# Relatively simple school project involving an NE555 timer

Discussion in 'The Projects Forum' started by Davidred7697, Dec 15, 2014.

1. ### Davidred7697 Thread Starter New Member

Dec 15, 2014
18
2
Hello, I just joined this forum today when I discovered that the community can provide help with school assignments. This is my very first post, so pleas go easy on me.

The instructions were essentially to build a relatively simple circuit and then present it in front of the classroom and turn in a detailed report. I am 20 years old attending a local community college in Illinois, with hopes of transferring to the University of Illinois at Urbana-Champaign or UIC. The class is called "Intro to Digital Systems". I had NO prior experience with electronics/electrical systems before this class aside from basic physics in high school. I think it would have been ideal to take a more basic class electronics class, such as Electronics 101 before jumping into ENG250, but unfotunately I was never recommended to do so (and ELT101 is NOT a requirement).

Unfortunately, I have been unsuccessful in building a functional circuit and would appreciate a little help with troubleshooting if possible. I will have to admit that I did not create/design the circuit myself and instead found it in a page that presents simple electrical/electronic projects for electrical engineering students.

I have attached the circuit diagram in this thread for reference.

I will also provide a link to the page where I found this project; it also includes a brief description of the circuit as well as the circuit diagram in case the attached file doesn't open for any reason: http://www.electronicshub.org/bike-turning-signal-circuit/

Here is also a link to a YouTube video that is embedded in the above page demonstrating how the circuit should work/look:

I am using a breadboard. I have built this circuit twice. The first time, the LEDs in the circuit lit, but did not pulse as they're supposed to. Also, I realized after a few hours of trying to solve the problem that 1) for some strange reason, the LED were brighter when I touched the top of metal transistors with my fingers. 2) I had not connected the ends of the 10k ohm resistors along the same path as indicated in the circuit diagram.

Because the above circuit did not work properly and I asked a laboratory manager for assistance to no avail (and an engineering instructor denied helping me at all twice and provided very little help a third time), I decided to re build the entire circuit once more. So I did just that. I re built the entire circuit, this time making sure that the 10k ohm resistors were placed properly. As I was building it, I was honestly expecting a fully and properly functioning circuit, thinking that I "fixed" a few previously made mistakes. Regrettably, this did not turn out to be the case.

This time, no LEDs lit up at all. Again, after hours of troubleshooting the circuit myself and then with assistance from a lab manager, no solution was found. In this instance, an electronics professor provided little assistance by saying: "check your ground". One strange phenomenon that I noticed was that when I touched the red/positive lead of a Digital Multimeter (DMM) to one of the legs of the LEDs (can't remember if it was the anode or cathode), that LED lit up! Also, if I touched the same lead to the following LED, both LEDs lit up. I could not get more than two LEDs to light up simultaneously. One more strange thing is that when I touched the same positive lead of the multimeter to one of the legs on the 330 ohm resistors, the voltage indicator on my power supply went from 12V down to nearly 0V.

These are the steps that I've taken myself and that others have helped me take to try to find a solution to my non-functioning circuit(s).

I would greatly appreciate any assistance based on the above information and I will be posting various photos that I will take of my circuits tomorrow, when I have access to it.

ANY feedback is greatly appreciated and I apologize for the long post. Please let me know if I forgot to include something in my post. Thanks!

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2. ### iimagine Active Member

Dec 20, 2010
129
9
Q1-Q5 emitters should be connected directly to ground, not R5, R8, R10 or R12

3. ### vu2nan Member

Sep 11, 2014
44
10
Last edited: Dec 16, 2014
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4. ### WBahn Moderator

Mar 31, 2012
20,068
5,666
This seems like a strange project to use in a course titled "Intro to DIGITAL Systems". Is your instructor aware that you are just copying a circuit from the internet for your project? If they are okay with that, then great. But I would think (hope) that the intent would be for you to actually design something (and preferably a something that involves some digital logic).

