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I'm a complete beginner to electronics, but I'm trying to follow Ben Eaters video series "Building an 8-bit computer". I tried to do the first part of an astable 555 timer, but the LED does not oscillate and on top of that the timer draws A LOT of current and heats up pretty fast. Does anyone have an idea what I did wrong and how?

I'm using NE555P, 1uF capacitor, 5V from a rigged phone charger.enter image description here enter image description here

Mr Filp
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    That's a very tidy wiring, congratulations, and thank you for the excellent photo! We'd still ask you to also (and: more importantly, even) draw a schematic (the question editor on this website has a schematic editor built in), because we can't see what voltage you supply this with, and to understand what this circuit is doing, the first thing I'd do myself is sit down and draw a schematic, starting by looking up the resistor values based on band colors etc – a lot of work that makes sense that you do, especially since intent and implementation might be different if a circuit isn't working! – Marcus Müller Aug 03 '19 at 23:09
  • Is pin-4 floating? It should be pulled up to Vcc. – brhans Aug 03 '19 at 23:15
  • @brhans In Ben Eaters video (https://youtu.be/kRlSFm519Bo) you can see at the start that pin 4 is floating. I also tried just now to connect it to Vcc, and it just dimmed the led but the NE555 is still getting extremely hot very quickly and the LED isn't oscillating. – Mr Filp Aug 03 '19 at 23:26
  • @MarcusMüller Thank you for the comment. I just drew the schematic, but (being new to the site) I don't know how to edit my question to add it. Should I just post it here to the comments? – Mr Filp Aug 03 '19 at 23:30
  • You can edit your original question to add the schematic. – Peter Bennett Aug 03 '19 at 23:36
  • @MrFilp in which shape/form do you have it? If it's an image, you can upload it to imgur.com (it's where stackexchange hosts images anyway), and add the link to your question by editing it :) As new user, there's a limit on pictures-per-post, but us older users can edit your question and embed the image after you've added the link to the question. – Marcus Müller Aug 03 '19 at 23:37
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    It's a good photograph, but the resistor colours are not clear, you cannot read a value on the capacitor nor see where its other leg goes. It seems to be the correct circuit for an astable. But, as has been mentioned, Pin 4 should be connected to +V and also pin 5 decoupled to ground by about 10nF. The 555 should not get hot at all! I've played with this circuit and I found that you could blow the 555 easily by accidentally reversing the power supply. Did you do this or plug in the 555 the wrong way round at some point? It's in the right way now. – Peter Jennings Aug 03 '19 at 23:42
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    @PeterJennings I feel so stupid now, I accidentally reversed the cords while rigging the USB cable. Thank you so much, I literally tried nearly everything except that. – Mr Filp Aug 03 '19 at 23:47
  • @PeterJennings or MrFilp: one of you two should post that result as an answer, so that Mr Flip can mark this question as answered. – Marcus Müller Aug 03 '19 at 23:50
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    @PeterJennings If you could post that as and answer I could mark this question as answered. – Mr Filp Aug 03 '19 at 23:52
  • @MarcusMüller I'm surprised how fast this community answer and am truly grateful for that. – Mr Filp Aug 03 '19 at 23:54
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    @MrFilp well, you put a lot of work into your question, an made sure your circuit and picture were clean, you reacted very politely and were generally a pleasure to interact with – it's not standard that we get so good questions from new users, so when that happens, that certainly boosts our motivation. But the thanks go to Peter! – Marcus Müller Aug 03 '19 at 23:59
  • @MarcusMüller I'll be sure to act as politely as before when I get stuck again and yes Peter cracked it! – Mr Filp Aug 04 '19 at 00:02
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    HI Mr Flip, I've posted it as an answer, but I fear I'm too late now. – Peter Jennings Aug 04 '19 at 11:51
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    Haven't scanned *all this text* to see if this is addressed: Those resistors appear to be 1% resistors...their colour-code value does not correspond to those in the schematic. The resistor between pin 6 and pin 7 looks to be 100k, not 100 ohms. – glen_geek Aug 04 '19 at 13:08
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    I am a bit late to the question, but I would like to agree with Marcus that it is nice to see a new user being polite, putting effort into a question and helping us by listening to the comments and doing the necessary steps to help solve the issue. I have seen some of the tutorial you are watching, the YouTube channel has some good tutorials. Be sure to watch them all to understand how everything works together! Good luck with your project! – MCG Aug 05 '19 at 13:53
  • I tweaked your image to make the colors on the resistors visible, as well as the notch on the 555. https://i.stack.imgur.com/8v4mj.jpg – DarenW Aug 06 '19 at 02:01
  • @glen_geek You're absolutely right, I mistakenly made it like that when I made the schematic in a hurry. Sorry for the confusion! – Mr Filp Aug 07 '19 at 18:19
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    @MCG I will try to understand as much as I can from it and with that learn stuff about electronics and the physics behind it. I've been eyeing this project for quite some time and I'm ready to take it steadily and go through it thoroughly. I'm also surprisingly happy to see all the positive attitude in the responses! – Mr Filp Aug 07 '19 at 18:23
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    @MarcusMüller Breadboard credit goes to Ben Eater! I've seen his videos and he really has very tidy work. However, replicating it accurately is also a feat. I've had to try it 4-5 times myself for it to work properly(plus 4-5 times more when soldering it on a perfboard). – cst1992 Oct 19 '19 at 13:36
  • @cst1992 glad to hear you've got it working :) Next step really is to learn e.g. KiCAD and produce your own PCB manufacturing files; there's pretty good low-cost manufacturers of PCBs (OSHpark (US), Aisler (EU), JLCPCB (China)) and if your circuit becomes a full 8-bit CPU one day, it'll be very much work (and hidden cost, from solder materials to ruined perfboard and components) if you need to do it on perfboard :) However, this is AWESOME and I congratulate you! – Marcus Müller Oct 19 '19 at 13:45
  • @MarcusMüller When I transferred the circuit(plus the 10nF capacitor and no. 4 pin connected to Vcc) to a perfboard using Fritzing, I found the circuit didn't work. At first I'd soldered the circuit the wrong way around ( mirror image for all the IC connections; burnt my finger when I touched it), but then I tried again and still nothing(atleast IC wasn't hot). Time to troubleshoot... – cst1992 Oct 23 '19 at 07:54

