If the pot is turned all the way to Vcc and the discharge transistor turns on you are shorting the power rails. You should put a fixed resistor between the pot and Vcc. The parallel resistor will change a linear pot in to a pseudo log one. Thanks for taking the time to make an informative video in to this useful chip. And to the people who say it is wrong for a clock, how come it is not obsolete.
Hi Julian, when using a 555 the supply must be decoupled really really well. Try a 100n ceramic directly across pins 1 and 8, actually straddle the capacitor directly over the top of the chip. Also increase the value of the tantalums across the supply lines to say 47uF each. The 555 is renouned for causing huge spikes on the supply, I seem to remember it has something to do with the totempole output transistors shorting the supply during output transitions. I have seen claims that a couple of amps are drawn during the output transitions, nasty! I have had similar decoupling problems with other chips too, the LM386 springs to mind. When breadboarding circuits I always connect a 100n cap across the power pins of each IC as a matter of course because it can save a lot of wasted headscratching time. Most 100n ceramics have leads long enough to straddle the chip so its easy and quick to do. The Texas Instruments LM555 datasheet shows the internal diagram of the 555 with the totempole output transistors.
Crocellian yes Julian comes across as not knowing much at first. But then if you actually watch the channel you realise he does not miss a beat and actually uses some real tricks of the trade to make things work. I have 30+ years in the industry as a design engineer (senior design engineer for an MOD contractor) through to head of design, and can tell you if you think he's an idiot it is you that lacks knowledge.
Crocellian - I understand your frustration, some electronics presenters on youtube bother me too but Julian is not one of them. I think his channel encourages experimentation which is fun and key for a hobbyist. As a child, decades before the internent was invented, I spent years reading electronics hobby magazines and books from the library, I did not have anyone to teach me and no one to ask when my experiments failed. Failing is frustrating but it also offers an incredable opportunity to learn. I think Julians channel encouages people to experiment and when his circuits don't behave as he expects he trys to investigate and solve the problem, this is good. As a hobbyist there are only a few 'secrets' we need to learn to enable success. Knowing these 'secrets' can allow us to achive a near professional understanding and the ability to design and build reliable working circuits. Of course they are not really secrets but they are subtle and not always easy to discover. I believe that proper power supply decoupling is one of those 'secrets'. I like to share what I know when I can, hence my post. Sincerly, I would encourage you to share your experience and help when you see something you believe needs clarifying. You may just trigger that 'light bulb' moment for someone, we all need that from time to time.
Your 1K resistor between pins 8 and 7 should be between pin 8 and the upper connection to your POT, that makes sure you are never shorting your Discharge tranny from Vcc to GND. I think that's what you meant to do anyway. If the bypass CAPs cleared the issue up, then it pretty much points to a issue with the voltage rails.
it probably locks up because VCC is short circuited to GND through 200k potentiometer (highest position of center point on schematic) and internal discharge transistor
You should only need one 1K resistor at the switch. Put it on the output side. Of course that's only valid if the switch opens before closing. I'd also ensure there's a resistor always between VCC and Discharge as you could sink a lot of current through the transistor otherwise. Move the spare 1K over there. :-)
I think the ESR of the 1uf is sagging the power supply when you have no 1k resistor and the discharge is doing the same thing on the other part of the cycle. To prove this you would need to scope the +ve and -ve of the 555 with your scope. I think it would be important to connect the ground of the scope directly to pin 1 and probe to pin 8 directly so the breadboard is not included in the equation. Another thing, the led has capacitance which can show up on the output. Disconnect the led to prove that.
If your momentary switch is break-before-make, there will be an instant where it's open-circuit and could cause it to act in free-running mode and cause some unexpected glitches in the future.
Yes, that could happen if the clock were set to run at a very high frequency. But this computer is more about blinking lights and processing instructions at a visible clock rate. Good point though :)
Make before break does not matter in this case because the switch has a 1uf capacitor, which effectively decouples the switch, giving it plenty of time to hold the voltage while the switch cycle completes.
you are in charge of your future, you seem to be against learning unless it conforms to a model that you like this sounds scary to me I think your a bit weird and I would imagine that anyone else reading this will get what I mean
What happened to poor pin 5? That's the 2/3 Vcc control voltage? 2/3 being a node on the 5k 5k 5k resistor divider network. 0.01uF to VSS in astable mode.