I don't have the bandwidth to watch the video, so I'm guessing at what the circuit is supposed to do from a quick glance at the schematic. I suspect it is supposed to turn on the LEDs sequentially, first one pair (D2,D3), then a second pair (D4, D5 in addition to the first), then a third pair (in addition to the first two pair) and so on.

But I don't see how the circuit in the schematic can work as given. I think that the bottom of R5, R8, R10, and R12 should be connected to ground (the negative terminal of the 12V battery) and that the bottom of R6 is NOT supposed to be connected to these four resistors. Otherwise you essentially have the emitter shorted to the base of Q1. But it isn't quite that simple and there appears to be other problems with the circuit. The values of R5 (and the others) seems too high for what I have in mind, which makes me suspect that they are supposed to be base resistors only and that the emitters are supposed to be connected directly to ground. Assuming that the intent is for the LEDs to come on as I described, I'm thinking that the resistors are supposed to be a voltage divider chain with each transistor base being driven by one of the junctions. This should result in the right-most pair of LEDs coming one and then progressively working it's way to the left.

It sounds like you are wiring up the entire circuit and then hoping that it will magically work when you apply power and, when it doesn't, trying to troubleshoot the entire circuit. Don't do that. Build up the circuit a little at a time and test it as you go. The first thing is to get the 555 timer part working, so omit D1 and everything to the right of it. See that you get a rectangular waveform at pin 3 of the 555 and that you have a signal that ramps up and down at the junction of R2 and C3.

You should bypass pin 5 of the 555 to ground with about a 0.1uF cap (I'm going from memory and it has probably been 20 years since I've done a 555 circuit, so you should check in the E-book for a solid recommendation on this).

Once you have that part done, put in D1 and you should see something that is more like a sawtooth waveform at the junction of R2 and C3. The bottom voltage should be about 0.7V (which is why D12 is used in the full circuit). Get that far and then lets go from there.

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5. ### vu2nan Member

Sep 11, 2014
44
10
Hi Iimagine,

In that case the LEDs will all light up together instead of in sequence!

Regards,

Nandu.

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6. ### Bernard AAC Fanatic!

Aug 7, 2008
4,534
476
That ckt. is doomed from the start-- except for the 555 part. If you want a short chaser try a 4017 IC with the modified 555.

7. ### vu2nan Member

Sep 11, 2014
44
10
Hi WBahn,

It'll work!

R6-R12 are the base bias resistors.

The first to switch on will be Q5, since its emitter is connected to ground through the 1N4148, then Q4, Q3, Q2, and Q1 in that order.

The emitter currents of Q1, Q2, Q3 & Q4 will flow through the bases of the transistors that follow.

Regards,

Nandu.

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8. ### ISB123 Well-Known Member

May 21, 2014
1,240
537
NE555 is mix of analog and digital components.Are you supposed to use ne555 or you picked it because you heard ne555 is "simple".

9. ### Alec_t AAC Fanatic!

Sep 17, 2013
7,024
1,453
I agree with Nandu. The original circuit should work as is. If it doesn't, you have an incorrect or missing connection somewhere.
Note that the 555's pins 4 and 8, and the top of R2, all connect to battery positive. The circuit shown is not drawn very clearly. Make sure you have the diode connected the right way round (cathode to battery negative).

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10. ### iimagine Active Member

Dec 20, 2010
129
9
OK, my bad, I apologize. It was late and I didn't bother to analyze it properly and was quick to make assumption. I agree that the circuit should work as you described.

That shouldn't happen! Are you certain that you are using NPN transistors? I would suggest that you disconnect those transistors form the 555 circuit and apply 12V directly to those 10k resistors, and see if they lit up; Testing sub-circuits this way will help identifying the problem faster.

11. ### Davidred7697 Thread Starter New Member

Dec 15, 2014
18
2
Hi, guys!

I just woke up and wanted to check up on my post. I must say this is surprising. I did not think I would get any replies. Instead I received so many helpful replies from seemingly knowledgeable people (sounds corny, but true) . Thanks!

