# Why is a transistor used in a night sensor circuit?

The following circuit is often used in introductory (high school level) physics courses to introduce transistors or to give an example of why transistors are useful:

Is it correct that one could achieve the same effect using just an LDR parallel to a resistor (and another resistor in series)? If so, what is the advantage of using the transistor?

• Just realized that my second option is just a simple voltage divider with bulb as load and LDR as second resistor. I didn't find a picture of this circuit in the web, however I have not enough rep to upload my own... – Ben Feb 28 '12 at 21:26
• Wow, they teach transistors at high-school! – abdullah kahraman Feb 29 '12 at 8:28
• @abdullah: They didn't in my high school. I had to get a book from the library to learn about transistors. The funny thing is that even in college there wasn't much about how to use real parts in real circuits. It was all high-falutin theoretical stuff. Don't get me wrong, that's all good information and is necessary to understand to create good designs. But, I was surprise how little time was actually spent on learning to use all that theory in practical designs. I learned a lot of that on the side by building things on my own, but much was learned on the job. – Olin Lathrop Feb 29 '12 at 13:13
• @OlinLathrop same here in Turkey. They teach how to calculate resistors and capacitors in parallel, but don't show what resistors and capacitors are for. And in the college, damn I learnt nothing from the classes! All I've learnt was myself and my dad who is an electronics engineer. Now I am buying a snap-circuits kit for a brother of a friend who is 10 years old. He is enthusiastic! – abdullah kahraman Mar 1 '12 at 10:07

The transistor provides gain. Basically it's a simple amplifier of the signal produced by the LDR and the pullup resistor. That signal has rather high impedance, so is generally not useful for driving something else directly, like a LED for example. Cheap transistors can easily be found that can be counted on to have a current gain of at least 50 in this case. If you want 5 mA thru a LED as a indicator, for example, then only 100 µA are required into the base of the transistor.

By the way, the 1 kΩ resistor in this schematic is silly and serves no useful purpose.

• Divider controls base current. May possible with LDR open circuit is set by the 10k to (Vbat-Vbe)/10k ~= (6-0.7)/10k =~~ 0.5 mA. // In other cases a resistor where the 1K is may be useful. eg with a largish capacitor across the LDR it would alter time delays. – Russell McMahon Feb 28 '12 at 21:15
• @Olin Lathrop - Omitting " ... is silly and ..." provides same meaning and MAY win more hearts and souls. Or not. I considered editing but concluded that you may consider my doing so to be silly :-). – Russell McMahon Feb 28 '12 at 21:17
• @dextorb: Normally a resistor is put in series with the base to limit current to a safe level, but in this case it's silly since the current is already limited by the 10 kOhm resistor. – Olin Lathrop Feb 28 '12 at 22:51
• @Russell: When something is bad design or silly, it's better to say so than to pussy-foot around. If whoever designed that circuit is offended, so be it. If they don't like that, they need to design a better circuit next time. – Olin Lathrop Feb 28 '12 at 22:53
• @Olin Lathrop - Designer offense hadn't occurred to me :-). For some readers "is silly" will mask "serves no useful purpose". People may not mind the very mild perjorative but the retained lesson may be that "Olin said it was silly" rather than "Olin said it was unnecessary". I'm not trying to be PC or 'touchy feely' - just trying to address what I perceive average brain most likely to retain and recover. If silliness is what you want to convey, lay on. I'd think that prompting "Why did he say that is unnecessary" is worth doing, given the choice. (You may be able to manage both :-) ). – Russell McMahon Feb 29 '12 at 1:14

The resistor divider R + LDR provides your signal: a voltage depending on the light level. So you need that. The 10k$\Omega$ means that the current can't be higher than $\dfrac{6V}{10k\Omega}$ = 600$\mu$A, which is probably a factor 100 too small to light the lamp. So you need a device which increases this current.
That's where the transistor comes in. A transistor will create a large current from collector (top) to emitter (bottom) if a smaller current flows from base (left) to emitter. How much current depends on the type of transistor. Current gains higher than 100 are not uncommon, and a Darlington transistor may even amplify 1000 times.
Like Olin says the 1k$\Omega$ resistor is not necessary here. It's often there, and then it's used to limit the base current, so that the transistor doesn't get damaged. In our case the 10k$\Omega$ already does that, so it's justified to call its use silly.

The reason why transistor it available in this circuit..because the transistor act as a current amplifier and switch,other wise this circuit won't functioning.regarding the silly transistor it can be replaced with the lowest value for example...1-10 ohm resistor try no harm.this circuit can be added with relay/SCR/solid state relay to control the high current equipment such as sport light etc.