Electronics > Beginners
Traffic light circuit with 556 timer?
electricMN:
--- Quote from: phennessey on December 23, 2019, 02:20:41 am ---...I don’t have any circuit software, so I’m drawing stuff with Adobe Illustrator....
--- End quote ---
Download Kicad for creating your schematics. It's free.
SmokeyTheElectrician:
--- Quote from: phennessey on December 23, 2019, 02:20:41 am ---... this circuit cannot be done with a 556...
--- End quote ---
With some minor change to the circuit, it can be done.
--- Quote from: phennessey on December 23, 2019, 02:20:41 am ---...Frankly, most of this is over my head....
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My apologies, with the schematic you presented, I over estimated your level of experience.
--- Quote from: phennessey on December 23, 2019, 02:20:41 am ---I was just hoping someone would sketch out a 556 timer circuit
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stay tuned ( I misunderstood and thought you wanted corrections so you could draw it )
--- Quote from: phennessey on December 23, 2019, 02:20:41 am ---Regardless of what happens, I plan on building this thing and I don’t care how long it takes me! Parts are ordered and that’s that!
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that's the spirit
--- Quote from: phennessey on December 23, 2019, 02:20:41 am ---I plan on attempting this circuit in 4 different ways:
1. 2x 555
2. 1x 556
...
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again, stay tuned
&
I look forward to seeing your posts as you realize
the other versions of the circuit
--- Quote from: phennessey on December 23, 2019, 02:20:41 am ---Everyone’s help has been incredibly valuable. I will post results here!
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I entirely agree
SmokeyTheElectrician:
Ok, here is the original circuit entered in LT SPICE
( LT SPICE is free for all to use, when you feel ready,
you can get off of Linear's website:
https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html
)
--Notice the 555's symbol follows the actual chip's pinout.
Also I modified the LEDs' current limiting resistors to have the LED current under 20mA.
When I ran the simulation, it does not quite work.
The orange LED stays on through the red LED's phase.
SmokeyTheElectrician:
Studying the waveforms from the prior post:
Notice I_red_led follows Timer1's output (Vtmr1), but is inverted
Also notice I_green_led follows Timer1's output (Vtmr2).
Then there is that space in time between when I(D_red) and I(D_green) is flowing
where we would like current to flow in the Orange LED - I(D_orange).
We ask ourselves is there anything unique in the relationship between
12v, Vtmr1, Vtmr2, and ground during that time interval that does not
( edit: /\ Discharge1, Discharge2, )
exist during the interval when the Orange LED should be off?
Well if you drive the Orange LED with Timer1 and sink it with Timer2,
you may observe Vtmr1 is high and Vtmr2 is low during the time
and only during the time the Orange LED should light up.
For all other time intervals Vtmr1 and Vtmr2 are both either high or low
simultaneously.
Ideally (other than that glitch) the waveforms we would like are as follows:
The only modification (correction to your drawing?) to the circuit
was to re-route the high side of the Orange LED circuit from 12v to
the Timer1 output ( V(tmr1)), like in the following diagram:
SmokeyTheElectrician:
You may recall Mr "schmitt trigger" pointed out in post 3 that Timer2 was powered by Timer1's output.
( generally bad form as small form IC outputs generally to not have the capacity to supply much current )
Then in post 15 Mr "schmitt trigger" advised exploring not powering up Timer2 in the above manner
as well as exploring what the 555's Reset functionality may do for us.
I made a quick scan of the datasheet for reset operation.
According to what I read and saw on the block diagram, reset is an active low input, upon which the
output would go low ( no current {=== no light} possible for the green LED)
and the internal open collector discharge transistor would be de-energized
(off ---> orange LED off).
Hmmm,
the Red LED is wired to be on when Timer1 Output is low
if that signal is fed into Timer2's reset, both the green and orange LEDs will stay off
because the second 555 is held in the reset state.
Then when Timer1 goes high, the red LED will turn off
and Timer2 will be enabled and operate as discussed in
posts 17, 27, and 29 (29 has corrections to 27).
Ok, by now we see that we would like the second 555 powered off of 12v
and
it's reset pin driven by Timer1's output, like in the following schematic:
( the waveforms are almost exactly the same as the last set posted
. -- timing is the same, just Vtmr2 and associated currents are different due to the more proper voltage supply for 555 #2 )
Now this circuit is entirely realizable in a single 556.
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