Door bell timer using 555 IC?

[Yorkie]

I have a chum who wishes to ensure that when a visitor presses the door bell button - even if only momentarily - the bell will continue to ring for several seconds. It seems to me that the 555 IC would be an ideal candidate for a timer, by using it to operate a small relay which would have normally open contacts, but would close when the door bell is pressed, and remain closed to power the bell for several seconds. I've found a simple circuit of a 555 monostable timer, which I've attached. My query is whether at pin 3, the voltage and current would be sufficient to pull in a small relay with modest current requirements, the contacts of which will power the bell via the bell circuit. IE, will the supply voltage or thereabouts appear at pin 3 when 'S1' (the doorbell button in this application) is pressed to start the timing period, sufficient to energise the relay coil, or have I lost the plot?

I've highlighted the relevant part of the circuit.

(I'm aware that the duration of the 'on' period  is determined by the choice of R1 and C2, and work out how to calculate that). 


Any thoughts please anyone?

[Ed_Dinning]

Hi David, best to use a 1K from the output to a BC107 or similar to drive the relay.
Also pin 5 should be grounded through 10nF or strange things can happen.
Reset pin (8?) should be grounded via 10nF and fed through a 1k (ish) resistor or you can get race conditions between reset and Vcc

I take it this is the c-mos version of the timer.

Ed

[Refugee]

It looks like it is wired up like a computer power supply MOSFET.
Keep R1 and C2 and connect the pin 6/7 point via a 1meg resistor to the gate. source to ground drain to bell direct and without the relay.
The button goes via a 10 to 47 ohm resistor across C2. swap the positions of R1 and C2.
FETS are like triodes with the gate voltage just above drain voltage for switch on and they amplify at 10s of amps per volt.

[Yorkie]

Thanks for reading my post and for your helpful responses guys. I found the circuit in a Babani book by Robert Penfold entitled 'Circuit Source Book #2'.

It seemed to have the merit of simplicity, always a plus point for a simply soul like me!!

It showed just the normal '555' - I think the CMOS version is the 7555?

[Yorkie]

I shoud have taken the trouble to check the 555 spec! It reveals that the output (pin 3) when high, is approx 2V beow the supply rail and will deliver up to 200mA - far more than is needed to operate a small relay. A diode across the relay coil maybe, to cope with back EMF?



  

[Ed_Dinning]

Hi David, most of the 555's used now are the CMos versions (7555 etc).
There can be problems with the bipolar type although it would drive a small relay coil directly provided you have a freewheel diode across it.

Ed

[Yorkie]

Thanks for the info Ed.

[Eddystonepete]

Hi David, I,ve been using a similar 555 cct to start a chime doorbell for about 3 years. The only problem I had was the 555 triggering on random spikes, i.e. it would trigger when I set the alarm. I used a much lower pull up resistor, your cct shows 10k, I think I ended up with about 1k. You may then have to alter the R and C that connect the push. Might be no bad thing anyway as most bell pushes like a bit of spark anyway!

Peter

[Mark Hennessy]

Honestly, I wouldn't bother with a 555 timer for such a simple application - but that's because I don't fully trust them

I've attached something that would probably work. Component count - not including switch or relay - is 8. This is the same as the 555 circuit, but it has the advantage that it's much easier to understand, and you have complete control over how it behaves.

Basically, a pair of transistors form a Schmitt trigger. The Schmitt action isn't strictly required, as the mechanics of the relay will ensure that it is either on or off, but it doesn't hurt... Omitting the 10 Ohm resistor will do away with the Schmitt action and save £0.01 at the same time

At rest, the capacitor charges up (to around 0.6V), turning on the first transistor. This turns off the second.

When pressed, the switch rapidly discharges the capacitor. The first transistor turns off, and the second transistor turns on.

When the switch is released, the capacitor starts to charge, and at some point, the first transistor turns back on, switching off the second one in the process.

Obviously, the relay coil must be matched to the supply voltage (as would be the case in the 555 circuit). The back-EMF diode is definitely recommended. The transistors can be any small-signal NPN devices - the BC108 is a perfectly good bet. Obviously, the second one must be able to handle the current of the relay coil...

The resistors are pretty arbitrary. The 22k needs to supply enough base current to the second transistor - if we assume a Hfe of 200 and a (generous) relay current of 50mA, then the base current is 0.25mA. The 22k supplies approximately double that, which is a decent margin for a one-off. Of course, the 100k and 100u RC combination set the time delay - feel free to experiment. Finally, the 100 Ohm resistor is just to limit the current seen by the switch contacts - depending on the switch, this might not be required (especially if it enjoys the "wetting current") - or perhaps the resistance of a long run of "bellwire" might present enough resistance.

The quiescent current consumed by this (if that matters) is mostly determined by the 22k resistor - about half a milliamp. Much lower than the non-CMOS 555 chip. Fine for a mains powered bell, but a bit high for a battery operated unit. Replacing the second transistor with a Darlington pair would help here. Also, finding the most power-efficient relay will help. For example, the NEC "MR82" types need 17mA, which means the 22k could be increased to 100k with no problems.

If you wanted to drive down the quiescent current still further, you could use a CMOS chip - e.g. the 40106 is good for this sort of thing.

Depending on the bell itself, it might be possible to omit the relay...

Anyway, just a suggestion...

Cheers,

Mark

[Yorkie]

Thanks Peter, for reading my post and outlining your experiences.

And thanks Mark for the alternative circuit, which will be simpler to construct than using a 555.

It will be powered from a mains transformer which supplies the bell, and will also be rectified to supply the DC for the timer circuit, so current consumption isn't an issue.

Another little diversion to add to my 'do list'!