I love the way you teach and the schematics do help me a lot to understand what's going on. I hope I will understand what your program in the memory of the Z80 really does. At the moment all I see are blinking LEDs ... Looking forward to see the breakup of your Z80 computer. Keep up the good work!
Strange 555 timer behaviour, Jules?! Have you considered trying to build a 555 using latches, comparators and resistors on a breadboard to tr and work out what's going on? You can scope out various parts of the timer, see why you're getting both the weird step glitch, and that slow rise time
The manual switch doesn't generate a single pulse. It turns the frequency generator on/off meaning pushing it can generate anywhere from no pulse to several depending on where the pot is dialed into. Is there a way to design it so the single switch generates one, and one only, pulse? I really like the single timer chip idea so I hope that's still possible without creating multiple input circuits.
I suspect the knee on the positive pulse a simpler explanation. The output of the 555 timer is a totem pole output. The output goes high into the load which has *some* natural capacitance which can be reflected back through P-N junctions to the input of the load multiplying that capacitance. The output rises to the V-CE of the output transistor, then the much lower base current I-BE charges the 'capacitance' of the load the rest of the way to source voltage.
I may be wrong but it could be the capacitor is not fully discharging. That might explain the fast rise to about 80% and the slow rise to vcc is the capacitor at full charge.
I made a 555 osc a couple months ago running on a 7805 and was getting 4.2vpp out, so I put a 7806 in there and got 5vpp. Also when I transfered it from breadboard to PCB, I had to change the caps values to get it back to the same frequency.
Just curious as to why a 555 was used. There is a simple two transistor astable that can be messed with to do the exact same job as was used decades ago for some homebrew 6502 and Z80 simple computers in the early 80's (with a toggle switch for single stepping). Just to reduce component count or some other reason?
Maybe, without the resistors the capacitor acts like a short-circuit for a short time when being charged and that pulls the overall voltage down until it charges a bit, thus causing the "stepped" output? But to be honest, I don't really know for sure either.
Is there a way to implement this with a 555 so that it has an even duty cycle through the whole frequency range? I only have a 10k pot so I had to use a 10uf capacitor to get a human-perceivable speed. The duty cycle is around 40% at the lowest frequency and 88% at the highest. The 88% duty cycle means that even if the clock period is, say, 32KHz, the time it spends turning off and back on again is equivalent to something on the order of 600 KHz. Do you think i'd have better results with a larger pot and smaller cap?
I suspect that you might gain some insight into your unanswered questions in this video by connecting your scope to the Vcc and ground pins of the chip, triggering off of the output waveform as you did before...
Forest Mims wrote a remarkably useful guide to the 555 back in 1992 and updated it regularly through ~1996. I bought it back then, and still have it, it's been that handy to have: archive.org/details/Forrest_Mims-engineers_mini-notebook_555_timer_circuits_radio_shack_electronics .
The curve probably has to do with the internal powersupply of the 555. Seems to me that the SR Flip Flop might not get the full voltage right away. Thats the only possible explanation. Ive never put one under an electro microscope,could be as easy as thin wires or induction which provide some resistance and thus not a sharp flank.
Crocellian Why not? The goal of the video is to demonstrate the principals of these electronic components in an easy to understand and visual way. If you don't like the channel, then don't go commenting on it
I've always have had good luck with the Cmos version of the 555 (7555). It has a much cleaner output. The drawback is a little bit lower drive power and to get the highest speed from it, it had to have a higher Vcc voltage (usually 15v). It depended on how fast I needed the output signal to be.
One other thing I noticed with my designs was, different manufacturers' 555s were different from one another. Each one's output would behave differently.
For the strange behavior I'd try tying pin 4 high (prevent false SR latching) . Looked at the datasheet for the 555 the output voltage appears within norms .
