# How to calculate base resistor value of a transistor Answered

Hello all. I'm trying to make autologht OFF and ON . Compnent: 1. Relay : 12 DC volt 2.tramsistor: BC547 3. LDR (I don't know specificatiom) 4. Source supply volt: 12 DC volt. 5. Resistor (need to know value) So how do I calculate of resistor? I have attache my circuit design n type of relay I'm using Thank you

## Comments

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I think I would try for a base current that is about (1/10) = 0.1 the magnitude of the saturated collector current.

For example if the current needed to turn on the relay coil is 100 mA, then I would try to make the base current to the transistor be about 10 mA.

I am guessing that will correspond to a value of approximately 1000 ohm = 1 Kohm.

I mean, in the event of complete darkness, I am expecting the resistance of the LDR to be 100 Kohm or 1000 Kohm, basically, so huge it looks like an open circuit (infinite reistance) compared to R.

Then I assume the base-to-emitter path looks like a forward biased diode (Vforward = 0.6 V), so that the current flowing through R, and into the base of the transistor, is

Ibase = (12.0V - 0.6V)/(1 Kohm) = 11.4 mA

Which is approximately the size of Ibase that I wanted.

In the event of light reaching the LDR, then R and LDR together will resemble a voltage divider, and the amount of base current, Ibase, will be much smaller. Will it be small enough to strangle the collector current, and turn the relay off?

I dunno. If the relay seems to never turn off, even with bright light on the LDR, then I suggest you try making R larger, e.g. 10 K, or 100K?

I mean, I cannot tell you for certain what is the optimal size for R, but I am guessing it will be somewhere between 1K and 100 K.

If you want to want to know more about your BC547, sites like alldatasheet,
http://www.alldatasheet.com/view.jsp?Searchword=BC...
have the data sheet for this transistor.

If you want to know more about how BJT NPN transistors work, the Wikipedia page for BJT (Bipolar Junction Transistor)
https://en.wikipedia.org/wiki/Bipolar_junction_tra...
can maybe give you some hints about that.

By the way, I am naively guessing the way this circuit is supposed to work (if it works at all) is the transistor is saturated when the relay is on, and in active mode (with a small current too weak to turn on the relay) during those conditions when the relay is off.