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Crosswalk project using 2 555 timers and can only be analog

ericpillon1

Mar 4, 2020
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I need help designing an Electronics project using only analog components. As well it needs to have 2 555 timers. I am trying to make it so that when you push a button the lights will go on but don’t really know how to implement that on LTSpice
 

Harald Kapp

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I need help designing an Electronics project using only analog components. As well it needs to have 2 555 timers.
A few more details? With two timers you can build a lot of things. What exactly do you want to achieve? And is this a kind of homework or assignment?

don’t really know how to implement that on LTSpice
As it happens LTSPICE has a model for a 555 timer in the MISC part of the component library. You can use this and wire it to your liking.
 

Hunter64

Nov 20, 2018
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Back to the datasheet, and try to explain what is digital about a 555.
 

ericpillon1

Mar 4, 2020
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Is this a school/college project?
So, this is a college project in which I'm choosing to do a crosswalk. it can only be an analog circuit using two 555 timers. we are given a 9V battery and some other basic parts. Other than that, the project is really open ended and we haven't learned anything about 555 timers and just started working with transistors. So I was wondering if someone could help explain a way for me to create a circuit and make the LED lights flash while people are theoretically crossing the street, then turn off after like 15-20 seconds.
 

ericpillon1

Mar 4, 2020
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A few more details? With two timers you can build a lot of things. What exactly do you want to achieve? And is this a kind of homework or assignment?


As it happens LTSPICE has a model for a 555 timer in the MISC part of the component library. You can use this and wire it to your liking.
So, this is a college project in which I'm choosing to do a crosswalk. it can only be an analog circuit using two 555 timers. we are given a 9V battery and some other basic parts. Other than that, the project is really open ended and we haven't learned anything about 555 timers and just started working with transistors. So I was wondering if someone could help explain a way for me to create a circuit and make the LED lights flash while people are theoretically crossing the street, then turn off after like 15-20 seconds.
 

Harald Kapp

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@ericpillon1 : Eric, I moved your thread in the appropriate sectin of our forum. Our policy in these cases is not to supply complete solutions but guide the op so he can find his/her own solution. There's more to learn by that way.

Therefore:
  • Use timer #1 as a monostable multivibrator. See e.g here how it is done. This timer will determine how long the lights wil blink.
  • Use timer #2 as an astable multivibrator. See e.g. here how it's done. This timer will determine the frequency of the blinking LEDs.
  • Use timer #1 to control the operation of timer #2, such that timer #2 is operating (blinking LEDs) only while timer #1 is active. This is called gating and explained e.g. here (scroll down a bit to where it says "astable gating").
That's basically all you need. There are a few points I'd like to point at, however:
  • Gating of the astable multivibrator can be done in several ways as shown on the page I linked. Select the method that matches the output signal of timer #1, notably observe which state the output of timer #1 is in when inactive and when active.
  • When driving the LEDs ensure you limit the current. The 555 can sink or source up to ~200 mA, tht should be good enough for a few LEDs in a demo project. If you need higher current, you'll have to add a driver. See or resource on driving LEDs for more details.
  • Simulating a switch or pushbutton in LTSPICE for activating timer #1 is a tiny bit tricky. See e.g. here how it can be done. The timing of the controlling waveform represents the pedestrians pushing the "walk" button ;).

Show us your results for further discussion or when you have questions.
 

ericpillon1

Mar 4, 2020
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Thank you. where do I connect the output of the monostable? does this monostable act as the entire cycle of the flashing lights, or does it simply make the lights stay on for say 1 second, then repeat with the reset? As well, the Gating website does not really help with gating the monostable to the astable, just mentions astable to astable.
 

Harald Kapp

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Thank you. where do I connect the output of the monostable?
You use it for gating the astable multivibrator, see my post #10 and below.
does this monostable act as the entire cycle of the flashing lights, or does it simply make the lights stay on for say 1 second, then repeat with the reset?
A monostable multivibrator is normally off. When triggered, it will be on for the time set, then turn off again. It will not automatically repeat the cycle - that is what the astable multivibrator does.
the Gating website does not really help with gating the monostable to the astable, just mentions astable to astable.
It is irrelevant whether the gating signal comes from an astable or a monostable multivibrator. In both cases the second timer (#2, the astable one) shall be active during the high period of the gating signal only. Simply use the monostable multivibrator instead of an astable one to create the gating signal you need.

With the information you've been given you should at least be able to create a block diagram of the circuit., i.e. a diagram showing the functional units and their interaction without going into the details. E.g. show a monostable multivibrator as a block labeled accordingly without drawing the internal circuit (no Rs, no Cs, no 555s etc.).
Show us how you think your system looks like on a high level in a block diagram. The next step will be to refine this block diagram into a real schematic.

While you're at it you can already start creating an astable multivibrator (short mv) and a monostable mv as (separate) simulation models in LTSPICE so you can find the required component values test both of the mvs individually,
The next step is then bringing the two models together and adding the gating function.
 
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