Julian. You should not let it float( it can be used to make a VCO ), tie it to GND via a 1nf cap at least, in your case there may not be any noticeable difference, but leaving it floating can easily cause problems. Also the discharge pin should not be allowed to go to V+ as it connects directly to GND. There are better ways to get symmetrical out put from a 555 but does it really need to be symmetrical ? as long as the falling/rising edges have the required duration, then you are "gravy". Just to say, I enjoy your videos :)
theres a far simpler way for a continuous free run oscillator, link pins 2 and 6, as above but do not use pin 7, capacitor from this point to ground, resistor from this point to output.... used this circuit loads of times.... as for adapting it for single shot, never yet tried...
Tantalums can exhibit weird behavior, might be worth doing the 'scope test again but pull the 555 out, and see if the shelf thingy happens without the chip in place.
Enjoyed the video, didn't enjoy the fact youtube played a Conservative Party political broadcast until I managed to get to the skip link. Surely this can't be right
If the pot is turned all the way to Vcc and the discharge transistor turns on you are shorting the power rails. You should put a fixed resistor between the pot and Vcc. The parallel resistor will change a linear pot in to a pseudo log one.
Thanks for taking the time to make an informative video in to this useful chip. And to the people who say it is wrong for a clock, how come it is not obsolete.
Shouldn't there be a small resistor between Vcc and the top of the trimmer? (to keep DIS from shorting Vcc to GND)
Hi Julian, when using a 555 the supply must be decoupled really really well. Try a 100n ceramic directly across pins 1 and 8, actually straddle the capacitor directly over the top of the chip. Also increase the value of the tantalums across the supply lines to say 47uF each. The 555 is renouned for causing huge spikes on the supply, I seem to remember it has something to do with the totempole output transistors shorting the supply during output transitions. I have seen claims that a couple of amps are drawn during the output transitions, nasty! I have had similar decoupling problems with other chips too, the LM386 springs to mind. When breadboarding circuits I always connect a 100n cap across the power pins of each IC as a matter of course because it can save a lot of wasted headscratching time. Most 100n ceramics have leads long enough to straddle the chip so its easy and quick to do. The Texas Instruments LM555 datasheet shows the internal diagram of the 555 with the totempole output transistors.
@Pete yep the 555 is an unstable little git if the supply lines are not decoupled adequately.
Crocellian yes Julian comes across as not knowing much at first. But then if you actually watch the channel you realise he does not miss a beat and actually uses some real tricks of the trade to make things work. I have 30+ years in the industry as a design engineer (senior design engineer for an MOD contractor) through to head of design, and can tell you if you think he's an idiot it is you that lacks knowledge.
Crocellian - I understand your frustration, some electronics presenters on youtube bother me too but Julian is not one of them. I think his channel encourages experimentation which is fun and key for a hobbyist. As a child, decades before the internent was invented, I spent years reading electronics hobby magazines and books from the library, I did not have anyone to teach me and no one to ask when my experiments failed. Failing is frustrating but it also offers an incredable opportunity to learn. I think Julians channel encouages people to experiment and when his circuits don't behave as he expects he trys to investigate and solve the problem, this is good. As a hobbyist there are only a few 'secrets' we need to learn to enable success. Knowing these 'secrets' can allow us to achive a near professional understanding and the ability to design and build reliable working circuits. Of course they are not really secrets but they are subtle and not always easy to discover. I believe that proper power supply decoupling is one of those 'secrets'. I like to share what I know when I can, hence my post. Sincerly, I would encourage you to share your experience and help when you see something you believe needs clarifying. You may just trigger that 'light bulb' moment for someone, we all need that from time to time.
Gordon Lawrence - Good description of the 555 :) I would rather use the CMOS version, 7555.
In a nutshell.........Fuck Off
10-10 till we do it again, takes me back to the 70's.
Jeff Pinter Yeah knock it up one, breaker. 😎
Your 1K resistor between pins 8 and 7 should be between pin 8 and the upper connection to your POT, that makes sure you are never shorting your Discharge tranny from Vcc to GND. I think that's what you meant to do anyway. If the bypass CAPs cleared the issue up, then it pretty much points to a issue with the voltage rails.
I haven't used a 555 in quite a while but this made me want to pull out a 7555 and try it out :)
it probably locks up because VCC is short circuited to GND through 200k potentiometer (highest position of center point on schematic) and internal discharge transistor
You should only need one 1K resistor at the switch. Put it on the output side. Of course that's only valid if the switch opens before closing.
