Bill, I don't know how to thank you enough!!!!! I guess I'm going to have to find a text book on Algebra. Again thank you for all the help.

One more question is a log potentiometer the type to use on this?

Thanks again, cary

 

Don't know. I don't know enough about your application. I lean toward linear though. If you were to draw hash marks on the case where the pot is mounted it would look a lot prettier maybe, but frequency is occasionally better displayed in log fashion.

BTW, on the algebra, you owe me 3 sheets of small notebook paper. I had some refreshing to do to figure it out.

 

Bill, the notebook paper is in the mail I'm going to use this to pulse a IR2110 mosfet driver for a home made EDM (electrical discharge machine) For my home machine shop. The original plans used a RC oscillator circuit with out mosfet, but the more modern machine are pulse mode.

Ran one of these for 13 years at work and decided to try and build one for my home shop. Have had the mechanical parts done for a long time but now that I'm retired I've got the time to devote to seeing the electronics through. Between you and Sgt.Wookie, I am probably going to finally get it done! Again I appreciate all the help!

cary

 

Keep the smoke in.

 

Keep the smoke in.

That's the biggest reason this project isn't done yet. I was a tool&die maker and knew what I was doing and the way or ways to do things.
Never made too many bad parts in all my years on the job. This electronics stuff is way harder and I want things to work, with out the smoke!

But I'm learning that you have to break some eggs to make an omelet.

cary

 

Hi Bill, another "dumb" question for you. On the PWM circuit it shows pin 3 giving the square wave out, But pin 7 isn't used in the schematic. Doesn't pin 7 have to be used to get the 1/3 - 2/3 voltage to make pin 3 work ?

Sorry to be such a pest, cary

 

This goes back to the basics of a 555. Pin 5 is a reference for the 1/3 and 2/3 resistor voltage divider that makes the 555 immune to power supply voltage. It is also a great place to quieten the internals of the chip from noise, which is why a cap is put there many cases. Depending on the application you can leave the cap off, but it can never hurt to have it on there.

Because it is a reference point if you vary the voltage of this point with an external source the character of the circuit will change. It can be used as a VCO input for example. In your case this would be a bad idea, but it does have it's uses.

Pin 7 is a simple switch to ground that isn't used on this circuit. It is meant to discharge a capacitor for many 555 circuits, and I have seen it used as an output pin. Don't recommend it though.

Couple of references for refresher courses.

555 Schmitt Trigger

555 Hysteretic Oscillator

When it comes to hysteretic oscillators the 555 is a chip of convenience, there are many other chips that would work as well. A CD40106 is a hex inverting Schmitt Trigger, a digital gate (6 of them). Each of the 6 Schmitt Triggers would work in this design, but the drive is much less.

 

I like the 4093 quad Schmitt-trigger NAND gate, arguably one of the most useful 4000-series CMOS IC's. You can build any other logic gate using NAND gates. You can do the same thing with NOR gates, but there isn't a Schmitt-trigger 4000-series NOR gate that I know of.

You can tie the two inputs to a 4093 gate together to make a Schmitt-trigger inverter, or use one of the inputs to gate the output, or a wide variety of functions. It's just a very handy device to have around.

The bjt (transistorized) 555 timer can certainly source/sink far more current than 4000-series CMOS can, but the output doesn't go higher than Vcc-1.3v, even under light loading due to the Darlington output configuration.

When using a resistor from pin 3 to pins 6/7 and a cap, it's not a bad idea to use a 24k resistor from pin 5 to ground. That compensates for the output (pin 3) not going all the way to Vcc, as it shifts the threshold and trigger points to a more suitable voltage range. This also slightly changes the RC formula, as the cap doesn't have to charge/discharge as far.