Door bell timer using 555 IC?

[Yorkie]

I can't see anything wrong with the basic circuit but I would put a 10k resistor in series with the variable as the 555 could be damaged if the pot is at minimum resistance. I always decouple pin 5, 10nF should be OK.

Is the relay turning on when power is applied? If so does it turn off after about 3 seconds?

Keith

Thanks for reading the thread Keith and for your comments.

The relay turns on the instant that I apply power and pin 3 goes high, but I've discovered that if I switch off for a couple of secs then back on again, pin 3 stays low and the relay doesn't come on until the bell push is pressed, at which point pin 3 goes high and the relay operates as it should. I wonder if the initial switch on charges up a cap, and when switched off and back on again, because the cap is charged, the 555 then behaves itself? Just clutching at straws really.

I need to revisit the combination of C1/R1 to get the time duration right, because the relay doesn't go off within a few seconds as it should.

(I added a 10nF cap to pin 5 of the IC, and I'll take your advice about adding a 10k resistor is series with the preset). 

[Keithstv]

Thinking about it I wouldn't drive the relay directly from the 555. Try disconnecting the relay and try again. If the circuit then works correctly you'd be better using a transistor to drive the relay.

However I've just checked the Signetics 555/556 handbook and they have a circuit driving a relay directly from the IC BUT there is a an extra diode between pin 3 and the relay, anode to pin 3 cathode to the relay and the option of a capacitor (value not specified) across the relay. As you've done the PCB I'd suggest trying the extra diode and if necessary a cap of say 10nF.

Keith

PS See you at NVCF next Sunday?

[Yorkie]

I think from what you say Keith, It's back to the drawing board! I'll perhaps redesign the PCB layout for a circuit to incorporate a transistor to drive the relay and etch a new PCB.

I've cobbled together such a circuit based on bits from internet offerings, which I've attached. Don't know how that looks! Any suggestions as to a suitable commonplace NPN transistor to drive the relay please? As I've noted on the drawing, the relay coil is 280 Ohms,. so at 12 V would pass 43 mA or thereabouts for maybe 5 secs or so when the doorbell button is pressed. (I'll most likely make the 22k resistor on pin 6/7 a pre-set to enable the 'on' time to be varied).

As to NVCF, yes, I'll be there with my wife, Jennifer, and I know that Gary will be there so I'll look out for you when 'doing the rounds'. I usually hover around the bring and buy at 12 noonish for an informal chat with any forum members should the mood take them, but it's  invariably a bit hit and miss because everyone seems to gets a bit engrossed in their hunt for 'unobtanium'  

I'll have my little 'Yorkie' badge on, and though the years are taking their toll, I still have a passing resemblance to my avatar pic, taken six years ago on my 70th birthday!

Hope to see a few forum members to pass the time of day with.

[Keithstv]

Any NPN transistor with a Icmax of >100mA should do. e.g BFY51 or BC337. You don't need the 100k pull up or the 0.1uF on the base. Also change the 1k series resistor on the base to 10k.

For a variable time change the 22k for a 10k in series with a 22k pot would give you a time between 2.5s and 7.5s approximately.

If you want a couple of BC337s let me know and I'll bring them to NVCF. I'll be wearing my avatar badge of our dog who looks the same now as when the picture was taken in 2006.

Keith

[Mark Hennessy]

I'd also suggest that the 1k base resistor is a bit low - trying to shove the best part of 12mA into the base is not the best plan. 10k would be kinder, while still giving plenty of margin to ensure the transistor saturates.

I like the BC337 - good for 800mA while having a decently high Hfe. Here, the ubiquitous BC108 or BC458 will do equally well. Just about any NPN small-signal transistor will fit the bill. What Elektor magazine used to call TUN - see http://www.sentex.ca/~mec1995/circ/tuptun/tuptun.htm

I'm afraid that I've been really busy lately, but have been meaning to ask - any idea why the simple transistor circuits didn't work? Had I been less busy, I would have prototyped one for you...

I'll be at the NVCF as well. But no badge - you'll have to guess

[Yorkie]

Thanks for your prompt response and advice Keith.

I've tweaked the circuit along the lines you suggested - how does that look?

Thanks for your offer to bring some transistors to the NVCF - I'm OK for both of those you suggest.

[Keithstv]

That should work, said he crossing his fingers.

Keith

[Yorkie]

Thanks for reading Mark, and for your helpful comments.

Ah, 'TUPTUNDUGDUS' - I remember it well! I've still got my old Elektor 'Summer Circuits' magazines from the '70s. Bat detectors, poltergeist scarers, Dalek voices...

Re the transistor circuit, I built that on Veroboard, and simple though it was, I probably made an error somewhere on one or another of the tracks.

I've kept the board and when I get a mo, I'll revisit it to see if I spot anything. I'd already embarked on a 555 PCB, so carried on with that.

I'm not a huge fan of stripboard - I  prefer to design a PCB, though I don't use any PCB design software, I usually furtle around with 'Paint'.

I've never really got to grips with programs such as Express PCB, though I know it's very popular and widely used. Some of the tracks and pads seem rather thin to my eyes. If space allows I try to use pads that are about 4mm diam, and tracks 3mm wide. The more copper left on the board, the better I find it.  All very low tech, but I don't do it often enough to invest the time in learning how to use PCB design software, (less still auto-routing!). Really, my tag-line sums me up

Well if you aren't wearing a badge at the NVCF, I'll just have to use your Facebook pic as an 'identikit' and accost anyone who seems to have a passing resemblance.

If you see a couple of old fogies who look like this, that's me & 'Mrs Yorkie':

[Mark Hennessy]

Hi David,

By way of much-needed distraction, I just knocked up the transistor circuit on a breadboard, and it works fine for me. I upped the timing resistor from 100k to 330k to stretch the delay to about 3 or 4 seconds, and I used a pair of BC108s. The relay coil takes about 30mA. I tested it with a bench power supply, and would probably test it with an unregulated noisy supply before committing to a build.

Which reminds me - without re-reading the whole thread, I don't think that anyone has mentioned an important detail about the power supply: if you take the 12VAC from the existing transformer and full-wave rectify it, you'll get about 16 to 18 volts DC, depending on how close to 12V the transformer actually is. The 555 will probably be OK, but the relay coil will take more current than you expect. Rather than going to the complexity of a voltage regulator, I'd use a series resistor, chosen to result in 10-12 volts across the coil.

Your earlier PCB layout suggests that the smoothing capacitor will be 100uF - if so, it's worth checking the ripple on the supply rail, and ensuring that it's not confusing the 555 IC. Of course, the sudden voltage drop when the relay coil energises might be a factor here...

Right - back to real life

Cheers,

Mark

[John M0GLN]

Which reminds me - without re-reading the whole thread, I don't think that anyone has mentioned an important detail about the power supply: if you take the 12VAC from the existing transformer and full-wave rectify it, you'll get about 16 to 18 volts DC, depending on how close to 12V the transformer actually is.

I was having a little problem with our door chimes, with the standard Friedland transformer they weren't loud enough, I measured the transformer which was a nominal 12VAC and as far as I can remember it was about 20VAC off load and about 8VAC with the bell push depressed, so I put a small bridge rectifier and a 220μF across the secondary and they work perfectly, just measured across the capacitor and it's 36V, so it's 'capacitor discharge chimes', they've been working like this for about 4 years.

John