# 555 PWM Circuit

Discussion in 'The Projects Forum' started by jwilk13, Jun 15, 2011.

1. ### jwilk13 Thread Starter Member

Jun 15, 2011
228
12
I know this topic has been beaten to death, but I haven't been able to think up a suitable solution to my design. Here's the breakdown:

My goal is to design a circuit that will generate a PWM signal at approximately 220 Hz with duty cycle ranging between 10% and 90%. The duty cycle is to be varied with a potentiometer. I got this far, and I can vary the duty cycle from approximately 0-100% (better than I need) with the potentiometer ranging from zero to its full resistance value. The one problem is that I want to be able to set the potentiometer in the middle of its range and adjust the duty cycle when the potentiometer is turned CW and CCW.

In other words, the potentiometer at its center would correspond to the lowest duty cycle. Then when turned full CW OR CCW, the duty cycle would increase to its maximum percentage.

One thing I thought about was jumping the two terminal ends of the potentiometer, then the center would correspond to 1/4 the value of the total resistance and would now be the maximum. Then when the pot is turned CW or CCW it would decrease to zero. But how I should apply this idea to a PWM circuit, I'm not sure .

I'm not in need of a full-blown solution, just some ideas to get my brain turning (I'm having designer's block). Any thoughts? Thanks in advance.

2. ### #12 Expert

Nov 30, 2010
16,314
6,818
Let me just look at this schematic for a moment so I can tell how you've labeled things...

Hmmm...no schematic.

How about helping me out with a schematic?

3. ### jwilk13 Thread Starter Member

Jun 15, 2011
228
12
I'm working with two schematics to figure out which one is better. The only differences with mine are changes to actual component values. The first one is shown, the other I don't have access to right now, but I'll post it later.

The second one is just a pair of 555 timers, one in astable operation and the other in monostable operation with the duty cycle being adjusted by an analog voltage to the control input of the monostable 555. The problem with this is that the duty cycle variation isn't linear with the applied control voltage. I'll explain more later . Thanks for the help.

4. ### iONic AAC Fanatic!

Nov 16, 2007
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It would also be good to know what function you would accomplish if this setup could be achieved.

Mar 24, 2008
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6. ### jwilk13 Thread Starter Member

Jun 15, 2011
228
12
@ionic: The goal would be to achieve the same PWM signal out of the 555 whether the potentiometer is turned counter clockwise or clockwise. Assuming the ends are jumped, the pot would start at its maximum value (now the center) and decrease in resistance if it is turned in either direction. Ideally, this decrease in resistance would increase the duty cycle. I'm not even sure if that's possible, but that's what I'm going for.

7. ### simo_x Member

Dec 23, 2010
200
6
@ jwilk13
In your scheme, if you put a diode in anti parallel to the motor, the transistor will be grateful.

Last edited: Jun 16, 2011
8. ### jwilk13 Thread Starter Member

Jun 15, 2011
228
12
@ simo: yeah the schematic is just a general idea of what I'm doing. I plan on using a "snubber" diode..."freewheeling" diode...whatever you call it across the motor.

I'm more looking for ideas on how to vary the duty cycle in the manner I described. I guess you can think of it similar to a RC servo controller with a joystick, only with a little bit of difference. I want the "joystick" center to correspond to low duty cycle, and full joystick forward, full joystick reverse to correspond to max duty cycle. Any ideas would be greatly appreciated. Like I said, I don't need a full blown solution, just some "maybe try this..." sorta stuff

9. ### Wendy Moderator

Mar 24, 2008
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2,536
Put the diode on the wiper, two other diodes on the edge (facing the same way on the edge). I think this will give you the effect I think you are looking for.

10. ### jwilk13 Thread Starter Member

Jun 15, 2011
228
12
Bill,

Thanks for the input. With what you're describing, would I short the ends of the potentiometer? I can't get the circuit to perform correctly, it's just giving me a constant high output.

Here's another idea. I have one 4-pin potentiometer (I think it's a center-tap from what I can tell, not center-detent). Would something like the image below work? I can't test it at the moment, probably sometime next week. Then when the pot is moved past center (forward) it would use D2 and D3, and when it is moved past center (reverse) it would use D1 and D2. Unfortunately the simulation program I'm using doesn't have the ability to simulate a center-tapped potentiometer, otherwise I'd know. Thanks again for all the advice.

11. ### iONic AAC Fanatic!

Nov 16, 2007
1,420
68
Ah Ha!, the answer you did not tell me in your reply! You want to vary motor speed in two directions!
Unless Bill's solution works, I's say you need an MOSFET H-Bridge for forward and reverse of a motor with the pot.

12. ### Wendy Moderator

Mar 24, 2008
20,766
2,536
Nope, my solution would not work. I didn't see any reference to a dual direction motor control. A standard PWM could do it with a H bridge, but for dual direction I think the H bridge is pretty much required. If I get time I'll sketch up something I think would work.