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Old 11-05-2017, 08:47 AM   #41
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Default Re: 555 countdown timer design question

set and reset with the same button, or different ones?
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Old 11-05-2017, 08:54 AM   #42
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Default Re: 555 countdown timer design question

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set and reset with the same button, or different ones?
I believe I stated that quite clearly Same button. The "trigger" signal thus needs to be toggled somehow between the trigger and reset pins.
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Old 11-05-2017, 10:39 AM   #43
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Default Re: 555 countdown timer design question

Found THIS which would be a very quick, albeit "dirty" solution to my problem. I just find it a bit cumbersome to use a relay for this purpose....I'm thinking of something less "bulky" and more "electronic" like a transistor.
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Old 11-05-2017, 12:11 PM   #44
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Default Re: 555 countdown timer design question

look at a chip called 74ls74,
it can be configured in interesting ways.
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Old 11-05-2017, 03:17 PM   #45
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Default Re: 555 countdown timer design question

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But this won't have any negative, possibly destructive effect either, right ?

Yes, you're right, precision is not critical up to the millisecond (though it would be nice to have, yet impossible with such imprecise components). A tantalum cap would require a much larger resistor, possibly in the order of mega ohms, since its value is by default lower....really, what WOULD be the maximum available tantalum cap ? On second though, you could perhaps parallel them...
There's no reason why keeping the button pressed should have any detrimental effects. It will just keep the output high until the switch is released.

100uF 16V (or higher voltage) tantalums are commonly available. I'm not sure what you mean when you say "...since its value is by default lower...".
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Old 11-05-2017, 03:34 PM   #46
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Default Re: 555 countdown timer design question

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Originally Posted by Dannyx View Post
...I'm now struggling to implement a feature to set AND reset the timer with the same button. You press the button once, the countdown starts, then if you want to cancel it before the time is up, you press the same button again and it resets it. The thing I'm actually using is in the picture (I removed the relays for hacking purposes). The relays have a common + and are switched on/off by that driver IC which is essentially a series of transistors that pull the other pin of the relays to GND. In our case, it'd pull the trigger pin to GND which is perfect (the action is momentary, so it doesn't latch the pin to GND), but I now need to figure out a way to switch that "pull to GND" from the TRIGGER to the RESET pin once the circuit is on. I've been looking at Dave's vid here which is close to what I want, except I don't have physical switch, but rather a transistor which can only pull to GND (can't switch a positive signal), so I'm trying to wrap my head around this...
As stj says, you need to look for a flip flop or sequential switching bistable circuit to integrate into the design, so that when the trig pin is taken low, the reset pin is taken high & vice versa. There would also need to be some kind of feedback from the timer circuit to reset the flip flop in instances when the timer isn't reset during the timing period.
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Old 11-05-2017, 05:48 PM   #47
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Default Re: 555 countdown timer design question

Thrown together possible solution (push timed on, push turn off)
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Old 11-05-2017, 07:36 PM   #48
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Default Re: 555 countdown timer design question

Annotated schematic with sample values and component designations for discussion if desired... also this is not a microwave oven, though it may still say so...(was too lazy to change the name after abandoning the microwave reverse engineer effort.)
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Old 11-08-2017, 07:34 AM   #49
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Default Re: 555 countdown timer design question

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Originally Posted by eccerr0r View Post
Annotated schematic with sample values and component designations for discussion if desired... also this is not a microwave oven, though it may still say so...(was too lazy to change the name after abandoning the microwave reverse engineer effort.)
Nice. As well as the toggle function with a single push switch, it does the timing as well, with just half a chip.

Along similar lines, you could do away with the shorting transistor & use the Q output to charge the cap (with the addition of a diode). The time delay is 0.7CR, where C is the cap on the R pin of the chip (22uF) & R is the resistor in parallel with the diode (330k).

