Inverting 555 timer high and low

Thread Starter

Dragonetti

Joined Jun 12, 2020
3
Hi, hopefully I've posted this in the right section. Please let me prefix this post with a disclaimer: I've never worked with circuits or electronics before. My interest is purely in scale modelling and I'm trying to use a 555 IC to get some blinking fibre optics as authentic as possible, as opposed to just using a 1hz blinking LED. I literally have no idea what I'm doing so please explain any answers like I'm a five year old.

I've followed this YouTube video exactly:
except that my components are:

R1: 39k ohm
R2: 2.7k ohm
R3: 230 ohm
Capacitor: 47 mF

This gives me a very short low time and a longer high time. I require this to be inverted. This is just for a single LED.

I'm sure this is fairly simple and I have googled it and found some suggestions that its quite simple for a single LED but I don't quite follow the explanations. If someone could refer to the circuit in the above video and tell me what to swap to where, that would be ideal.

Many thanks for your patience and help
 

ericgibbs

Joined Jan 29, 2010
10,208
hi D,
Welcome to AAC.
One option is to connect the LED from the 555 Supply to pin #3 via 220R resistor.
Try it, it should be inverted as required.
E
Update;
Excuse poor sketch.
Capture3.PNG
 
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dl324

Joined Mar 30, 2015
11,000
Welcome to AAC!
R1: 39k ohm
R2: 2.7k ohm
R3: 230 ohm
Capacitor: 47 mF

This gives me a very short low time and a longer high time. I require this to be inverted. This is just for a single LED.
The image shows Ra, Rb, and RL, but you have R1, R2, and R3.

Assuming you're interested in learning, it would be easier to understand what you're talking about if you used the same component designators. That's why we use them in the first place.

Electronics is a fairly precise discipline and using an incorrect prefix can give wildly different results. You say you're using a 47mF capacitor. That's 47,000uF and I'm almost certain you meant 47uF. mF was once understood to be microfarads, but now it's no longer an accepted practice.

Here's a picture that could be even more confusing:
clipimage.jpg
2000 MFD could be interpreted as 2,000uF or 2,000,000,000F. Common sense would dictate that the intended value be 2,000uF, but without the picture, it wouldn't be as clear.
 
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Thread Starter

Dragonetti

Joined Jun 12, 2020
3
Hi dl. Thanks for your reply. And yes as I've stated and I'm sure you can tell, I have no idea what I'm doing :)

I've been building scale models for a good while, but been too intimidated by the intricacies of installing lighting until now so I am just muddling through the best I can and yes I am interested in learning.

As far as my incorrect designators go, I appreciate the corrections. I wasn't sure what 'RL' meant so that's why I replaced it with R3 just to make more sense to myself. With the microfarads you are correct again but I couldn't find the mu symbol on my phone's keyboard :)

Thanks again for the help. I do really appreciate it.
 

dl324

Joined Mar 30, 2015
11,000
As far as my incorrect designators go, I appreciate the corrections. I wasn't sure what 'RL' meant so that's why I replaced it with R3 just to make more sense to myself. With the microfarads you are correct again but I couldn't find the mu symbol on my phone's keyboard
RL is usually used to indicate a load resistance. Ra and Rb are the component designators used in the datasheets.

Regarding the mu symbol, we understand lower case 'u' to be the micro prefix. You can also spell it out: microfarads.
 

dl324

Joined Mar 30, 2015
11,000
BTW, the symbol used in the video is one of the better that I've seen. For symbols in schematics, the preference is to have the inputs on the left and the outputs on the right. This facilitates the left to right flow that's preferred.

For a 555 timer, it makes sense to put reset and Vcc on the top and ground on the bottom. I prefer to put the control pin (5) on the left. The creator of that video didn't bypass it to ground. Datasheets should all recommend that to minimize noise being coupled into that input. In general, it's a good design practice to not allow any inputs to float.
 
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