555 timer astable questions. - Page 1
Forum section: electronics / beginners

author: MarkusAnd
author_rank: Contributor
timestamp: March 31, 2019, 01:12:36 pm
content: Hello!As I'm continuing my testing of simple circuits I have some questions about using the 555-timer in astable mode. In most of the circuits that use a 555-timer to output PWM-signal, there are two diodes connected between R2 and the pot. When I remove the diodes as in the second picture, i can only adjust the output between 2v and 3v(5v input), and when i use the diodes as in the first schematic i can perfectly dim a LED. Why are the diodes used and why can't I get PWM-output without them?//MA

author: LateLesley
author_rank: Frequent Contributor
timestamp: March 31, 2019, 02:23:49 pm
content: In the first circuit, the way it is wired, you are changing the mark-space ratio of the oscillator. In the second, you are only changing the frequency of oscillation, not the mark-space ratio, so it's always roughly50%, no matter how fast it oscillates.The 555 is a relaxation oscillator, it works by charging and discharging C1. When charging, it goes through R2 and R1, when discharging it'll go through R1.Where the diodes come into play, is to help increase the variation in charge and discharge time.when charging, it'll go through R2, D2 the right half of R1 to the cap C1.When discharging it'll go through the LEFT half of R1 and D1 and to ground through the discharge.as the wiper moves left, it takes longer to charge and discharges faster, giving short pulses.as the wiper moves right, it charges faster, and takes longer to discharge, giving longer pulses.The second circuit can vary the mark space ratio, but not as much. it'll just vary the frequency more as both the charge and discharge times get shorter with decreased resistance.

author: rdl
author_rank: Super Contributor
timestamp: March 31, 2019, 03:39:12 pm
content: Dave built a version of the first circuit in EEVblog #392 and there is some explanation of how it works.

author: MarkusAnd
author_rank: Contributor
timestamp: March 31, 2019, 04:11:38 pm
content: okay, so when I don't use the diodes I'm changing the frequency. So the only reason I'm using the diodes is to give myself a bigger range or? And do I change the frequency when I'm using the diodes?

author: LateLesley
author_rank: Frequent Contributor
timestamp: March 31, 2019, 04:21:43 pm
content: It might change the frequency a little, but not by much at all.With the diodes, the frequency hardly changes, but the mark-space ratio will.Without the diodes, the frequency changes, but the mark-space ratio hardly changes.

author: MarkusAnd
author_rank: Contributor
timestamp: March 31, 2019, 04:46:19 pm
content: 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?

author: LateLesley
author_rank: Frequent Contributor
timestamp: March 31, 2019, 06:31:49 pm
content: 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.

author: rdl
author_rank: Super Contributor
timestamp: March 31, 2019, 08:28:45 pm
content: In the first circuit with the diodes, an additional variable resistor in series with R2 could be added for frequency control.

author: MarkusAnd
author_rank: Contributor
timestamp: April 01, 2019, 09:41:52 am
content: Thanks for the help!

author: LateLesley
author_rank: Frequent Contributor
timestamp: April 01, 2019, 01:40:41 pm
content: 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