Correct way to turn on and off a sine wave

Thread Starter

Horacio Dos

Joined Dec 28, 2016
14
Hello.

I'm building a Capacitive Touch Morse Keyer. The main parts are allready done (Arduino and MPR121 integration and the Wien bridge oscillator)

For the Wien bridge oscillator I used a LM741 and a ICL7660 to get a negative voltage and after some calculations and a voltage divider I managed to get a quite beatifull 860Hz sine wave above 0V.

Now I want to turn the wave on and off in order to get the dots and dashes with a pwn without modifing the wave. I always get the signal from the MOSFET Source (BS170) when It is supposted that there is no voltage going out if there is no pwn.

I'm very new to electronics and transistors (NPN, PNP and MOSFET) still are a headache to me. I don't know if these are the correct components to do that. I allready tried to turn on and off the power suppy but it seems that the BJT affects the wave, also tried to use an optocoupler with a very strange result that I can't even translate into words. Also I flipped the transistors to make it work (of course, it didn't) and I'm trapped in a trial and error loop.

Wienv2_schem.png
 

Sinus23

Joined Sep 7, 2013
248
I've never done so myself but how about using the switch at the output not the power rail? Might give you a less of a headache?

Just an idea...
 

Thread Starter

Horacio Dos

Joined Dec 28, 2016
14
Hello.
Please ignore J1, J2 and J3. These are reminders for connections in the breadboard. J3 does nothing, I took signal to measure from there, thats all.
Thanks
 

Sinus23

Joined Sep 7, 2013
248
Hello.
Yes, that's what I'm trying to do. Switching on and off the op-amp output.
Thanks!
.
Is J4 the connection to the amp/headphone/line out ?

Tap the line(circuitry) into the speaker/headphones with the switch(put the switch before the receiving outlet) and see if you can get a clean on/off signal(if not we might need to get some more experienced people up in here) :oops:
 

Thread Starter

Horacio Dos

Joined Dec 28, 2016
14
Yes, J4 is the output wave to be transmitted. I'm sure I will have to transform it a little bit more to match the radio input specs.
Thanks!
 

Sinus23

Joined Sep 7, 2013
248
Good to hear. However how clear is the signal from the Morse code circuit? because from your original post it wasn't too good. And the problem is probably what Bertus stated since he has expertise in this field of electronics far beyond me.
 

Thread Starter

Horacio Dos

Joined Dec 28, 2016
14
Good to hear. However how clear is the signal from the Morse code circuit? because from your original post it wasn't too good. And the problem is probably what Bertus stated since he has expertise in this field of electronics far beyond me.
Hi.
The signal is good. The problem is that it is always on.
 

Attachments

bertus

Joined Apr 5, 2008
22,120
Hello,

As I said, you do not have a collector resistor on Q1.
Q1 never can switch the signal the way you made it.

Bertus
 

Sinus23

Joined Sep 7, 2013
248
Hi.
The signal is good. The problem is that it is always on.
Then switch it off after your Opamp output. Sorry if I wasn't very clear. I've never built a Morse code machine but If I did my first instinct would be to go on/off after the amplification.

Any criticisim welcomed.
 

Thread Starter

Horacio Dos

Joined Dec 28, 2016
14
Hello,

Q2 will never be switched as Q1 has no collector resistor.
Q1 is in danger to be blown.

Have a look here how it is done here:
http://www.solorb.com/elect/sidetone/index.html

Bertus
Hi
I have added a 10K resistor between 5V and Q1 collector and nothing happened.
As far as I can understand in the link, the op-amp output feeds two NPN gates and there is a third involved. I seems too complicated.
 

Thread Starter

Horacio Dos

Joined Dec 28, 2016
14
I replaced Q2 and now I see a change. The circuit seems to work now but in an inverted mode. There is always a signal in Q2 Source until there is a PWM in Q1 base, then the wave is off. Also found that the crest and the trough are cut. Image attached
 

Attachments

Sinus23

Joined Sep 7, 2013
248
"I would switch the power to the oscillator, rather than the output. A MOSFET between ground and the oscillator ground.

Bob"

This is the criticism I was asking for. Now why would you do that?
 
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