# Astable 555 circuit not oscillating

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.

• 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! Commented Aug 3, 2019 at 23:09
• 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. Commented Aug 3, 2019 at 23:42
• @PeterJennings If you could post that as and answer I could mark this question as answered. Commented Aug 3, 2019 at 23:52
• @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! Commented Aug 3, 2019 at 23:59
• 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
Commented Aug 5, 2019 at 13:53

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.

• Thank you, I would have blown all of my 555 timer chips without you guys! Commented Aug 4, 2019 at 10:23
• Thanks for point #6. Should come in useful. Commented Oct 19, 2019 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 Commented Oct 19, 2019 at 13:48

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.

1. The LED flat edge cathode is not towards gnd. So it is backwards.
• the leads are also stressed beyond recommended in spec.

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 .

• If Peter is correct, this is only a partial answer...
– user105652
Commented Aug 4, 2019 at 3:17
• @Sparky256 absolutely! Hence I asked OP and/or Peter to post a full answer. Commented Aug 4, 2019 at 8:57

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.

• ...or, the math can be done, and we can know the freq to within cap tolerance. Shortcuts make us sloppy Commented Aug 4, 2019 at 19:12
• 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. Commented Aug 4, 2019 at 20:57