Door bell timer using 555 IC?

[Keithstv]

Our door bell, a standard ding dong type, had been modified by the previous owner to give a ding on the front door bell push and a dong on the back door bell push. It wasn't very satisfactory so a few years ago I modified it to a capacitor discharge circuit driving the solenoid. This is controlled by an 8 pin PIC which gives 3 ding-dongs for the front door and 2 ding-dongs for the back door with a bi colour LED indicating which door push was pressed. It also inhibits the door pushes for about 10 seconds to prevent repeated ding dongs. It's a bit of an overkill but it was an interesting exercise and kept me amused for a while!!

Now back to Davids 555.

Keith

[Yorkie]

Well I've amended the circuit to incorporate Keith and Mark's suggestions, and have redesigned the PCB accordingly.

The circuit, PCB artwork and component overlay are attached, and hopefully, will fit the bill!

[Mark Hennessy]

Don't see a smoothing capacitor

In light of John's comment, it's definitely worth measuring the off-load voltage of the transformer - the 555 is only good for 18V DC.

Cheers,

Mark

[Keithstv]

If you are going to use the 12VAC supply to power the 555 via a bridge and smoothing cap I'd add a 330ohm resistor from the smoothing cap to the +ve supply of the 555 and add a 12V zener across the 555 +ve and -ve supplies with a parallel 47 to 100uF cap. The relay should be run from the output of the smoothing cap.

The zener should prevent the 555 supply from rising to too high a level. I'll amend your circuit later and post it. Or if you're in no great hurry I'll see you on Sunday.

Keith

[Keithstv]

Here's a very quick modified circuit

[Nick]

In light of John's comment, it's definitely worth measuring the off-load voltage of the transformer - the 555 is only good for 18V DC.

The bipolar NE555 (the original) is good to a max Vcc of 18VDC but the CMOS versions (which are generally "better"), e.g. the TLC555, are only typically good to 15VDC.

Not all "555"s are the same

With 12VAC from the transformer you'll get a peak under load of about 17VDC (Vpeak=Vrms*sqroot(2) for a sine wave), subtract 2 x 0.7V for the two diodes in the bridge that are always in series and you're left with about 15.5VDC.

Note also that in the diagram above, the relay has 15.5V across it, not 12V , so with a 280ohm Rdc that's now 47mA through the coil - actually, a bit less due to the Vce of the transistor.

Using a 12V zener you'll be dropping 15.5V - 12V, i.e. 3.5V,over the 330R resistor which will dissipate about 40mW, so use an 1/8th or 1/4W for safety - the zener will need to carry this current which is about 11mA.

A zener is a good choice here as the difference between the unregulated output (15.5) and the regulated requirement (12) is small at 3.5V. If you use an ordinary small 12V linear regulator, e.g. a 78L12, it has a Vf (forward dop voltage) of 1.7V, i.e. it requires an absolute minimum of 13.7V in to get 12V out which only gives us 1.8V of headroom - not a lot. Its in situations like this that LDO (Low Drop Out) linear regulators are useful having a typical Vf of only 0.5V, though using an LDO here would be gross overkill

[Yorkie]

Thanks so much for your sound advice guys, and the time you've taken to make suggestions for amending the circuit.

I've now re-drawn the PCB layout to incorporate those suggestions, and have appended the circuit, artwork and component overlay.

I've added the 330 R resistor, zener and 47 uF cap across it, plus a smoothing cap as recommended. (I'd actually included one in the first layout, but had omitted it in this one).

I'm checking with my chum to find out whether the bell transformer is actually 12V AC rather than 8V (which many of them are).

I guess that if it's 8V AC, I'd be safe to omit the zener, the 47uF cap across it, and the 330R resistor?

How does that look  fellas?

[Yorkie]

I've just had confirmation that the bell transformer is indeed 8 Volts - not 12V as I'd originally thought.

[Mark Hennessy]

Hi David,

Afraid to say that I've spotted a mistake with the PCB. The output from the bridge rectifier should feed the relay coil and the 330R resistor, plus the 220uF smoothing capacitor. Currently, it doesn't...

Your schematic looks correct at a glance, so it's a question of making the PCB match the schematic.

Looking at the PCB as it stands, it might be a little bit tricky to route the +UNREG (as I hereby christen it) to the relay coil without a jumper. So - a top tip here - do just that; use a jumper. Why? Because, as I mentioned earlier, depending on the exact details of the transformer, and hence the exact value of +UNREG, you might find that a resistor in series with the relay coil is beneficial. And it's a lot easier to replace a jumper link with a resistor than it is cutting a track. Does that make sense? That sort of "forward thinking" is a valuable skill when doing this sort of thing.

Irrespective of the transformer, I would still retain the Zener diode and series resistor. Why? Because without building and debugging a prototype, I'm not 100% sure about how the 555 responds to a ripply supply in this application (I alluded to this in post #29). And even if there is no problem, the Zener provides protection to the 555. Mains surges happen

Additionally, I'd be a little wary of the trigger input (as mentioned by Peter in post #8). I think I'd provide the pads and footprint for a small capacitor between the switch contacts - perhaps 100n or similar? The wiring to the switch will act like an antenna, and might pick up all sorts of nonsense. A useful "dirty" test for this sort of thing is to hold a mobile phone next to the wiring while it is in a call.

I'd also be tempted to replace the link (into pin 2) with a resistor, as that will limit the current that can flow in the event of an electro-static discharge. Especially if you are using the CMOS version of the timer. Of course, the value of that resistor (at least 1k) might have an impact on the pull-up...

Isn't it amazing just how complex things become when you want them to work in the real world? That's why my 2-transistor circuit appeals to me - despite the apparent simplicity, it's actually pretty well "sorted". Mostly because you know how it works. Who knows what's actually inside the 555 - that is, all versions, from all vendors? Been there, got the t-shirt

All the best,

Mark

[Keithstv]

Marks suggestions are good ones. If the transformer is 8V then the 12V zener could be replaced by a 9.1V one with no ill effect. It's always worth adding the extra pads for components you might need on a pcb and not fitting them much easier than not adding the pads and needing them later. Our PCBs at work usually have provision for extra components which we think may be needed.

Keith