Hello! I'm trying to build a circuit from the first schematic on this page:
http://www.allaboutcircuits.com/vol_6/chpt_7/6.html

I fairly sure I built it all correctly, but when I turn it on, the first LED lights up and just stays lit! It doesn't shuffle through the LEDs, lighting each one in turn, like I thought it would.

I did make a couple of adjustments to the schematic:

• Used distinct LEDs
• Changed the resistor for each LED to 220 ohms
• Used a 9V power supply

I'm not sure if any of those adjustments would cause a problem, but I don't think they should. Does anyone know what the problem could be?

 

Make sure pins 13 and 15 of the 4017 are connected to GND.

 

Check pin 3 of the 555 and be sure that it is switching. If not, make sure pin 4 of the 555 is tied high.

 

If you understand how breadboard connects wire then try to make your own layout from the schematic. This way you can be sure you have connected it up correctly. Although the pictures of the breadboard layouts are nice, I find it somewhere confusing trying to count and follow the number of holes. Start by placing the ICs in the breadboard across the middle channel. Then make sure that you connect the power supply pins for both of the ICs. Then you can work your way round the IC's pin by pin ensuring you have connected everything correctly. Remember that the tracks run in sets of 5 contacts vertically usually. This article should help you understand how breadboard connects components together.

Try using this 555 astable calculator to work out a nice frequency like 1Hz. Easy values could be:

- R1 = 1K
- R2 = 10K
- C = 47uF

Make the 555 astable up with these values and connect a 680R resistor and LED to the output (pin 3) to see that the 555 bit is working correctly. Then once you have established this you can check that it is inputting to the 4017 counter correctly.

It's a very nice article too, I have to admit I have never looked at the articles by AAC very much.

 

I built a circuit using the first half of the schematic in the link I had above, with the output connecting to an led with a 220 ohm resistor, in order to see if the 555 part was working correctly, but the led doesn't even light. I switched out the 555 for another one just in case the first one was malfunctioning, but that didn't solve the problem. I checked all the connections as well and they should all be good.

 

I suggest taking a picture for more eyeballs here to see. Probably a "stupid mistake" you can't see yourself. We all make them.

 

I hesitated to mention this because I supposed that circuit has been built before and therefore must be correct. But I have never seen pin 7 connected directly to pins 2/6 as is shown there. Maybe that is ok, but attached is an schematic of an astable circuit that I know will work.

 

I built another circuit to test the 555 from the schematic on this page:
http://www.ohmslawcalculator.com/555_astable.php

I omitted the 10 uF capacitor coming from pin 5. For my values I used:
C = .1 uF
R1 = 1M
R2 = 1M

I connected the output to an led with a 220 ohm resistor and the led didn't light. Do you think the 555 could be malfunctioning? Maybe the 555 I switched it with didn't work either.

 

Oooh, excellent point (in #7). I don't think you'll be able to see the flashes, and the OP's circuit might be working fine. For one thing it's 13 Hz, which isn't super easy to see but also the duty cycle is so high that you'd have to see the very brief off period at that frequency. Much harder than seeing short flashes of light at 13 Hz.

 

R1, R2 = 1M seems rather high. Start off with something like R1 = R2 = 10k and C = 100μF.

 

Do you think the 555 could be malfunctioning?

I guess I'm back to asking for a picture of your build.

 

I built another circuit to test the 555 from the schematic on this page:
http://www.ohmslawcalculator.com/555_astable.php

I omitted the 10 uF capacitor coming from pin 5. For my values I used:
C = .1 uF
R1 = 1M
R2 = 1M

I connected the output to an led with a 220 ohm resistor and the led didn't light. Do you think the 555 could be malfunctioning? Maybe the 555 I switched it with didn't work either.

The circuit you describe here works fine. You may have your LED connected backwards; you want the anode toward pin 3 and the cathode to ground.

ETA: The capacitor from pin 5 to ground is 10 nF, not 10 μF. I always put it in if I am not using pin 5 for control.

 

Based on my test results, I think the original drawing referenced in post 1 is wrong. There should be another 1M resistor between pin 7 and pins 6/2.

 

It should produce a negative going edge for the 4017, as is. I suppose they left off "R2" to minimize the parts count.

 

I guess I'm back to asking for a picture of your build.

Haha sorry, I'll have to get back to you tomorrow, I don't have the circuit with me right now.

 

It should produce a negative going edge for the 4017, as is. I suppose they left off "R2" to minimize the parts count.

Okay, I shorted pin 7 back to pins 6/2, and I can see the negative pulse on my old scope, but it is so brief that I can't trigger on it. It looks like it might be less than 5 μS. For the cost of a resistor, I would put one in and make the pulse long enough to see.

With R1 and R2 at 1M, and C1 at .1 μF, you get a nice 4.8 Hz clock with a 67% duty cycle.

 

The pulse width when 2,6, and 7 pins are shorted would be dependent on the 555 used.

I'm sure with a modern DSO, and auto triggering, it's much easier to find the pulse.

CLocking with a sub-10 microsecond pulse is extreme when the frequency is 1 Hz.

 

I finally fixed the circuit! It turns out the 4017 was malfunctioning; I replaced that and everything works fine now.

Thanks for all the help!