Now that I look back at my post, I realize that one thing I forgot to mention in the OP is that, since the lab managers said that they did not have any BC547 transistors and suggested I use alternatives that I could find online. So I did just that. I went online and found the following thread:http://www.edaboard.com/thread181945.html

I used the 2222 transistors as alternatives for the first circuit I built and used the 3904 or 4401 on the second circuit. I'm unsure if this will make a difference, but I think its worth mentioning.

Essentially, what I'm understanding is that the circuit I posted will not work and that I either have to make a few modifications or build a similar working circuit (that I can see at instructables. Com)?

I am typing this through the mobile platform. Needless to say, it is very cumbersome to do so, to say the least. Bear with me. Ill be at school soon. Thanks again.

12. ### Davidred7697 Thread Starter New Member

Dec 15, 2014
18
2
Hi, guys. My deadline is in two hours. I also have a final exam today, so I will end up studying a little more for that if I don't get this circuit fixed. As promised, I will upload a few pictures of both circuits that I built. Remember that the first one I know for sure that it isn't wired properly and the second one is the one I am unsure about.

The circuit with the red-colored LEDs is the first circuit I built and, of course, the circuit with the green-colored LEDs is the second. I apologize if my circuits look a little messy.

13. ### WBahn Moderator

Mar 31, 2012
20,068
5,666
I'm not too keen on the combined emitter currents for four transistors having to flow through the base of Q5. That's going to be a base current of somewhere around 60mA to 70mA. Strikes me as poor design.

14. ### WBahn Moderator

Mar 31, 2012
20,068
5,666
The bottoms of your 10kΩ resistors aren't connected to anything. Be sure that you understand how the slots on your breadboard are connected!

15. ### Davidred7697 Thread Starter New Member

Dec 15, 2014
18
2
I noticed that, in my second circuit (the one with green LEDs), one of the transistors (the one at the very bottom in my pictures is actually connected to two resistors and not just one like the others. So I immediately tried to connect a resistor between the *base* of the transistor and along with the rest of the resistors, but realized that I no longer had space. So I came up with a way to connect both resistors on another column. I hope it's right.

Another thing I saw is that the NE555's Vcc input was not connected to the power supply! So I made the proper adjustments.

I then proceeded to test my circuit once more and, of course, it did not work.

I'll post a few pictures of my updated circuit below:

Another thing I've noticed is that voltage is being dropped through the LED's, which means that current is lowing through them, but they don't light up. I have also tested one of the LED's individually and it does light up. There must be some other obvious thing I'm failing to understand.

16. ### wayneh Expert

Sep 9, 2010
13,435
4,273
Where's D12?

Oh, I see it. Is it backwards?

I was thinking you could work on each LED pair individually by moving D12 along. Make the first pair work, then move to the second, and so on. Divide and conquer.

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17. ### WBahn Moderator

Mar 31, 2012
20,068
5,666
So have you gotten the 555 portion working yet? Or are you still building up the whole thing and then wondering why the whole thing doesn't work?

18. ### Davidred7697 Thread Starter New Member

Dec 15, 2014
18
2
Yes, thank you for the suggestion. As I stated earlier, I am aware that the wiring is wrong on the first circuit (the one with red-colored LED's). The one that I am currently trying to troubleshoot is the second circuit I built (green LEDs).

19. ### Davidred7697 Thread Starter New Member

Dec 15, 2014
18
2
Sorry. I am not entirely sure exactly what each pin of the NE555 timer does. All I know is that the timer is supposed to send a "pulsing" signal, which makes the LEDs alternate from a HIGH state to a LOW state repeatedly.

I have attempted building only one part of the circuit (the pair of LEDs that are attached to D12) and used a pulse generator instead of the NE555 timer and got it working.

Two pairs of LEDs now light up, but no pulsing. I'm guessing there is a problem with the wiring around the NE555 timer.

What other information can I provide for more assistance while I try to figure out what I'm doing wrong?

To be clear, I do not want "someone else to do my project for me". I have spent about a dozen hours trying to build this simple circuit and it doesn't seem to be working. I'm pretty much out of ideas and have no tutors at my school to talk to. I hope you guys understand.

20. ### wayneh Expert

Sep 9, 2010
13,435
4,273
That's all I saw in that first video.