Adjusting frequency NE556

"Dave, I can't do that" wrote in news:a08a31d0-129e- snipped-for-privacy@z6g2000pre.googlegroups.com:

Try this;

R3 R4 and C2 control the rate of the PWM

as you want it to run about 12 times faster you need to decrease the capacitor (C2) or the resistors (r3,r4) by a factor of 12.

I'd replace C2 with an 8.2nF capacitor which should get you fairly close to 5Khz, (if a more precise 5khz is needed also replace r3 with a 27K with a 10K trimmer in series.)

C6 and r5 are also responsible for the on-time of the pulses and one of those should also be adjusted down by a factor of 12 (probably easiest to change the capacitor again)

there is a different, simpler, PWM cricuit (that works well especially if precise control or stability of the frequency is not needed) that uses just a single 555 (or half of a 555) and places the variable resitance with a pair of steering diodes inside the oscillator circuit.

Bye. Jasen

Thanks Jasen and Jerry, I will give them a try.

Dave

with

And then again, you can use a dual comparator/op-amp, one unit as the triangle wave generator while the other is used as the comparator. With the above, you can obtain 0..100% PWM at what ever the triangle wave is set for with out screwing with duty cycle factors.

with

And to add to my post since JF got so sentimental about it.

The above can be replaced with a comparator or op-amp that employs the totem pole outputs. Meaning, they source and sink on the output. This can eliminate the need for the high side R that is being used now since the LM339 id open collector on the outputs.

Personally, I would use a LM358 or something in that line which is what I normally use..

Many others will work of course, that is just an example.

And John, I don't have time to translate schematics to ASCII art because I would have to do so by hand.

This would be very easy to implement with a PIC, but then you would need to learn how to program using PIC code, which is not all that simple. Microchip has an app note for motor control using a PIC16F684, which I have used for a step-up DC-DC boost converter. The document shows the schematic and basic operation of a motor control kit which I got as a prize at a Microchip seminar. You can have a look at:

Another good app note is:

These show a full bridge controller, which is more complex than a single MOSFET with PWM, but it is really much better because it can reverse direction under software control, and you can also use dynamic braking which slows the motor more quickly and provides more precise control.

If you really want good control, a stepper motor is best. The PIC16F684 is also usable for stepper motors, and I have had some experience with that device if you want to go that route.

But stepper motors are even more complex, and I think you want something very simple. Still, the PIC16F684 would be able to provide a nice PWM drive to a logic level MOSFET at 5 kHz, and you could add a pot across the 5 VDC power supply to an A/D pin, and you could have a complete PWM circuit with only a few components. And you could change the frequency just by reprogramming it, or add other things like pushbuttons, LEDs, and even literally "bells and whistles".

You can use a PICkit 1 (which I have) to build your entire motor controller. It connects by a USB cable to your computer for programming, and it has a prototyping card that can be snapped off and used separately. There is also a PICkit 2 that has more features. Here's more info:

And for the PICkit 2:

I could give you a PC board that could be used to make a simple PWM motor control using a PIC16F684 or the similar PIC16HV616 (which I am using). The board is designed to produce up to 60 VDC at 750 mA as a boost converter, but you could replace the inductors with a motor (and diode), and you could add an external pot for speed control. But it's only about 1" x 2.5", and mostly SMT. I got about 100 boards for about $150, and I could spare a few.

I also have about 100 boards that were designed for an LT1247 or UC1843a PWM controller, and they might work also. It's a simple IC, and you could get a free sample in DIP package from Linear Technology or TI. The boards are essentially scrap, and I have some extra parts, so let me know if you (or anyone else) is interested. Again, this was for a boost converter, and it works, but not quite right.

Sorry for getting carried away, but these are just some other ideas that might help you with your project, and maybe you might enjoy a more software-oriented approach to circuit design with PICs.

Good luck,

Paul

Most likely no one John! But then again, you should know how that feels.

Isn't it amazing that a 'programer' can't write a little coherent text?

I agree, but neither of you had a sig file bragging about your programing skills. You just do it. :) You two have different styles, but are doing the work, rather than bragging and pointing to a crude 'brag' website.

I used to write software but with my failing eyesight, it's just too much trouble to only be able to work for about fifteen minutes a day, and end up with a headache. Hopefully, I'll finish some modifications to my house before winter, and have room for the 22" monitor again. I need to sit about 12 inches from it, and use a larger font than I used to. I used to have about 90 lines of text on the screen when writing code. No more. :(

The project I need to finish is a EEPROM reader for video monitors, to see what modes they are capable of without connecting them to a computer.