I'd also ensure there's a resistor always between VCC and Discharge as you could sink a lot of current through the transistor otherwise. Move the spare 1K over there. :-)
I think the ESR of the 1uf is sagging the power supply when you have no 1k resistor and the discharge is doing the same thing on the other part of the cycle. To prove this you would need to scope the +ve and -ve of the 555 with your scope. I think it would be important to connect the ground of the scope directly to pin 1 and probe to pin 8 directly so the breadboard is not included in the equation. Another thing, the led has capacitance which can show up on the output. Disconnect the led to prove that.
If your momentary switch is break-before-make, there will be an instant where it's open-circuit and could cause it to act in free-running mode and cause some unexpected glitches in the future.
Yes, that could happen if the clock were set to run at a very high frequency. But this computer is more about blinking lights and processing instructions at a visible clock rate. Good point though :)
Make before break does not matter in this case because the switch has a 1uf capacitor, which effectively decouples the switch, giving it plenty of time to hold the voltage while the switch cycle completes.
you are in charge of your future, you seem to be against learning unless it conforms to a model that you like this sounds scary to me I think your a bit weird and I would imagine that anyone else reading this will get what I mean
oh yeah, you go away !
What happened to poor pin 5? That's the 2/3 Vcc control voltage? 2/3 being a node on the 5k 5k 5k resistor divider network. 0.01uF to VSS in astable mode.
I agree, I always put 0.1uF pin 5 to gnd to help bypass.
I love the way you teach and the schematics do help me a lot to understand what's going on. I hope I will understand what your program in the memory of the Z80 really does. At the moment all I see are blinking LEDs ... Looking forward to see the breakup of your Z80 computer. Keep up the good work!
I'm curious as to how he simulates Ram pack wobble ;)
Strange 555 timer behaviour, Jules?!
Have you considered trying to build a 555 using latches, comparators and resistors on a breadboard to tr and work out what's going on? You can scope out various parts of the timer, see why you're getting both the weird step glitch, and that slow rise time
I guess so. But I don't want to get too bogged down with the clock circuit - there's a lot more computer circuitry to design.
Indeed. the devil's in the details, and all that noise! :P
Fun future project though :D
The brilliant Ben Eater just recommended you in his latest 8-bit cpu video
+makebabymake Oh, cool - I feel quite honoured :)
The manual switch doesn't generate a single pulse. It turns the frequency generator on/off meaning pushing it can generate anywhere from no pulse to several depending on where the pot is dialed into. Is there a way to design it so the single switch generates one, and one only, pulse? I really like the single timer chip idea so I hope that's still possible without creating multiple input circuits.
I suspect the knee on the positive pulse a simpler explanation. The output of the 555 timer is a totem pole output. The output goes high into the load which has *some* natural capacitance which can be reflected back through P-N junctions to the input of the load multiplying that capacitance. The output rises to the V-CE of the output transistor, then the much lower base current I-BE charges the 'capacitance' of the load the rest of the way to source voltage.
I may be wrong but it could be the capacitor is not fully discharging. That might explain the fast rise to about 80% and the slow rise to vcc is the capacitor at full charge.
I made a 555 osc a couple months ago running on a 7805 and was getting 4.2vpp out, so I put a 7806 in there and got 5vpp. Also when I transfered it from breadboard to PCB, I had to change the caps values to get it back to the same frequency.
Just curious as to why a 555 was used. There is a simple two transistor astable that can be messed with to do the exact same job as was used decades ago for some homebrew 6502 and Z80 simple computers in the early 80's (with a toggle switch for single stepping). Just to reduce component count or some other reason?
One reason I can think of is the variable frequency of the clock, which can be useful when stepping through the program.
Maybe, without the resistors the capacitor acts like a short-circuit for a short time when being charged and that pulls the overall voltage down until it charges a bit, thus causing the "stepped" output? But to be honest, I don't really know for sure either.
