Need some help with a 555 timer circuit

[MK14]

I just wanted to thank everyone who contributed to my final design, all your help has been greatly appreciated.

Attached are my final schematic (Doesn't include connectors, Indicator Led or 24v to 12v convertor), my pcb and the ground plane to make the slots where traces need to cross easier to see.

I have ordered the necessary components to bread board the circuit before printing the pcb, so until then any and all feedback in regards to optimizastion, design, design checking, layout, etc is helpful.

If I remember I will make sure to come back and post an image of the assembled pcb.

I haven't really been following this thread, so may have missed important details. But at a quick glance, is the poly fuse (F2), in the right position ?
It only seems to protect some fairly high value (relative to a poly fuse) resistors (i.e. low current bits of the circuit). Presumably you intended to protect the entire Vcc rail ?

[Pizzashape23]

That looks good to me. The decoupling capacitors are near the supply pins to the ICs, which is great.

Is there any reason why you're not using the NE556 or better still the CMOS TS556/ICM7556? The 556 means it has two 555 timers in the same package, which saves space and makes one of the 10µF capacitors unnecessary. The CMOS versions (TS/ICM7) use much less power and don't draw such big power spikes, when the output changes state, so can use much smaller 100nF decoupling capacitors.

I kind of overlooked the 556 from the get go when Ian.M designed it using 2 555's and I saw that they were cheaper than a 556 (Tiny amount, but it just stuck with me) and also space was never really an issue.

I'm not using the cmos versions because power draw was never an issue (It is used in conjunction with a high power furnace) and because originally I had planned to drive the relays directly off the output pin until I realised it would be better to use one central board to drive all the solenoids and then switched to a transistor. (I am using those little machined sockets for the 555's so it's easy to change down the line with an ic puller)

I haven't really been following this thread, so may have missed important details. But at a quick glance, is the poly fuse (F2), in the right position ?
It only seems to protect some fairly high value (relative to a poly fuse) resistors (i.e. low current bits of the circuit). Presumably you intended to protect the entire Vcc rail ?

I am pretty sure it is, it is only rated at 50ma, much less than the theoretical max 2A draw of the rest of the circuit (including what is being switched by the relays) and as it is a polyfuse it is only intended to temporarily disable the circuit anyway. When it trips it will "cut" power to the switch disabling inputs to the first 555, while also pulling the reset pin of the second 555 to ground disabling it.

Take this explanation with a grain of salt as the circuit was designed by Ian.M, and only in "reverse engineering" the circuit (Not going to produce something I don't understand) I came up with the above explanation, it could very well be a design error but I feel like it makes sense.

[Ian.M]

The 50mA polyfuse is there to protect against shorts from the button wiring to ground.  In that event, the second 555 is also reset for 'belt & braces' lockout of the feed system, persisting long enbough to avoid false triggering on glitches if the fault clears and the polyfuse recovers.   I assumed the DC-DC converter already has short-circuit protection.

Zero999's suggestion of a CMOS 556 dual timer is a good one. It lets you eliminate the 10uF electrolytics from the BOM, as a 330nF caps as used elsewhere will be suitable for decoupling it.   As both 555 configurations leave the control voltage at the standard 2/3 Vcc, if you were using a 556 you could also tie the two control voltage pins together and save an additional capacitor.

If there is any significant risk of vibration or repeated severe thermal cycling, if you are going to socket the chips, you should also provide a plain PCB hole at either end of the package large enough to get the smallest size of cable tie through, looped over the top of the chip for retention in its socket.  Otherwise there's a risk it will tend to 'walk' out of its socket and you'll have to open the unit up annually to reseat the chips.

[MK14]

The 50mA polyfuse is there to protect against shorts from the button wiring to ground.  In that event, the second 555 is also reset for 'belt & braces' lockout of the feed system, persisting long enbough to avoid false triggering on glitches if the fault clears and the polyfuse recovers.   I assumed the DC-DC converter already has short-circuit protection.

Unfortunately, that doesn't really make sense..... I understand now, see below.

Also, isn't the driver transistor (bottom right hand corner of schematic) unprotected, as regards the back emf's from the relay coils ?
Assuming the relays don't have built in diodes.

EDIT: It makes sense to me now, the 50ma polyfuse, and worries about switch shorts to ground.

[Pizzashape23]

The 50mA polyfuse is there to protect against shorts from the button wiring to ground.  In that event, the second 555 is also reset for 'belt & braces' lockout of the feed system, persisting long enbough to avoid false triggering on glitches if the fault clears and the polyfuse recovers.   I assumed the DC-DC converter already has short-circuit protection.

