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Electronics => Beginners => Topic started by: TinkerFan on February 25, 2016, 09:41:10 pm

Title: LDR Transistor Switch Bad Circuit? But why?
Post by: TinkerFan on February 25, 2016, 09:41:10 pm

G'day guys,
I just came across this schematic in a physics lesson (we didn't test it) and I was wondering, because the circuit seems to be wrong (or impractical). I built it up with LTSpice and a bit of a playaround with it. The load was not specified (probably a light bulb  |O; LED's seem to be unknown in physics lessons)

Original schematic: https://kitronik.co.uk/blog/how-an-ldr-light-dependent-resistor-works/ (https://kitronik.co.uk/blog/how-an-ldr-light-dependent-resistor-works/)

(http://Unbennant.png)

The upper base resistor is a fixed value, while the lower one is the LDR (it was said before, that its value is between 5k to 20M). The load is between collector and supply rail (So a normal collector circuit; transistor as a switch, right?).
I then tried to choose the fixed resostor(31k), so that the transistor is just switched off, when the LDR is at its lowest value. But the Emitter resistance is much to low, so that the LDR can have any effect on the bias for the base. And indeed, something is not really working here: The voltage is just above Vbe, so the transistor is turned on and saturated, but it doesen't depend on the value of the LDR. But if the LDR has 5k (its lowest value), the transistor will get into its active region, having the base at 3.6V (what is happening here?).
How could one improve this circuit? Put a large resistor at the base?
Could someone please explain how this (is ment to) works and how far I am right?

I'm sorry, if this is a really stupid question, but I am kinda stuck here and my English could also be better...

Thank You
TinkerFan

PS: The output at the collector is shown in the attachment (green is at 5k and 1meg is grey)

Title: Re: LDR Transistor Switch Bad Circuit? But why?
Post by: Seekonk on February 25, 2016, 10:08:16 pm

These overly simplistic circuits seldom work very well.  However,if you replace the transistor with a LM431 it would.  It has a sharp transition at 2.5V and can easily drive a 50ma load like a small relay or LED.  Understand that the saturation voltage will be around 2V.

Title: Re: LDR Transistor Switch Bad Circuit? But why?
Post by: ataradov on February 25, 2016, 10:17:30 pm

Why is there your profile form in the first picture?

Title: Re: LDR Transistor Switch Bad Circuit? But why?
Post by: TinkerFan on February 25, 2016, 11:23:13 pm

Thank you for that info, I didn't check it... |O

So this will never work due to the low impedance od the emitter? The opamp idea is overcoming this problem, right?