I have simulated both circuits. The only JK FF in the library is a generic TTL (5V on the outputs) -ve edge triggered one, so it starts when the switch is opened, rather than closed. I've adjusted the timing components for 5 seconds, as the simulation is not in real time. It takes around 40s on my i5 pc & around 90s on my old Athlon pc. I've replaced the relay with a LED.

http://tinyurl.com/y97u6ev6

http://tinyurl.com/y9emb9nf

Last edited by Radio Fox; 11-08-2017 at 08:11 AM.. Reason: Extra info
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Old 11-08-2017, 07:49 AM   #50
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Talking Re: 555 countdown timer design question

Just had a look at the past posts. You guys seemed to have ditched the 555 entirely and moved to a flip flop of sorts ? A 4027 like this one ? Now I just need to figure out how it works Meanwhile, I've been working on an overcomplicated and overengineered circuit that achieves the one-button function on the 555 as well, but the part count is ridiculous and might not work in real life, so I'll probably have to go down your route. Just for fun, I'll simulate it and share it if it works even barely
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Old 11-08-2017, 08:04 AM   #51
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Default Re: 555 countdown timer design question

Quote:
Originally Posted by Dannyx View Post
Just had a look at the past posts. You guys seemed to have ditched the 555 entirely and moved to a flip flop of sorts ? A 4027 like this one ? Now I just need to figure out how it works Meanwhile, I've been working on an overcomplicated and overengineered circuit that achieves the one-button function on the 555 as well, but the part count is ridiculous and might not work in real life, so I'll probably have to go down your route. Just for fun, I'll simulate it and share it if it works even barely
Yep, that's the chip. Also known as CD4027 & MC14027.

My initial thoughts were to incorporate the flip flop into the 555 circuit, but eccerr0r's idea is much neater. If your going to use a flip flop to toggle the push button switch, why not get it to do the timing as well. Much more compact.
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Old 11-08-2017, 08:16 AM   #52
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Default Re: 555 countdown timer design question

just use this:
https://www.ebay.co.uk/itm/291693366485

32bit ARM power!!
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Old 11-08-2017, 09:54 AM   #53
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Default Re: 555 countdown timer design question

I only chose the 4027 because it was the first classic CMOS (i.e. 12V tolerant and high impedance inputs) FF that came to mind...others will work. Though I did do "head spice" to make sure it works, the only caveat is that I'm not sure how the 4027 deals with the slow rise and whether it will self immolate. Something that you have to think about that simulations don't show

I was trying to get the 555 to work but realized it was superfluous, I never liked its behavior when having to trigger it multiple times, almost made me want to try a 74ls123 or 4047, but decided against it.

The caveat of my solution is that the light is turning off once RC expires, you can't hold the button down to keep it on.

Yes, you can do so much more with a microcontroller.
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Old 11-08-2017, 11:08 AM   #54
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Default Re: 555 countdown timer design question

There's another small problem which is not compatible with the remote module I'm using, though it can be EASILY overcome: from what I can make out, the 4027 requires a POSITIVE signal to trigger it, whereas the signal my board sends out is effectively a "pull to GND", therefore I THINK using a PNP is required: the remote module pulls the base of the PNP to GND, emitter-collector path becomes conductive and sends VCC into the trigger pin...just a hunch...haven't tested it.
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Old 11-08-2017, 11:41 AM   #55
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Default Re: 555 countdown timer design question

I would just use a NPN Transistor common Emitter to make Inverter circuit.
Output of the Controller board is HIGH so the NPN Transistor will be on = Collector Voltage will be LO, and when controller output goes to GND hte NPN Transistor will be off so Collector Voltage will go HIGH.
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Old 11-08-2017, 12:57 PM   #56
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Default Re: 555 countdown timer design question

this is a fun thread, but the truth is that microcontrollers have reached the price where they simply beat-down any analog solution!!

maybe not the ARM pcb i linked , but stuff like the PIC 10F series cost almost nothing!!
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Old 11-08-2017, 05:25 PM   #57
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Default Re: 555 countdown timer design question

I also wonder if the prices of these 555's and 4027's (or 4013 in this new revision which I think is cheaper than 4027) makes the microcontrollers an even better deal...

however you assemble these analog^H^H^H^H^H^H^H hardware-only solutions, they're ready to go (wait a minute, aren't CMOS/TTL digital logic? what's this analog?) The microcontroller needs to go through a flash programming cycle...

