Electronics > Projects, Designs, and Technical Stuff
Need some help with a 555 timer circuit
Pizzashape23:
This is my first post, so go easy, anything i have missed/done wrong just let me know and i would be more than happy to correct it.
Essentialy, I am trying to create a circuit which outputs a fixed pulse width (1 sec) for all input widths, both longer and shorter than the output. I have tried a bunch of different circuits with discrete components, but after a bunch of failures a friend recommended I use a 555 timer. I ended up finding a circuit I thought would integrate well with my existing project on the Electronics Stack Exchange, and modified it to suit my needs. I don't currently have access to the components needed to fully test it, but i need to finalise a prototype so I was hoping someone with a bit more experience could give some input/advice. (I know the best way would probably be to breadboard/veroboard it but this was my second best option.
I have already made a post on the Electronics stackexchange with what I hope are all the details needed, I am going to link it here, but if linking to another forum is frowned upon here i would be glad to repost all the information.
https://electronics.stackexchange.com/questions/514223/will-my-555-timer-circuit-work-the-way-i-want-it-to
Will this circuit work?
Would you recommend a different circuit (preferabbly without an MCU)?
Any modifications I should make circuit or safety wise?
Buriedcode:
I would suggest using a circuit simulator like LTspice (which has a 555 model I believe). I'm sure there are simpler and easier ones about, all free, but I have most experience with this.
It sounds like you want an edge triggered monostable, I haven't even thought about such things in years. So, say, a falling edge on the input will trigger a pulse, so it doesn't care when the rising edge is, so the pulse can be as short or as long as you like, it'll only trigger on the start of the pulse. If you want to change the polarity you can use a bipolar transistor to invert the input or output.
I'm pretty sure the 555 monostable is non retriggerable (multiple edges during the output pulse are ignored), but I can't remember off hand.
Honestly, playing about with a circuit simulator, even just copying schematics from the web to see what is going on can be interesting.
This page has an example of one:
https://electronicsclub.info/555monostable.htm
Pizzashape23:
Thanks for the reply, I'm pretty sure a falling edge monostable circuit is exactly what im after.
--- Quote from: Buriedcode on August 11, 2020, 04:21:10 am ---
Honestly, playing about with a circuit simulator, even just copying schematics from the web to see what is going on can be interesting.
--- End quote ---
I tried using LTspice but got nowhere (inept at using it for anything but circuit diagrams). I also used tinkercads circuit program and got it to work exactly as intended, my one issue is that it is aimed at begginers/kids and initial concepts. It doesn't give exact values for transistors or mosfets and it's difficult to monitor what is going on, so i was unsure whether the result I got was because I had the right values or because of the programs leeway.
Ian.M:
In the real world, you can never expect your components to have the 'right value' as *everything* has a tolerance. Resistors used to be +/-10%, or +/-5% but now 1% is far more common and nearly as cheap. 0.1% is readily available at a moderate premium. Capacitors are much worse - a typical electrolytic may be +/-20%, and +/-10% is typical for many other types. Most inductors aren't precision components either. The situation with discrete semiconductors is even worse - typically there will be at least one datasheet parameter for which a 2:1 variation is entirely normal. Therefore, if you don't want to have to hand-trim every circuit to find its 'sweet spot' then re-trim it when it inevitably drifts you need to get into the habit of designing defensively so your circuit tolerates non-ideal component values.
Its worth persevering with LTspice, though the learning curve is admittedly fairly steep if you haven't previously used any other SPICE software. If you get stuck with simming something, ask here. Although your circuit maybe harshly criticised, if you are willing to learn, you wont be! ;)
Pizzashape23:
I get what you are saying in regards to component tolerance, does this mean the leeway in TinkerCad simulation would carry over into real life, at least to some extent? What I mean is can the simulation be trusted?
Also I was curious if you had an opinion on the overall circuit itself, is there a better way of achieving the same thing or should I go ahead and submit the prototype?
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