5 Answers5

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This answer is a summary of existing good answers plus various comments. The OP supplied a good image and schematic. Several issues stand out or were a potential past problem.

  1. Breadboards are known for odd behavior, however this circuit should be stable with just a 4.7 µF capacitor across the 555 power and ground pins.
  2. Inputs should NEVER be left floating. As Marcus mentioned in his answer the active low reset pin should be tied to Vcc for stable operation.
  3. As Sunnyskyguy mentioned in his answer it is very possible the LED was inserted backward. If so it may or may not have been damaged. Replace it when possible.
  4. Peter Jennings mentioned that you may have inserted the 555 IC backwards initially or had Vcc and gnd reversed at the power connector. If so consider it toast and try a new one. Reverse polarity can damage most any IC and cause it to get very hot even with no load connected.
  5. While it is not mandatory, inserting a 10 nf cap from the control pin to ground helps the 555 reject noise on the Vcc line.
  6. It is good practice to route ALL ground connections first, then power, then inputs, then outputs. Much better chance of getting connections right the first time, and having even complex boards work right the first time. Plug in your ICs last after testing your power feeds with a DVM.
  7. Do NOT bend LED or other component leads close to the body of the part, as this can cause internal stress and damage. Use needle-nose pliers to create a 1/16th inch minimum gap before the bend.

I would replace the LED and make sure the cathode goes to ground. Use a new 555 timer and please pay attention to component orientation. Add the extra capacitors mentioned for stability. This is a simple 555 timer IC. Pay attention to details and it should work just fine.

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    Thank you, I would have blown all of my 555 timer chips without you guys! – Mr Filp Aug 04 '19 at 10:23
  • Thanks for point #6. Should come in useful. – cst1992 Oct 19 '19 at 13:40
  • @cst1992 I don't follow that advice myself, often, when designing boards where I do connections on both the top and bottom side of the board – bottom side is practically ground-only if possible at all. If not possible, be very sure to not cut the ground planes (i.e. make only short connections and make sure it's still one contiguous ground plane). For high-frequency circuits, you actually need to route signal and ground together, because you need to know the current return paths, but with an NE555 you'll not get close to that – Marcus Müller Oct 19 '19 at 13:48
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As has been mentioned, Pin 4 should be connected to +V and also pin 5 decoupled to ground by about 10nF.

The 555 should not get hot at all! This is the big clue. I've played with this circuit and I found that you could blow the 555 easily by accidentally reversing the power supply. Did you do this or plug in the 555 the wrong way round at some point?

It's in the right way now.

Peter Jennings
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  1. The LED flat edge cathode is not towards gnd. So it is backwards.
    • the leads are also stressed beyond recommended in spec.
Tony Stewart EE75
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You didn't connect the inverted RESET pin.

To cite TI's NE555 datasheet:

To prevent false triggering, when RESET is not used, it should be connected to VCC .

Marcus Müller
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Try to change the size of capacitor. Try various sizes. Some might be osscilating too fast that you cannot see it. Say you have 1 micro farad you change to 100 mf. Then try lesser values. than what you used.

Joshua Edzie
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    ...or, the math can be done, and we can know the freq to within cap tolerance. Shortcuts make us sloppy – Scott Seidman Aug 04 '19 at 19:12
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    If you had read what was already posted as comments to the question you would have seen that the *actual* issue (power supply polarity wired to the breadboard reversed) was identified 19 hours before you posted this. Such random guess advice as you are giving here is really the domain of comment, not answers. – Chris Stratton Aug 04 '19 at 20:57
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    Please try to use proper grammar or risk having your answers ignored or downvoted. –  Aug 04 '19 at 21:25
  • Or the resistors may be smaller than claimed. We can't see the color bands in the originally posted photo. But I fixed that - see my link to an adjusted photo on imgur in the comments following the question. – DarenW Aug 06 '19 at 02:32