555 Timer...not timing?

[Mad Mart]

Hi all

Although I'm getting on a bit in age I've just started to get interested in Electronics, so have spent the last week or so reading and watching videos on YouTube.

Anyway, I bought a starter kit and decided to make a circuit using the 555 chip. The circuit has two LEDs. No.1 LED is on until you press the momentary switch at which time no.1 LED goes out and no.2 LED switches on. The problem is no.2 LED should stay on for a certain length of time before the timing cycle ends and then goes out, turning on LED no.1. What my circuit does is just to toggle between no.1 & no.2 LEDs when I press the switch. I'm using a 100k pot and a 1000uF 25V Cap.

I've attached a photo to hopefully clarify my circuit...please be gentle with me.

[rob77]

posting the schematic would be better in fact most of the guys here will not even have a look without a schematic diagram posted (and i fully understand them )
anyways... is that resistor in the top left corner connected to the + rail ? - if not then that's your problem. and do i see it correctly as 220R ? - should be bigger (few Kohm) and connected to the + rail.

[Mad Mart]

Yes, the resister is connected to the +ve rail. It was a 220R but I've now swapped it for a 2K2, it made no diference though.

I agree I should have posted a schematic...I have now. Borrowed from a Rob Winter tutorial video.

[polishdude20]

It looks to me like in the picture, your ic isnt connected the right way. You should have pins 6 and 7 tied together but it looks likeyou tird together 2 and 3 . Im just looking at that dot there and it looks like its inthe wrong place!

Edit: nvm stupid me. The dot confused me but since your chip has a cutout id base it off that. So your picture is actually correct. My bad.

[polishdude20]

Try grounding pin 5 with a 0.01uf cap. Also try changing the resistors of the leds around

[TerminalJack505]

I'm guessing your pot isn't wired-up quite right.  To test that theory you can try replacing it with a 4.7K resistor.  That should give a ~5 second delay.

[c4757p]

If the pot's disconnected, the output should stick high. Toggling will be observed if either the discharge pin (pin 7) or the capacitor are disconnected. Check those.

[Mad Mart]

I've tried all your suggestions so far. Still no difference. I measured the voltage for pin 6 and when I press the switch it goes immediately to 0V. If, however, I keep the switch depressed then the voltage will gradually build up to ~2.3V over a period of about 16 seconds.

I've just noticed that if I take the pot out, it has no effect on the circuit.

[Bryan]

Are you sure you have the polarity of the power supply correct.?

[rob77]

Are you sure you have the polarity of the power supply correct.?

according to the picture he has the polarity right - he's even strictly using RED wires for + rail and black wires for - rail , and other colors for the "signal" wires. furthermore on the picture you see that a wire with a white strip (+) from the wall-wart is connected to the red rail on the breadboard.

[adi101]

It looks like the pot is marked 104, right? This means the value is 100K, not 3.3K.
My suggestion is to use a 10uF capacitor instead of 1000uF.

[rob77]

probably the 555 is dead, try a different one.

[Bryan]

Are you sure you have the polarity of the power supply correct.?

according to the picture he has the polarity right - he's even strictly using RED wires for + rail and black wires for - rail , and other colors for the "signal" wires. furthermore on the picture you see that a wire with a white strip (+) from the wall-wart is connected to the red rail on the breadboard.

One of my walwart's has a stripe on the negative (-) wires. Worth a double check. 

[eetech00]

hi

Here's something to check.

Sima shows R2 as a 3.3k resistor.
Note that the output eventually returns to 0v.

Now look at simb
Simb shows R2 as a 100k resistor.
Note that the output never returns to 0v.

I'm guessing something wrong with potentiometer.

eT
 

[Electronics-Repairman]

Having used the 555 for a long time 1000 uf seems a lot to me ,pin 6/7 are wired together  someone suggest they weren't ,pin 5 should have a 0.01 uf ceramic to ground, if you scope pin 3 you should get a timing pulse, as suggested you may have toasted it,pin 3 is the output side , so you should get a signal off that, the pot seems to bother me two, i've always used a 3 pin pot, with the center pin wire to one of the others !!!

[rolycat]

Your description of the behaviour of the circuit sounds as though it is connected correctly except the timing elements VR1 and C1. If all else has failed, it can be worth reassembling everything from scratch - at least you are likely to encounter a different type of fault 

Since I have far too much time on my hands at the moment and haven't played with a 555 for a while, I recreated the circuit you gave using a similar layout but shorter wires, which will hopefully show the connections more clearly.

If C1 is disconnected it exhibits the same behaviour you describe for your circuit, otherwise it works as expected.

VR1 is a 100k trimpot and currently C1 is a 100uF electrolytic, although I tested a 1000uF and that works too, with a long maximum delay. Other resistor values are as in Rob Winter's schematic.

With these values, the green LED D1 lights for around 15 seconds after SW1 is pressed with VR1 at its maximum.

Hope this helps.

[edy]

Have a look here... Lots of schematics using the 555 that may help:

http://www.talkingelectronics.com/projects/50 - 555 Circuits/50 - 555 Circuits.html

Lots of free eBooks on that site, good to learn from!

