Door bell timer using 555 IC?

[Yorkie]

Thanks once more fort your valued input guys.

I've now amended the layout along the lines suggested, having made the following changes:

The relay coil and 220uF smoothing cap are now connected via a link to the unregulated DC before the 330R resistor.

I've replace the link on pin 2 of the IC with a 1k resistor.

I've changed the zener diode from 12V to 9v1 to reflect the AC input of 8V rather than 12V.

Not sure whether Marks suggestion of a small value cap 'across the switch contacts' meant the relay contacts or the bell push.
Presently I've added it across the relay contacts, but can easily amend that to be across the bell push contacts.

So how is that looking please?

[Keithstv]

Looks better. Adding a cap across the relay contacts would help supress any interference from the bell but I'd add a 10nF cap across the bell push terminals as well.

Keith

[Mark Hennessy]

Hi David,

I was talking about the bell push contacts - the subject of that paragraph was concerning the risk of false triggers caused by noise pickup from the bellwire.

Arguably though, a switch on the relay contacts might not be a bad plan as the bell will be a rather inductive load.

I must admit that personally, as a general principle, if I was committing to a PCB, I would thoroughly debug a Veroboard or breadboard prototype first - even for something as simple as this. But hopefully it's all good now

Cheers,

Mark

[Keithstv]

I'll await a full report on Sunday.

Keith

[Nick]

More on this - no one has commented yet that as long as pin 2 (trigger) is kept low, the output will be active and the bell will keep ringing.

This MAY NOT BE DESIRABLE

If you want to enforce the length of ring to being only the monostable pulse length, regardless of how long the button is pressed, a few simple extra components will ensure this:

   

Please excuse the rough diagram - I only have LTspice on this computer - What happens here is that when the button is pressed, a short negative spike is generated on pin 2 (trigger), followed by a +ve spike as the button is released - the +ve spike is clamped by D1 to 0.6V above Vcc and is unimportant. With the values given, you will have a pulse width range from about 2.5s to 7.7s. See the simulation below of what will be seen on pin 2 (trigger) when the button is pressed...

   

This simple addition will add a whole bunch of noise immunity to the circuit for about 10pence with the additional benefit of not driving you nuts if someone keeps their finger on the button !!

Most data sheets also recommend decoupling the supply to the 555 as close to the chip as is possible with a 100n ceramic plus a 1uF electrolytic. Again, cheap, so do it.

From a "Muntzing" point of view, it's worth noting that 555s can source 200mA and sink 50mA from pin 3, so the output transistor is not necessary at all as we are only sourcing about 50mA - the freewheeling/flyback diode around the relay is still required.

ALSO: A point often missed - The 220uF capacitor discharges rapidly through pin 7  as soon as it reaches 2/3rds of Vcc - internally, there is just an NPN transistor to GND - peak currents of over 1A can be expected - although there seems to be no specification of how much pin 7 can sink, the other pins sink much less, so I have included R5 for safety (a common thing to do) - this limits the discharge current to about 100mA  (*) and doesn't impact the timing. Some of the manufacturers' datasheets for their variants of the 555 state that you don't need to current limit pin 7 so long as the total package power budget is not exceeded - I include limiting as a) it's cheap, and b) you don't necessarily know whose implementation of the 555 you are using, so being sure is both cheap and good practice !!

EDIT: Updated schematic and full LTspiceIV card deck attached as "555.txt" - rename to "555.asc" and open with LTspiceIV

   

...But hopefully it's all good now

Hah!

(*) I = V/R = (0.666*Vcc-0.3)/47 = 120mA. The "-0.3" allows for the Vce[sat] of the 555's internal discharge transistor. In practice, the ESR of the capacitor and other losses reduce the current to about 100 to 110mA.

[Radio Fixer]

This is fun  , design by committee.  Used to get involved with those when I was working.  I would like to take part but this Philips Monoknob is taking all my attention.
I had a boss who said you are one of those people who are totally  task orientated.  I thought it was a compliment at the time but my wife isn't too happy about it when things want doing around the house.

Be interesting to see the final circuit and does it work on site?  A number of times we built things in the Lab and thought we had covered everything but on site, well red faces.  It was at a time when EMC practice was in its infancy and so those who have expressed to take concern for this, particularly for the long bell push leads, are spot on.

Gary

[Yorkie]

Well I have to say that I'm overwhelmed and really very grateful for all the interest, advice and trouble that everyone has gone to, for what - for me - had started out as a simple 555 timer!

I'm indebted to Nick for the time he's taken to outline possible problems if a called hangs onto the push button, and for the suggested mods and simulation.

I've developed the PCB to a point at which there's just no more space for additional components, so I'm going to give it a go and see what happens.

Serves me right for not persevering with two transistors, 4 resistors and a cap!

I'll report back in due course after other diversions over the next few days - weekend away for NVCF, Sheffield Monday,  Stately Home visit Tuesday, friends visiting Weds,  and so on it goes...

[Yorkie]

This is fun  , design by committee.  Used to get involved with those when I was working.

Be interesting to see the final circuit and does it work on site?  A number of times we built things in the Lab and thought we had covered everything but on site, well red faces.  It was at a time when EMC practice was in its infancy and so those who have expressed to take concern for this, particularly for the long bell push leads, are spot on.

Gary

It brings to mind the saying that "the difference between theory and practice in practice, is far greater than the difference between theory and practice in theory"!

And don't we know it!

My problem is that my practical skills, abilities and experience far outweigh my technical knowledge.

That's fine for making things just to look at, but when they're things that are supposed to work as well as be ornamental, that's when the fun starts!

[Refugee]

Things that don't work when they get to site can be fun but this one never made it out of the factory.
The story was told by my father from the early 1960s when he was working in the fledgling packaging industry.
It was one of the first heat sealing machines ever with a safety cut out based on photo resistors with lots of OC71s in the circuit. Being hand made and still under development the PCBs had been handled quite a bit and the lacquer had come off some of the transistors.
Cue customer and on go the covers for the demo at your peril.
The rep put his arm into the machine to demonstrate the new magic eye safety cut out and the machine tried to seal his hand into a bag. Ouch!!
The poor guy desperately struggled not to scream and just about managed to hide his singed hand from the customer.
Always test again with the covers on or else you will be in trouble.

[Yorkie]

Pleased to report that I've now built and tested the PCB and can report 100% successful outcome!

The range can be adjusted from 3 seconds to 10 seconds on the pre-set pot.

Enjoyable little project which grew like topsy from the original concept!

Pic of the completed PCB attached, and yes, before anyone mentions it, that resistor bottom right is wonky! Also, if I'd have checked my stock of caps I'd have designed the PCB layout for radial 220uF caps rather than axial, which is why the + wires from those caps snake across the PCB (brown insulation). I only have radial caps to hand. While I was designing and etching the timer PCB, I decided to design and etch a PCB for a little 555 tester. I've attached a pic of the two PCBs, and when the tester is finished, I'll post a thread on it. 

Many thanks to all who helped, advised and encouraged me along the way.