New version: uses more common CD4013/HEF4013/MC14013B .
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Old 11-11-2017, 04:13 AM   #58
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Default Re: 555 countdown timer design question

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Originally Posted by eccerr0r View Post
...The caveat of my solution is that the light is turning off once RC expires, you can't hold the button down to keep it on...
Was that a design consideration? I'm loosing track. LOL.

How about an AND gate that keeps the relay transistor turned on if there is still a high on the 4027 clock pin? The other input to the AND connecting to the notQ output which goes high at the end of the timing cycle.

Something like this (I've put an inverter on the clock pin to simulate a +ve going clock trigger which the 4027 requires):-

http://tinyurl.com/yacso6tq

The AND gate can be created using a couple of NPN transistors instead of another IC if preferred.

I don't know how much these components cost in other countries, but in the UK, the 4027 can be got dirt cheap on Ebay.

Last edited by Radio Fox; 11-11-2017 at 04:19 AM.. Reason: Additional info
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Old 11-11-2017, 11:05 AM   #59
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Default Re: 555 countdown timer design question

I don't think it was a design constraint, though it was thrown around as an issue "what happens if you hold the switch down"...

That's what happens when we (as engineers) get incomplete design constraints and then surprise constraints come up after we get our first designs out...

"Back to the drawing board" as they say

BTW, was looking at Mouser for 4013 and 4027. For 1-unit quantities, IIRC, 4013 is $0.51 and 4027 is $0.53 (USD) or something like that (looks like there are choices that are cheaper). This is "expensive" as it's getting into the cheap microcontroller territory... However in the 1000-unit quantities, the price break starts taking effect, except you still need that capacitor, resistors, and at least one extra transistor...

Last edited by eccerr0r; 11-11-2017 at 11:14 AM..
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Old 11-11-2017, 01:13 PM   #60
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Talking Re: 555 countdown timer design question

Ok guys, here's the crazy, overbuilt and GROSSLY exaggeratedly complex version of this project Using nothing but trannies I managed to achieve the following: A initiates a 60 second countdown, B keeps the lamp on permanently, C and D stop the thing entirely regardless of the state.......you got that ? The relay that will be used to actually power the light is represented by the purple LED at the top.

The operation:

Switch (channel) B, Q3 and Q2 form a simple latch:
-when B is closed, it drives the base of Q3 high, which drives the gate of Q2 low, turning on the FET
-as the FET is now conducting, current flows through the 100k resistor back into the base of Q3, essentially keeping it on as if your finger was still on button B.
-B also drives Q1 which pulls the RESET pin to GND to cancel any remaining time, if any left, on the 555 timer


When A closes, it drives the bases of Q6 and Q9:
-Q9 pulls the TR pin low, imitating the 60-second countdown
-Q6 stops Q3 and therefore Q2 in case it had been latched on previously by switch B (it cancels the latch if it was switched on first). This ensures the lamp turns OFF after the countdown and doesn't stay on via the latch

C and D are the two remaining buttons on the remote and I figured I'd have them both do the same thing - stop the whole circuit, like so (used a single switch to limit clutter as much as possible)
-when either is pressed, it drives the base of Q4 and Q5 HIGH
-Q4 pulls Q3 LOW and cancels the latch if it's already on
-Q5 drives the RESET pin LOW to cancel the countdown if it's in progress
You got that ?..........Good, all we got to do now is build it

Cheers guys. As always, I really don't care - it's all about having fun, ain't it ?
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