Burglar Alarm to Door Buzzer Using a Monostable Circuit

Relay it is. I keep putting more than 1W through the simulated buzzer and destroying it (no idea what the real one can handle). The transformer seemed to be building up voltage, but with this configuration it does not. Am I the only one who thinks the transformer reverse connections are odd? Maybe it's a common thing to do.

Hi Simon, Your present real life transformer buzzer arrangement has proven functionality. I would not change anything there except to use the relay for the switch in place of the door switch which will go to triggering the 555 circuit. The D-2 in your latest schematic is a mistake and should not be used as it will tie your low voltage to your mains. I would put the relay switch into the circuit on the same side of the transformer as the buzzer and leave the transformer hooked directly to the mains. I am not sure but this is probably the way it is hooked up in your present real life application as it is the safest and most professional way to hook it up. The transformer relay part of the circuit does not have polarity ( + signs ) as it is AC.  At this point don't worry about the power handling ability of the buzzer as your simulation buzzer may not accurately emulate the real one. I also see that you have upped the R-1 to give yourself about 1 second of buzzer when the door is open. This is probably a good idea. Now it is time to get your schematic approved and build it. I am looking forward to good results.

John

Hi Simon, Your present real life transformer buzzer arrangement has proven functionality. I would not change anything there except to use the relay for the switch in place of the door switch which will go to triggering the 555 circuit. The D-2 in your latest schematic is a mistake and should not be used as it will tie your low voltage to your mains. I would put the relay switch into the circuit on the same side of the transformer as the buzzer and leave the transformer hooked directly to the mains. I am not sure but this is probably the way it is hooked up in your present real life application as it is the safest and most professional way to hook it up. The transformer relay part of the circuit does not have polarity ( + signs ) as it is AC.  At this point don't worry about the power handling ability of the buzzer as your simulation buzzer may not accurately emulate the real one. I also see that you have upped the R-1 to give yourself about 1 second of buzzer when the door is open. This is probably a good idea. Now it is time to get your schematic approved and build it. I am looking forward to good results.

John

What do you mean by "use the relay for the switch in place of the door switch"? I'm hesitant to mess with the door switch, as it took three iterations for the electrician to get one that was reliable installed. They would work for a few times then stop working, and the current one has remained working.

Sorry if I confused the issue. Leave the door switch as it is in the door but run the wires from it to the switch position in our schematic. Run the wires from the relay contacts to the position in the old door bell circuit where the door switch was originally. Here is a revised and completed schematic of how your system should look.

John

Hi Simon, The relay should be rated at about 9 Volts DC and no more than 200 mA. This would mean the relay coil resistance will have to be more than 100 ohms. If it is more it will be better as this will put less load on the 555 Timer.

John

If my Ohm's Law is correct, wouldn't I need (if using a 12V DC supply) 60 Ohms of coil resistance? Nonetheless here is one I found that may be suitable, possibly overkill (but at least not UK stock.)

HE721A1200 - HAMLIN HE721A1200 - REED RELAY, SPST-NO, 12VDC, 0.5A, THD | Newark element14 US

Hi Simon, I really like the way you are researching and checking things out. Your Ohms law is corrent but I do not want the relay to end up drawing the max current that the 555 can deliver without damage. I want the relay to draw mabe half or 100 mA if possible. Also we will loose some of the 12 volt supply inside the 555 timer so the actual voltage on the relay may be 10 or 11 volts. If we have 10 volts and 100 mA then our resistance should be 100 ohms approx. The other consideration when we choose the relay will be the Amp rating on the contacts. I would estimate that 5 amps will be adequate. If the contacts are rated for more, even better. I will try to look at your parts list later.

John

The relay you showed me does not have contacts that can handle the likely current of the buzzer. The fact that you had trouble with the door switches indicated that the buzzer may be drawing a fair amount of current.

John