Electronics > Beginners
555 timer astable questions.
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MarkusAnd:
OK, it kinda makes sense now. So if I, for example, want to blink a LED at a certain frequency I don't use the diodes because I just want to change the frequency but when I want to dim a LED with PWM I use the diodes?
LateLesley:
Yes, because it's the mark-space ratio which will decide how much "ON" time an LED will get, adjusting the brightness. The frequency just decides how fast it is being turned on and off, not how long it is on and off in each individual cycle.

You'll also note, the LED brightness in the second circuit should match the midpoint of the first circuit. The first circuit should be able to make it brighter.
rdl:
In the first circuit with the diodes, an additional variable resistor in series with R2 could be added for frequency control.
MarkusAnd:
Thanks for the help!
LateLesley:
I was looking at  post about simulation software, and someone linked to an online simuator for basic circuits. I found one with the guts of a 555, so I modified a circuit to your first schematic. It might shed more light on how the circuit works with the diodes.


http://www.falstad.com/circuit/circuitjs.html?cct=$+3+0.000005+5.023272298708815+64+7+50%0Aa+288+168+384+168+9+5+0+1000000+5.218515538001821+3.3333333333325195+100000%0Aa+288+264+384+264+9+5+0+1000000+6.666666666665039+5.218515538001821+100000%0Ar+240+56+240+104+0+5000%0Ar+240+104+240+152+0+5000%0Aw+240+152+240+280+0%0Ar+240+280+240+328+0+5000%0Aw+240+152+288+152+0%0Aw+240+104+272+104+0%0Aw+272+104+272+280+0%0Aw+272+280+288+280+0%0Aw+464+176+464+192+0%0Aw+384+184+384+192+0%0Aw+384+192+464+240+0%0Aw+464+240+464+256+0%0Aw+384+240+384+248+0%0Aw+384+240+464+192+0%0AR+240+56+240+16+0+0+40+10+0+0+0.5%0AR+88+56+88+24+0+0+40+10+0+0+0.5%0Aw+88+120+216+120+0%0Aw+216+120+216+352+0%0Ac+88+248+88+352+0+1.0000000000000001e-7+5.218515538001821%0Ag+88+352+88+368+0%0Ar+384+368+464+368+0+10000%0Aw+464+176+496+176+0%0Aw+464+368+496+368+0%0A153+384+256+464+256+1+2+0+5%0A153+384+176+464+176+1+2+5+5%0Aw+496+176+496+368+0%0AO+464+256+544+256+0%0At+328+368+296+368+0+1+0.5667282541267074+0.6567151313054329+100%0Aw+216+352+296+352+0%0Ag+296+384+296+408+0%0Aw+328+368+384+368+0%0Ax+120+115+189+118+4+16+discharge%0Ax+129+178+174+181+4+16+trigger%0Ax+120+242+185+245+4+16+threshold%0Aw+272+104+272+32+0%0Ax+272+22+320+25+4+16+control%0Ab+514+398+200+40+0%0Aw+240+328+240+384+0%0Aw+240+384+296+384+0%0Aw+288+248+184+248+0%0Aw+288+184+184+184+0%0Aw+184+184+184+248+0%0Aw+184+248+88+248+0%0A174+128+232+56+256+0+100000+0.3119+Resistance%0Ad+128+144+128+232+2+default%0Ad+48+232+48+144+2+default%0Ac+600+32+600+96+0+0.000001+6.666666666665039%0Ag+600+96+600+128+0%0Aw+600+32+272+32+0%0Aw+88+256+88+264+0%0Aw+88+264+88+248+0%0Ar+88+56+88+120+0+1000%0Aw+88+120+88+144+0%0Aw+128+144+88+144+0%0Aw+48+144+88+144+0%0Ao+20+16+0+4099+10+0.00078125+0+2+20+3%0Ao+28+32+0+4106+5+0.00009765625+1+1%0A
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