Is there a way to implement this with a 555 so that it has an even duty cycle through the whole frequency range? I only have a 10k pot so I had to use a 10uf capacitor to get a human-perceivable speed. The duty cycle is around 40% at the lowest frequency and 88% at the highest. The 88% duty cycle means that even if the clock period is, say, 32KHz, the time it spends turning off and back on again is equivalent to something on the order of 600 KHz. Do you think i'd have better results with a larger pot and smaller cap?
I suspect that you might gain some insight into your unanswered questions in this video by connecting your scope to the Vcc and ground pins of the chip, triggering off of the output waveform as you did before...
Not heard that sign off in years Good Buddy!! :oD
Yeah, the youngsters won't understand that one ;)
Hi julian is it possible to construct with an accelerometer a circuit for a car breaking light to fast flash, when you heavy break?
Forest Mims wrote a remarkably useful guide to the 555 back in 1992 and updated it regularly through ~1996. I bought it back then, and still have it, it's been that handy to have: archive.org/details/Forrest_Mims-engineers_mini-notebook_555_timer_circuits_radio_shack_electronics .
I had forgotten about that. Thanks dude.
You are more than welcome. I can't begin calculate the number of 3rd year EEs that passed mid-term labs because of Forest Mims' handy notebooks :)
The curve probably has to do with the internal powersupply of the 555. Seems to me that the SR Flip Flop might not get the full voltage right away. Thats the only possible explanation. Ive never put one under an electro microscope,could be as easy as thin wires or induction which provide some resistance and thus not a sharp flank.
The one person who disliked this video probably fried his chips.....
Shiladitya Ray Once the magic blue smoke is released... Electronics don't work anymore.
Shiladitya Ray It was probably Angelica
Crocellian Why not? The goal of the video is to demonstrate the principals of these electronic components in an easy to understand and visual way. If you don't like the channel, then don't go commenting on it
Crocellian Believe me.....You are not even close to Julian Sir's knowledge..... That's why you are spreading rubbish......
I always fry my chips as the oven ones are not as nice.
bueno trabajo amigo, felicitaciones!
Saludos de tu amigo Marcos Borges!
I've always have had good luck with the Cmos version of the 555 (7555). It has a much cleaner output. The drawback is a little bit lower drive power and to get the highest speed from it, it had to have a higher Vcc voltage (usually 15v). It depended on how fast I needed the output signal to be.
One other thing I noticed with my designs was, different manufacturers' 555s were different from one another. Each one's output would behave differently.
On the side.... Could the step be due to the current drain on the Eneloops when the LEDs are lit at all?
I just checked the eneloops - they're all at 1.28 volts - so they're good. Easily capable of supplying an amp or two.
OK Good to check though.
Hey can you make a high voltage capacitor charger using IC 555?
Winding the pot wiper up to Vcc isn't going to make the discharge transistor very happy!
2:10 If a potentiometer is marked as 203 that means 20 x 10^3 = 20K and not 200K. Thanks
For the strange behavior I'd try tying pin 4 high (prevent false SR latching) . Looked at the datasheet for the 555 the output voltage appears within norms .
did you leave pin 4 (reset) floating?
I did.
Julian. You should not let it float( it can be used to make a VCO ), tie it to GND via a 1nf cap at least, in your case there may not be any noticeable difference, but leaving it floating can easily cause problems. Also the discharge pin should not be allowed to go to V+ as it connects directly to GND. There are better ways to get symmetrical out put from a 555 but does it really need to be symmetrical ? as long as the falling/rising edges have the required duration, then you are "gravy". Just to say, I enjoy your videos :)
theres a far simpler way for a continuous free run oscillator, link pins 2 and 6, as above but do not use pin 7, capacitor from this point to ground, resistor from this point to output.... used this circuit loads of times.... as for adapting it for single shot, never yet tried...
probably could do it the same way as in the video, dont see why not...
Tantalums can exhibit weird behavior, might be worth doing the 'scope test again but pull the 555 out, and see if the shelf thingy happens without the chip in place.
At 2:10, 203 is 20K, not 200K.
Sorry - slip of the tongue - the pot is a 204
No worries.
I read that the output doesn't go high enough to turn off a PNP transistor.
@4:45 .... Slick pen-change :-)
Enjoyed the video, didn't enjoy the fact youtube played a Conservative Party political broadcast until I managed to get to the skip link. Surely this can't be right