[Mad Mart]

 
I have just rebuilt the circuit similar to rolycat's, and guess what...it's just the same as before. I've swapped the 555 out about 4-5 times now and that has made no difference either. When pin 2 goes low it should make pin 3 go low also, which it does. But it should also start the timing on pin 6, which it doesn't. That's obviously where the problem lies. It can't be the pot as I've swapped that out several times for a straight resistor. The only other component is the cap and I've swapped that around I don't know how many times.

[rolycat]

I have just rebuilt the circuit similar to rolycat's, and guess what...it's just the same as before. I've swapped the 555 out about 4-5 times now and that has made no difference either. When pin 2 goes low it should make pin 3 go low also, which it does. But it should also start the timing on pin 6, which it doesn't. That's obviously where the problem lies. It can't be the pot as I've swapped that out several times for a straight resistor. The only other component is the cap and I've swapped that around I don't know how many times.

It definitely sounds as though your capacitor isn't charging properly via the pot. When you say you've swapped it around, do you mean you've tried alternative values?

Try breaking it down. Attach your multimeter across the capacitor pins, and disconnect the link to the 555 pins 6 and 7. Briefly short the capacitor pins and then watch the voltage. Does it climb rapidly to almost 9V with the pot at its minimum limit, and via the expected charge voltage curve at intermediate settings?

Leave the pot at an intermediate value and reconnect pins 6/7. What happens to the voltage? It should start at around 0V, climb steadily to about 6V after SW1 is pressed and while D1 is lit, and then jump back to 0V.

Another possible source of problems is the breadboard. Cheap ones especially can introduce mysterious problems due to intermittent or even failed contacts. One way to check this is to move connections systematically to different strips.

[Mad Mart]

Try breaking it down. Attach your multimeter across the capacitor pins, and disconnect the link to the 555 pins 6 and 7. Briefly short the capacitor pins and then watch the voltage. Does it climb rapidly to almost 9V with the pot at its minimum limit, and via the expected charge voltage curve at intermediate settings?

Done that and the voltage climbs as you described. Not sure about the curve?

Leave the pot at an intermediate value and reconnect pins 6/7. What happens to the voltage? It should start at around 0V, climb steadily to about 6V after SW1 is pressed and while D1 is lit, and then jump back to 0V.

It doesn't do that. What happens is, if you keep the switch held down the voltage will climb to the limit set by the pot. When you let go the voltage drops to 0V.

[rolycat]

Try breaking it down. Attach your multimeter across the capacitor pins, and disconnect the link to the 555 pins 6 and 7. Briefly short the capacitor pins and then watch the voltage. Does it climb rapidly to almost 9V with the pot at its minimum limit, and via the expected charge voltage curve at intermediate settings?

Done that and the voltage climbs as you described. Not sure about the curve?

When a capacitor is charging the voltage rises quickly at first, then more slowly as it gets closer to the supply voltage. Google "Capacitor charging" for the details.

Leave the pot at an intermediate value and reconnect pins 6/7. What happens to the voltage? It should start at around 0V, climb steadily to about 6V after SW1 is pressed and while D1 is lit, and then jump back to 0V.

It doesn't do that. What happens is, if you keep the switch held down the voltage will climb to the limit set by the pot. When you let go the voltage drops to 0V.

The pot isn't setting a voltage limit, it's controlling how much current flows into the capacitor, and thus how quickly it charges up. When the voltage at IC pin 6 reaches 2/3 of the supply voltage it ends the timing period, so the trigger voltage will always be 6V with a 9V supply.

You shouldn't need to press SW1 for more than an instant.

You will get the behaviour you describe if the pot is at its minimum setting, since the capacitor will be connected almost directly across the supply rails and will charge up immediately. Are you sure you are setting it to a halfway position? If you turn off the supply and disconnect the capacitor, what resistance do you read between IC pin 6 and pin 8?

[Mad Mart]

I'm getting 1.1k across pins 6&8.

[rolycat]

Sounds reasonable. What value capacitor are you using with that?

If you try my earlier suggestion again and time how long the capacitor takes to charge to 6V through that resistance when disconnected from the 555, it should be about the same as the monostable timeout when in circuit.

It may be worth checking the resistance between the capacitor negative lead and, say, IC pin 1 and the positive lead and pin 6, just in case the breadboard connections are faulty.

[Mad Mart]

I'm not sure if I understood correctly but I took the capacitor (470uF) out of the circuit and just connected a 1k resistor between it and the +ve. It didn't really take anytime at all to reach 6V. cap -ve to IC pin 1 is 0 ohms.

[rolycat]

I'm not sure if I understood correctly but I took the capacitor (470uF) out of the circuit and just connected a 1k resistor between it and the +ve. It didn't really take anytime at all to reach 6V. cap -ve to IC pin 1 is 0 ohms.

A 1k resistor will take less than a second to charge a 470uF capacitor to 6V, which may explain your problem. Set your 100k pot at about halfway, check the resistance is around 50k, and reassemble your circuit. Press the button briefly and D1 should light, and then go out about 30 seconds later. If not, double check that the capacitor leads are making good contact.