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555 timer chip question - Page 2

post #11 of 37
Thread Starter 
Quote:
Originally Posted by radodrill View Post
No; the only affect that they have is on the timing of the flashes



The output of the 555 timer on pin 3 uses TTL (CMOS) logic signals where a false is a voltage ~0V and a true is an output voltage ~3V. As such, it it possible to light an LED directly from the output; however, the 555 timer can only provide a limited amount of current to the output as it is intended for logic/signaling (e.g. clock generator), drawing too much current from the output will cause the timer to fry or possibly explode. By connecting the output of the timer to the base of a switching transistor, you can control higher current (and higher voltage) devices without overloading timer.

Essentially you would wire the base of the transistor to the 555 output and the emitter of the transistor to ground then the light array would be connected between the 9V and the collector of the transistor. In this situation the transistor is operating as a high speed switch that allows current to pass through when the output of the timer is high (true).

Looking on the RadioShack website, they do carry a TIP120 transistor, which is nearly identical to the TIP122 and will work interchangeably.
Ok I get most of what you're saying. But I don't get where to wire the LED's and transistor related to the rest of the diagram. If you could edit that diagram real quick in paint that would be great. Sorry, I know you're always the one helping me with this
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post #12 of 37
Quote:
Originally Posted by Boris4ka View Post
But I don't get where to wire the LED's and transistor related to the rest of the diagram. If you could edit that diagram real quick in paint that would be great
Rather than drawing it in paint, I found a circuit diagram that shows what I'm talking about; this is the same timing circuit but with different values of R1, R2, and C1 (but you can replace those with the values the other circuit uses). You would wire in your LED array in lieu of the transformer that's in the circuit below. For your application, the pull-up resistor (50R in the diagram) between the 555 and the transistor isn't necessary, but I'd probably use a 150R-300R resistor to ensure you don't overload the 555 timer.

BTW, here is a good short read on the 555 timer that also includes the equations for the timing of the output signal.

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post #13 of 37
Well rado already replied, but this is what I thought he meant. His diagram above is more elegant however.



with 5 sets of two LEDs (only two drawn there...)

I'll be keeping this thread referenced, I know I've needed diagrams like this in the past.

Edited by otterpopjunkie - 1/21/10 at 7:41am
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post #14 of 37
Quote:
Originally Posted by otterpopjunkie View Post
Well rado already replied, but this is what I thought he meant. His diagram above is more elegant however.

...

with 5 sets of two LEDs (only two drawn there...)
essentially yes; except I would have put the resistor and the LEDs between the +9V and the collector of the transistor and shorted the emitter of the transistor directly to ground
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post #15 of 37
OK here's a pretty version


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post #16 of 37
Thread Starter 
Quote:
Originally Posted by radodrill View Post
OK here's a pretty version

[]http://www.overclock.net/attachment.php?attachmentid=138129&stc=1&d=1264113 986[/IMG]
Thank you so much I think I understand how to wire it now. But one thing first, you have pin4 going to +9v, while the original diagram says to not connect pin4 to anything.

Also what do the double arrows for the LED's mean?

And in your diagram, for the TIP120, would pin3 go to B, -9v to E, and the -LED's to C

And what about pin1, where do I ground it to if my case for the project is plastic?

Edit: and what about the 0.22 uF capacitor - the lowest I found was 1.0 uF. Does he mean 22uF, or 220uF?

Thanks again.
Edited by Boris4ka - 1/21/10 at 5:14pm
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post #17 of 37
Quote:
Originally Posted by Boris4ka View Post
Thank you so much I think I understand how to wire it now. But one thing first, you have pin4 going to +9v, while the original diagram says to not connect pin4 to anything.
When wiring the 555 timer as an astable oscillator, pin 4 should be tied to the Vcc, but they did not depict that

Quote:
Originally Posted by Boris4ka View Post
Also what do the double arrows for the LED's mean?
The main part that's wired in is simply a diode element; when the 2 arrows or 2 wavy lines are added like that, that indicates that the diode is giving off optical energy and identifies it as a LED.

Quote:
Originally Posted by Boris4ka View Post
And in your diagram, for the TIP120, would pin3 go to B, -9v to E, and the -LED's to C
yeah, pin 3 of the 555 goes to the TIP120 base, Gnd goes to TIP120 emitter, and the - side of the LEDs go to the TIP120 collector

But looking at the diagram I put in a PNP transistor instead of a NPN transistor

Quote:
Originally Posted by Boris4ka View Post
And what about pin1, where do I ground it to if my case for the project is plastic?
I used the ground reference symbol to show that was the 0V reference in the circuit. The standard chassis ground is shown below.


Quote:
Originally Posted by Boris4ka View Post
Edit: and what about the 0.22 uF capacitor - the lowest I found was 1.0 uF. Does he mean 22uF, or 220uF?
You should select a capacitor that sets the duty cycle such that it matches your desired flash rate, rather than just blindly selecting a capacitor. The page I linked earlier has the necessary equation(s) to determine the best values for R1, R2, and C1.
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post #18 of 37
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Alright, well I built the circuit on a bread board, twice (completely rebuilt it twice). Radodrill, unfortunately your method doesn't work. The TIP120 lets the LED's flash like they're supposed to only for a second, and then they just stay normally lit. It works fine without the transistor. Also, pin 4 does have to stay not connected to anything - if I follow your diagram, it doesn't work. I'll post a pic once I solder everything to a circuit board.
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post #19 of 37
Quote:
Originally Posted by Boris4ka View Post
Radodrill, unfortunately your method doesn't work. The TIP120 lets the LED's flash like they're supposed to only for a second, and then they just stay normally lit.
Got any pics of the wiring? It's possible that you're using the wrong pins on the transistor; as the TIP 120/122 transistors use BCE while the small transistors use CBE.


I'll try it out on a breadboard myself later on, but the circuit I provided should work.
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post #20 of 37
my money's on Rado
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