Zero999's suggestion of a CMOS 556 dual timer is a good one. It lets you eliminate the 10uF electrolytics from the BOM, as a 330nF caps as used elsewhere will be suitable for decoupling it.   As both 555 configurations leave the control voltage at the standard 2/3 Vcc, if you were using a 556 you could also tie the two control voltage pins together and save an additional capacitor.

If there is any significant risk of vibration or repeated severe thermal cycling, if you are going to socket the chips, you should also provide a plain PCB hole at either end of the package large enough to get the smallest size of cable tie through, looped over the top of the chip for retention in its socket.  Otherwise there's a risk it will tend to 'walk' out of its socket and you'll have to open the unit up annually to reseat the chips.

I should stop assuming what components do and ask next time im unsure, and yeah the DC-DC converter does have short-circuit protection.

I won't really be able to remove the 10uF from the BOM as they are also used in the RC network for the second 555, and saving one or two components was never an issue, rather the amount of individual components as they will be bought in bulk anyway. Either way, if I find the bipolar versions have issues they will be easy to change down the line.

While I like the zip tie idea, I'm pretty sure it's unnecesarry as the main board will be in the control room, I could have put the solenoid drivers on the same board but by switching their 12v supply, distance/heat/vibrations shouldn't be an issue. All the wire used between boards has high temp insulation and is more than thick enough.

Unfortunately, that doesn't really make sense to me. But don't worry, best to leave me out of it (I don't want to spend the time to study all this thread).

Also, isn't the driver transistor (bottom right hand corner of schematic) unprotected, as regards the back emf's from the relay coils ?
Assuming the relays don't have built in diodes.

They definitely do have diodes in built, it was one of the selling points for me when I bought them for another project. There is also an inline fuse from the 24v supply into the DC-DC converter rated for 75% of it's max current.

[MK14]

They definitely do have diodes in built, it was one of the selling points for me when I bought them for another project. There is also an inline fuse from the 24v supply into the DC-DC converter rated for 75% of it's max current.

That's good!
Fusing is a nice extra touch as well.   

[MK14]

Sorry, I didn't know about the thread on the other forum, so couldn't reply there. I didn't realise I needed to refer to it, to get the details, I assumed, they would be on this forum.

Now I've read it (other forum), I have a question.

Why not simply use a 1 second (timing amount adjustable, via switches), off the shelf timing relay (powering additional relay(s), if necessary, to meet the output specs) ?

E.g. for around £17 + VAT (if applicable), from RS. There are others they sell, and other suppliers. So, I am just giving an example.

https://uk.rs-online.com/web/p/timer-relays/1785021/

Product datasheet:
https://docs.rs-online.com/e6ce/0900766b817142df.pdf

It seems it can be triggered by a switch, and will refuse to be started again, until it has seen the switch turn off, and then back on again. I.e. holding or shorting the switch, will not cause the 1 second to be exceeded, if I understand it correctly. It seems to have a basic +/- 0.5 % accuracy.

Function B: Single shot timer / delay off with trigger input
(leading edge) - as function “C” but the relay contacts energise and
timing commences on the immediate input to pin 5 (leading edge).
Also unlike function “C” if the trigger is still present after timing
has been completed this will not have any effect, in all circumstances
the trigger will need to be removed and reinstated for the function
to happen again. If the trigger input to pin 5 is present before the
connection of the supply to pins 2 & 10, when the supply is
connected the

I feel bad, because I came to the thread very late, and the design(s), have already taken place. But, as the thread on the other forum, seems to say. 555 timers, are not especially robust and reliable as such. As nice as designing a custom circuit might seem to be. There are all sorts of gotchas, with such circuits.
If you can just buy a ready made, (presumably) robustly/tested version, somewhat cheaply, and replacements are available. It perhaps deserves, some consideration.

But feel free to do what you think is best. It is your decision. Others seem to have put a lot of effort into this already, so I feel             

If it was me. I'd be tempted to use it as a belt and braces type of unit, even if you still use your circuit. As it should mean that you don't exceed 1 second, even if the control circuitry tries to exceed the time limit.

tl;dr
I really like 555 timers. But, I've also had some bad experiences with them. They can trigger, possibly a bit too easily, and potentially be a bit haywire as regards the timings. I had a poorly suppressed DC motor, which basically turned a 555 timer, into a random number generator, rather than a precise timing unit, it was suppose to be.
It was a very long time ago, in my early hobby days.
But in fairness, 555 timers are basically reliable. But one needs to take care, if using them.

I haven't used timing relays like this (that I can remember). Maybe they have their own problems, I don't know.