the transistor used has an actual gain considerably greater than that it should make a good switch.
That rule of thumb implies a base current of 5mA-10mA here.

More like 0.5 mA to 1 mA.

ak

 

[Alec_t]

Oops. Can't edit my earlier post.
Base current implied is indeed ~ 0.5mA-1mA, and actual is ~0.43mA.

 

[bertus]

Hello,

@Alec_t , I have added the line.

Bertus

 

[Alec_t]

Thanks Bertus.