What is the difference between these two circuits - night lamp circuit with LDR

Using Arduino General Electronics
1

Hello Electronics experts,
How do these two circuits behave in terms of operation?
Trying to understand if the extra resistor to the transistor's base is really needed ? and does the LED on collector or emitter make any difference? I breadboarded both circuits, and they seem to work fine.

PS: I'm computer science engineer, fascinated by wonderful world of electronics

Circuit-1:
image

Circuit-2:
image

Thanks,
SA

2

Did you forget to post the schematics ?

3

Circuit 2 is the best circuit as the transistor switches ON/OFF better.

Circuit 1 has more of a chance of having the transistor being operated in its linear region.

The R6, 220 Ω is not really needed.

R2 of 200k is way to large.

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4

Thanks Larry.

Is circuit 2 better, because the LED is on the collector side?

Removing the base resistor R6 220, would make it similar to circuit 1 except that the LED is on collector side. Wouldn't it?

How do you figure out the collector/emitter side that the load should go?

The variable resistor R2 200k was added to adjust the brightness of the LED. What would be the recommended value here?

Thank you

5

The two circuits can't be directly compared, as the transistor is operated in different configurations - #1 is common collector (emitter follower) and #1 is common emitter. They operate fundamentally differently.

If you're an engineer, researching those configurations should answer most of your questions.

6

Is circuit 2 better, because the LED is on the collector side?
Yes, Goggle common emitter and collector
Removing the base resistor R6 220, would make it similar to circuit 1 except that the LED is on collector side. Wouldn't it?
220 Ω limits the base current, however, 220 is so small it’s not doing much in that respect
How do you figure out the collector/emitter side that the load should go?
For switching placing the load in the collector side takes advantage of a transistor saturating switch.
The variable resistor R2 200k was added to adjust the brightness of the LED. What would be the recommended value here?
A 5k potentiometer will give more easily seen changes, add a 220 Ω series resistor with the adjusting potentiometer.

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7

Appreciate it Larry. Great answers. They put me onto right path to explore further.
Please do share if there are any easy to understand references (for non-electronics engineers) that come to your mind

Thanks again
Have a wonderful day

8

Of course, in both circuits, the presence of light on the LDR turns OFF the LED, but ...

  • Circuit 1 has a wide range adjustable dimming control (R2) and adjustable sensitivity control (R1) for the LED. There's no LED current limit protection (R2 can be set to 0Ω).
  • Circuit 2 has a fixed 220Ω resistor to limit maximum current (LED brightness) and fixed 150K resistor for sensitivity.
9

You mean #2 if common-emitter.

The simplest way to think of the difference:
first circuit has no voltage gain, only current gain, the latter has both voltage and current gain. This means the second circuit will react more definitely to changes in light, the first can only dim slowly in response to light changes.

If you were doing this with an Arduino you'd use PWM for the LED, and measure the light using an analog input. The mapping from light to LED brightness would be completely arbitrary as you code it up - for instance hysteresis would be easy to add.

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10

Thanks Mark, great answer. Easy to understand for non-electronics engineers like me.

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