Which components do I need to change? (555 timer)

[Croboy]

http://www.sentex.ca/~mec1995/gadgets/servo3.htm

So this is a shematic. I need to put flex sensor (about 40kOhms) instead of potentiometer (2k7Ohms). Can I change something in circut to get the same PWM output signal?

SORRY FOR MY BAD ENGLISH.

 

[(*steve*)]

Increase R2 in the same proportion (40/2.7)*68k and reduce the capacitor (2.7/40) * 390 nF

You may have some trouble if the capacitor gets too small or the resistor too large -- and you'll be verging on both of them I expect.

 

[Croboy]

Thanks for a very fast response. I am impressed.
What about this 2k7Ohms (R1) does he need to be changed to 40kOhms?
Thanks again.

 

[Croboy]

In Electronic Workbench I had make a change.
So this 390nF capacitor is now 700nF.

Potenciometer is now resistor (20kOhm)
and this 68kOhm is now potentiometer (0-40kOhms).

I get a nice 2mSec PWM signal on 40kOhms. As you can see on this attachemt.
DOES IT MAKE ANY SENSE?

 

[(*steve*)]

DOES IT MAKE ANY SENSE?

No, did you try what I advised?

 

[Croboy]

So this is what I get from your advice. On first picture potenciometer is set to 20% and on the second is on 100%.


Is this okay?


I am feeling very stupid but need to ask.

 

[(*steve*)]

I think that looks about right.

You need to look at the duration of the pulse, but it should change from 1/1000 sec to 2/1000 sec.

By eye, it looks to be a little long (and that is what I suggested might happen). You may be able to reduce the capacitor a little more.

I presume this is simulation? It may not behave quite as well in real life.