RE: Basic Transistor Switch Circuit- Expert Advice Required

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1

Hi

I’m new to electronics and would be grateful for any expert advice on the following situation

I’ve built a basic circuit to try show how a transistor works as a switch. Basically, I have an NPN transistor hooked up to an LED, 6v power supply and 10k potentiometer.

When the pot is turned up enough the LED glows brightly- until it is turned up to that point, it glows not very much.

My question is: does this circuit demonstrate the transistor acting as a switch, or no? If no, is there some way with minor changes I could modify it (e.g. using a fixed resistor instead of the pot)?

Thank you for your time

Tarek

2

An LED almost always requires a series current limiting resistor. A 1000 Ohm resistor is usually fine. Post a circuit diagram of how you wired everything and state the transistor type.

Hint: google “transistor switch circuit” for many introductory pages.

3

Thanks JRemington

Sorry all I have is this frizting. The pot is 10k and the NPN transistor is a B547

4

You didn’t try google, and need to do some more reading and reseach. The circuit shown will ruin both the transistor and the LED.

Avoid fritzing diagrams.

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AllAboutCircuits.com has some tutorials - read them or find some others (as jremington suggested - USE GOOGLE)

http://www.allaboutcircuits.com/textboo … witch-bjt/

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Thanks for letting me know

Is there too much voltage/current? Should I expect the LED to blow out and the transistor to fry?

7

Why don’t you search and look at other schematics to answer your own question? That is how you will learn.

8

Thank you for your advice

I tried and tested the following circuit and it seemed to work

Here is the circuit diagram and (sorry) the fritzing to show how I assembled it on the breadboard

Does this demonstrate the switch function of the transistor now? And is it safe?

9

Here is the Fritzing

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Does this demonstrate the switch function of the transistor now?

Tell us what you observe about the function, and what you think those observations mean.

11

There is a clear on-off effect when the pot is turned up to a certain point- not a range of brightness. I think that is the transistor working as a switch. Am I right?

Also, with the previous circuit: would the components be damaged? As I shared this circuit (wrongly) and need to make ppl aware

12

There is a clear on-off effect when the pot is turned up to a certain point- not a range of brightness. I think that is the transistor working as a switch. Am I right?

Correct. The transistor needs a certain voltage/current to be fully switched on. The point of a transistor is to switch a high voltage/current device from a low voltage/current device. An example would be trying to get an Arduino to turn on a motor or a high wattage LED. The Arduino cannot supply enough voltage or current and if you tried it, it would smoke the Arduino. So we use a transistor to carry the current and keep it away from the Arduino.

Also, with the previous circuit: would the components be damaged?

You already know the answer, so why ask it again? If you wanted to keep the wrong schematic away from others, why not delete it?

13

Thanks man

14

Just one more question

This circuit needs a resistor to protect the LED, right?

The capacitor is 1000uF and more than 9v. The first switch is held down to charge up the capacitor, and the second to flash the LED (using the stored charge)

Again, sorry for the fritzing

15

You need to provide a current limiting resistor nearly always (I’m sure there are cases where it’s not required although I really can’t think of one). Plan on it and design it in!

16

This one is ok, right?

17

I don’t know if your circuit will do as you say.

The resistor is now limiting current for capacitor charging, but not current limiting through the diode from the cap.

Unless you are pressing both switches at the same time in which case the cap is not really doing anything.

Both switches are N.O.?

18

The switches are the ones (excuse non-technical language) that have to be held down to be ‘on’

I tested it a few times and it worked