Hi

I built a pulse generator with some components for the local radio shack, 555 on a breadboard and ...
I tested it and it puts out a 50% duty cycle square wave. With a variable resistor I am able to control the frequency.
I want to build one where I control the duty cycle as well as the frequency.

how do I do this? any diagrams?

Thanks

 

There are other circuits to do this, but you might find this one interesting - http://www.allaboutcircuits.com/vol_6/chpt_6/9.html

 

Thanks.

the breadboard shows one op-amp but the schematic shows 2.
I am thinking to use a resistor instead of 3 diodes to drop the voltage. actually I have a 4.5v dc supply and may not need much resistance. and instead of the 12v light, I have a LED.

I will be looking for another schematic to replace the op-amp with comparator(s) since this is my first go and I am learning.

 

Thanks.

the breadboard shows one op-amp but the schematic shows 2.

The 1458 is a dual 741 opamp. It has two complete operational amplifiers in a single package. You could use an LM324 instead; it is a quad opamp that Radio Shack carries.

I am thinking to use a resistor instead of 3 diodes to drop the voltage.

That won't work. The output voltage of a 741-type opamp won't go low enough to turn the transistor off. That's why they used three diodes; each will have around 0.7v drop across themselves until the current level gets very low.

actually I have a 4.5v dc supply and may not need much resistance. and instead of the 12v light, I have a LED.

You will need to use a resistor in series with the LED to limit current.
Rlimit = (Vsource - Vled) / DesiredCurrent

I will be looking for another schematic to replace the op-amp with comparator(s) since this is my first go and I am learning.

Why not just build it as-is for the moment?

 

Greeting samjesse,

I think that sgtwookie's suggestion that you build the circuit as shown is best. I sense that you are just beginning to get acquainted with electronics. There are enough variables in any circuit that you are better off avoiding the introduction of any new ones. Once you have successfully built the circuit as it is shown, you can then indulge your curiosity and investigate the effect of the changes you have suggested.

hgmjr

 

OK, I will have a go at it.
but which is the control for the frequency and which is for the duty cycle? I see only one potentiometer.

 

That circuit has a fixed frequency and can only vary the duty cycle.

 

My original post said:
"I want to build one where I control the duty cycle as well as the frequency."

I was going to buy more components today to finish this project, but how can I get it to vary the frequency as well as the duty cycle?

thx

 

Hello,

I have made some changes to the eBook schematic.

The 1M pot will change the frequency.
The 10K pot will change the duty cycle.

Greetings,
Bertus

 

Thank you.

 

Will the updated schematic make the frequency and duty cycle adjustment independent?

 

Hello,

Yes they are independend.
The 1M pot will change the frequency only.
The 10K pot will change the duty cycle only.

Greetings,
Bertus

 

I built the circuit as indicated but am getting a bad looking signal. here is it. if you need me to post the signal I will.

http://3marulanway.com.au/images/circuit1.jpg
thx

 

Have you read these, they both have examples for what you're wanting to do...

LEDs, 555s, Flashers, and Light Chasers

The 555 Projects