ok I made a mistake, this circuit isn't working after all.....

I have a pulse that somewhat resembles a square wave coming from pin 1 of the op amp, I feed it into the 2N3904 acting as a buffer
so I can drive an LED from the collector for the rest of my circuit (not shown).

Although I have this pulse on the op amp side of the resistor, I have absolutely nothing on the collector?.....shouldn't I see
the pulse on the collector as well?

my ground position is the middle horizontal line on the scope so my pulse is actually starting at around 1.7 volts at it's lowest point
and rises to around 8.4v at it's highest point, the scope vertical says 500mv per division but it is actually 5v per division, I have only had the scope 2 weeks and I need to learn how to use the thing and set it up so that it reads properly.

any help would be welcome as I am close to giving up and growing vegetables instead

Neil.

 

The output from the opamp never goes low enough to allow the transistor to switch off. Make the 1k resistor 10k and add a 2k2 resistor between the base and emitter.

 

I'll get on that in a couple of hours.....

Thank you.

Neil.

 

......so my pulse is actually starting at around 1.7 volts at it's lowest point

You need to better understand transistors.
How do expect 1.7V to turn the transistor off?
That voltage will keep the transistor on, as the base-emitter voltage of a BJT is about 0.7V when on.
You need get the base voltage below 0.6V to turn it off.
Try putting a 5kΩ in series with the base, in place of the 1kΩ, and add a 2kΩ resistor from the base to ground.

Here's an Ltspice simulation of that:
Notice that the OFF base voltage, V(b) is now below 0.5V.

Incidentally, that's not an emitter follower circuit, it's a common-emitter.

 

Yes, my current knowledge of transistors is very limited, after I have finished my current project I intend to teach myself analogue electronics, until then I need help.

Thank you.

Neil.

 

Yes, my current knowledge of transistors is very limited, after I have finished my current project I intend to teach myself analogue electronics, until then I need help.

Okay, I can understand that.
But in the meantime you shouldn't be surprised that circuits you throw together don't work as you think they should.

 

If you tie the collector directly to the 9 V, and put the LED and its current limiting resistor between the emitter and GND, that will be an actual emitter follower and will do what you probably want.

Or leave the resistor in the collector leg, and put just the LED between the emitter and GND. Either way, the LED will flash when the opamp output is high.

ak

 

Okay, I can understand that.
But in the meantime you shouldn't be surprised that circuits you throw together don't work as you think they should.

Can you point me in the direction of a webpage or something I can read transistor theory with some experiments?

 

Can you point me in the direction of a webpage or something I can read transistor theory with some experiments?

For starters, go to the top of this page and click on Textbooks.
There's a lot of material to cover but I don't know of any shortcuts to learning electronics.
Any questions, feel free to post here and we'll do our best to answer them.
Good luck.

 

ok so I replaced the 1k with a 10k and fitted a 2k2 from the base of the transistor to the emitter
and nearly peed my pants with excitement when I turned on the power and my LED sprang to life slowely winking at me.

next step is to finish building the tremolo unit for my guitar of which this circuit is a part then I am going to
get some transistor theory under my belt.....thoroughly fascinated now!

Thankyou so much for all your help guys, I am delighted

Neil.

 

I'd like to see your latest schematic. Something in your description is not quite right.

ak

 

My current schematic is the same as in the original post but with the changes in post #10

 

I'd like to see your latest schematic. Something in your description is not quite right.

It's the circuit in post #4 with R2=10kΩ and R3=2.2kΩ.

 

And as I said, something about the description of the changes is not right.

ak

 

And as I said, something about the description of the changes is not right.

ak

What do you think is not right?

 

And as I said, something about the description of the changes is not right.

So, you just want to keep us guessing?