555 Timer

Thread Starter

Bertus2

Joined Mar 20, 2009
5
Hi

Is there a way of generating a fixed frequency signal, but with a variable dutycycle, with a single LM555. The Dutycycle must be variable from about 10 - 90% .
 

hgmjr

Joined Jan 28, 2005
9,029
Pin 7 is not required since the output square wave on Pin 3 is charging and discharging the capacitor through the diode and resistor network.

hgmjr
 

Metalfan1185

Joined Sep 12, 2008
161
ok, just wondering...


is it possible to do the same, but have a variable frequency and the fixed duty cycle?
 

Wendy

Joined Mar 24, 2008
21,840
That is a subject for another thread, hijacking is discouraged.

It could be done, but not with a single 555.
 

SgtWookie

Joined Jul 17, 2007
22,201
Actually Bill, if R2 is 1/10 the value of R1, it will limit the PWM range to what the O.P. requested.

I suggest that R2 is not bogus; it limits the peak charge/discharge current through the diodes to/from C if R1 is at one end or the other.
 

thatoneguy

Joined Feb 19, 2009
6,349
Actually Bill, if R2 is 1/10 the value of R1, it will limit the PWM range to what the O.P. requested.

I suggest that R2 is not bogus; it limits the peak charge/discharge current through the diodes to/from C if R1 is at one end or the other.
The schematic shows R2 as variable though, so it too can be zero Ω. The way it's drawn doesn't make it stick out as a pot right away. It should be a small fixed value, not variable.
 

Wendy

Joined Mar 24, 2008
21,840
It was a recycled brain fart, and we all know how those smell. The base circuit is a variation of this, but the diodes force different charge/discharge paths. Since the resistance always adds up the same, the frequency shouldn't change much.
 
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