how does this circuit work

[hammer6]

how does a circuit work

hi i have just started basic electronics could someone please explain how each section of this circuit works. all i understand is as the light the ldr recieves changes the led wil turn on or off. but dont undertand for example y other resistors are necesary...

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[Audioguru]

bc549 base resistor

1) The 100k potentiometer and the LDR form a voltage and current divider. The potentiometer is adjusted so that the LED turns on and off with a suitable amount of light on the LDR.

2) The transistor is a switch. Base current through the 2.2k resistor causes a much higher collector current. The 2.2k resistor limits the base current if the pot is turned to zero ohms.

3) The 270 ohm resistor limits the current in the LED.

 

[hammer6]

how does a light circut work?

thanks but dont understand about the base current through the 2.2k resitor causes higher collector current

 

[barrybear]

does trnasistor work like a resistor

This circuit is actually a building block from a school modual pack called alphs kit .
You need to do some basic transistor theary to now that a small change in base current will cause a large change in collector current.
The resistors are there to limit the amount of current , and to control the conduction

 

[Reuel2]

12 volts slow glowing led circuits

The 2.2K resistor prevents high collector current. A transistor can make an interesting pop and cloud of smoke and glow if the current through the base is not limited.

 

[Audioguru]

how does a light circuit work

The 2.2K resistor prevents high collector current.

No.
The 270 ohm resistor in series with the collector and LED prevents high collector current.
The 2.2k resistor in series with the base prevents high base current.

 

[umery2k75]

how does a basic circuit work

@A transistor can make an interesting pop and cloud of smoke and glow if the current through the base is not limited.

Even 20mA sourcing MCU can destroy it. I remember once I forget to put the resistors for the base transistors on my PCB. I had several of them, they were directly sourced by MCU. They didn't die instantly, but it was more like a slow posion, typically they got dead around in 5-10 mins and I remember one transistor plastic case was damaged and you could feel the plastic of TO-92 package being rashed up for as low as 20mA directly on base! then I realized I missed those base resistances.

 

[Audioguru]

how to work circuits with potentiometers

20mA is not too high for the base current of a transistor. The power dissipated is only about 0.016W which is almost nothing.
The max allowed peak base current in a little BC549 transistor is 200mA.

But a micro-controller would source about 50mA to the base of a transistor which is too high for the tiny output transistor in the micro-controller which would try to dissipate 0.21W and it would fry.

There is a high power mood light project where the designer forgot to use current-limiting resistors between the micro-controller and the bases of the driver transistors. The micro-controller got so hot that its stopped working.

 

[Reuel2]

how does a ciruit work

My description above happened when I was using a 12 volt source with something like 90 available amps. I accidentally shorted my limiting resistor, and pushed 12 volts through the base. The transistor glowed for several seconds until I managed to disconnect the power.