Has anyone been able to get this to work?

Thread Starter

jordanwb

Joined Jun 22, 2008
30
I'm trying to build this circuit(the first one) for my traffic lights, but it's not working in real life. It works in KTechLab on my computer but not a real breadboard. What's happening is that both LEDs stay on and don't cycle.

The only thing I did different was that I substituted 47K ohm resistors for the 100K. I tested all the individual components and they all work.
 
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Wendy

Joined Mar 24, 2008
22,146
It is a basic circuit, the kind they taught in my college. Both LEDs dark?

The schematic is a bit unclear, in that the crossing wires should not be connected, but like I said, it is a text book drawing. My first thought is the caps may be flawed, or a transistor. How sure are you of your components?

Try shorting each 100KΩ resistor with a 4.7KΩ-10KΩ resistor, this should cause the matching transistor to light up the LED, verifying the gain of the transistor and checking out the LED and resistor. Short the Collector Emitter of the transistor, this will verify the LED polarity is correct and the resistor is doing it's job.
 
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Wendy

Joined Mar 24, 2008
22,146
Sorry, missed that part, lift the base resistors, the LEDs should go out. It sounds like the gain of the transistors may be higher than the original design called for, if they go out try increasing them to 1MΩ. If you get a slow flash rate then bump the capicitors values down by the same ratio (0.1uF).
 

Thread Starter

jordanwb

Joined Jun 22, 2008
30
I took out the resistors connected to the base and the LED's did not go off. Could it be I mixed up Base and Collector? I always forget which is which.
 

Wendy

Joined Mar 24, 2008
22,146
It might still oscillate, check this circuit out for example.

Just curious, do you have something against 555 timers? I came up with this circuit a while back for the same application.

 

Thread Starter

jordanwb

Joined Jun 22, 2008
30
No I don't have anything against 555 timers. I didn't look for a schematic using one because I don't have any 555 timers. The Source (Canadian version of Radio Shack) has them, I'll go pick one up tommorow. That seems even easier than the one I was using.

Regarding your double post (shame), I did check to make sure they were NPN transistors. I'll change the Base and collector around and see what happens.

[Edit] It works! I got base and collector backwards.
 

Wendy

Joined Mar 24, 2008
22,146
My suggestion, use what works for you, don't spend any more money than you have to. My tagline says it all.
 

SgtWookie

Joined Jul 17, 2007
22,210
2N3904 transistors are mighty inexpensive compared to 555 timers. Of course, they're not giving capacitors away either nowadays... :rolleyes: Good job on getting it going :)
 

Thread Starter

jordanwb

Joined Jun 22, 2008
30
Well $3.99 for a 555 at The Source isn't a big deal for me. Thanks.

Oh BTW I hooked up two red and two green LEDs in parallel on each side. The red LEDs turn on but the green don't. If I removed the Red LEDs, the green LEDs turn on. Do Red LEDs take more power than Green LEDs?
 
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Wendy

Joined Mar 24, 2008
22,146
For that much you could order 10 from Jameco, look for the TI versions (lots of people manufacture 555's).

The red leds drop a different voltage than green leds, and there is a difference between older generations and newer. Shouldn't make much difference to this circuit, but the 555 version cares even less. You can put a variable pot for the resistor to tweak to exactly the rate you like on the 555 circuit.
 

Thread Starter

jordanwb

Joined Jun 22, 2008
30
No I don't have them yet. They're very small lights, so I don't think they'll take a lot of power.

I just thought of something. What if the traffic lights I get have a yellow light? Then I don't think this circuit will work.

I found this one though: http://home.cogeco.ca/~rpaisley4/20step.html

The thing is I don't have the materials to make PCBs.
 
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SgtWookie

Joined Jul 17, 2007
22,210
No I don't have them yet. They're very small lights, so I don't think they'll take a lot of power.
The thing is, that you will need to limit the current through your lights so that you don't instantly vaporize either the light filament itself or the transistor(s). You will probably want to run them at a slightly lower voltage than they were designed for in order to make them last a long time.

I just thought of something. What if the traffic lights I get have a yellow light? Then I don't think this circuit will work.
No, it won't work as-is. It could be modified to add another RC timer and transistor driver, though. [eta] See the attached for an initial stab at a green-yellow-red-yellow-repeat sequencer. C1 and C3 should be reduced to get a shorter time for the yellow lights to be on.

I found this one though: http://home.cogeco.ca/~rpaisley4/20step.html

The thing is I don't have the materials to make PCBs.
Funny, that's the exact same page I suggested to you in an earlier post on your other thread; next to last on this page:
http://forum.allaboutcircuits.com/showthread.php?t=12515

Rob sells the bare boards for $12/ea, and the complete kit for around $25.

However, Rob's circuit is designed to drive LEDs. I suggest that before you experiment further, you obtain the specifications for the bulbs in the signals that you will be receiving.

Or, do you have a link to the page where you ordered them from?
 

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Thread Starter

jordanwb

Joined Jun 22, 2008
30
The thing is, that you will need to limit the current through your lights so that you don't instantly vaporize either the light filament itself or the transistor(s). You will probably want to run them at a slightly lower voltage than they were designed for in order to make them last a long time.
I have a 6 Volt DC transformer which I think will work nicely. Plus the transistors I bought today are rated for 30 volts.

Funny, that's the exact same page I suggested to you in an earlier post on your other thread; next to last on this page:
http://forum.allaboutcircuits.com/showthread.php?t=12515
:D

However, Rob's circuit is designed to drive LEDs. I suggest that before you experiment further, you obtain the specifications for the bulbs in the signals that you will be receiving.
The lights in the traffic lights are very small, about 3/16" in diameter, although I am aware than by nature, Incandescent bulbs take more power than LEDs. I found a store on my side of town that specializes in electronics.
 

Thread Starter

jordanwb

Joined Jun 22, 2008
30
Not where I live - oh wait you said modern. Lol. I went to the store today and they had a PCB with all the parts (you have to solder it together), needed to make of all things, a traffic light controller. I'm still not sure which to go for because I don't know if the semi-assembled one will handle non-LED bulbs. I one I/you found looks like it would.
 

SgtWookie

Joined Jul 17, 2007
22,210
Not where I live - oh wait you said modern. Lol. I went to the store today and they had a PCB with all the parts (you have to solder it together), needed to make of all things, a traffic light controller. I'm still not sure which to go for because I don't know if the semi-assembled one will handle non-LED bulbs. I one I/you found looks like it would.
You can always use a driver transistor or Darlington IC for that sort of thing.

Actually now that I've had a chance to think more on this, I'd drawn up a circuit for a fellow months back that with some minor alterations should work quite well for something like this. I'll re-visit it and post something later.
 
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