Astable 555 - Weird behaviour? - Page 1
Forum section: electronics / beginners

author: Grytpype
author_rank: Contributor
timestamp: January 05, 2022, 09:55:50 am
content: Hi All and Happy New Year!I'm working through the Electronics for Dummies book and have run into some, at least to me, weird behaviour on project 23 'LED Flasher with set / reset".I think I understand the principals of how it should work so wondered if anyone can explain what's happening:When powered from the 9v rail (pin eight), the astable 555 on the right works as expected; the LEDs turn on and off in sequence. My understanding of this is that when the output is high, the 555 supplies the voltage and grounds via the yellow LED to the negative rail. When low, the red LED takes its power from the positive rail and grounds through the 555. In my mind they are mutually exclusive (?)The 'weirdness' happens when you power it from the set / reset 555 (pin 3) output on the left. It still alternates the LEDs on / off, but the top one doesn't turn completely off. Swapping the LEDs / 555s around makes no difference so I don't think they're hooky.I'm sure it's something obvious but I can't see where the top LED is getting this intermediary voltage (if that's actually what's happening?) and why it doesn't happen when the 555 on the right gets its voltage directly from the 9v rail.Any help greatly appreciated! It's driving me nuts.Thanks!

author: jpanhalt
author_rank: Super Contributor
timestamp: January 05, 2022, 10:19:27 am
content: It doesn't look like pin 1 is connected to ground with wither IC. What looks like a capacitor from pin1 to ground is not what is needed.Please post the schematic you are using.

author: Grytpype
author_rank: Contributor
timestamp: January 05, 2022, 10:25:13 am
content: Thanks but both 555s are grounded from pin 1. The capacitor is on pin 2 / trigger of the right 555.Hopefully this image is a little clearer.Ta!

author: mcz
author_rank: Contributor
timestamp: January 05, 2022, 10:31:14 am
content: Quote from: jpanhalt on January 05, 2022, 10:19:27 amschematic might be copyrighted.The way I see it pin1 on both ICs is connected to the ground rail.Reply to the OP:If we look at the datasheet of the NE555 (https://www.ti.com/document-viewer/NE555/datasheet/specifications#SLFS0227382) under Electrical Characteristics -> High Level Output Voltage:We see that the IC can't drive the output all the way to the positive (9V) rail.Now with just one NE555 the 1-1.5V missing might not be able to turn on that LED but if you now have two times this voltage since both NE555s can't drive all the way to the 9V rail it becomes a problem.The difference between the output voltage and the 9V rail is enough to turn the LED on. I suggest modifying the schematic and connecting the upper LEDs positive leg to the PIN8 of your second NE555. See if that fixes the problem.cheers

author: Grytpype
author_rank: Contributor
timestamp: January 05, 2022, 11:29:58 am
content: Thanks MCZ. Your suggestion worked! I'm not sure I fully understand the difference but will go through it again this evening.I guess it's wired that way in the book so newbs like me can see something happening as soon as the circuit is powered i.e. the top led comes on straight away; I should have mentioned that!So now, no light comes on at startup but the set / reset works perfectly to start / stop the astable flashing

author: mcz
author_rank: Contributor
timestamp: January 05, 2022, 11:59:39 am
content: Quote from: Grytpype on January 05, 2022, 11:29:58 amOkay, just ask again if you can't figure it out.My solution was more or less just a quick solution to figure out if my hypothesis was correct.An alternative solution (there are many) would be to add one or two or even three diodes in series with that upper LED to increase the forward voltage drop and help it to stay off.Since this is just a fun learning circuit the original designers probably never thought of this "LED lights just a little bit" problem as a real problem. And they obviously want to keep parts count and complexity low.

author: Grytpype
author_rank: Contributor
timestamp: January 05, 2022, 12:09:37 pm
content: That's great. Will experiment.Thanks again!

author: jonovid
author_rank: Super Contributor
timestamp: January 06, 2022, 06:08:31 am
content: not sure what thecolors are but ,anyhow here is a free book full of 555 ic circuitshttp://www.talkingelectronics.com/te_interactive_index.html

author: Grytpype
author_rank: Contributor
timestamp: January 06, 2022, 10:30:32 am
content: Thanks jonovid!