Burglar Alarm to Door Buzzer Using a Monostable Circuit

So, um, the buzzer works from 12VAC, ya' figure? 

I really have no idea.

Hi Simon, Here is a simple monostable 555 timer circuit that I have used for similar applications. R-1 should be between 10K and 14 Meg and C-1 should be between 100pF and 1000uF. The formula for calculating the time that the output from pin 3 will be high is: Time( in seconds) = 1.10 * R-1 (in Ohms) * C-1 (in Farads). The power to the circuit should not exceed 15 volts DC. If you are going to power a buzzer with the unit you will have to use a relay or other means to boost the power as the output of the 555 timer is limited to about 200 mA.

John

I've seen that circuit, but been unable to simulate it before purchasing the needed parts.

I would also need to invert the momentary switch output and add to it it to get something that will work with my input and output volatage and current needs.

Hi Simon, From your description of the circuit and the application of the switch you should not have to invert the input. While your switch is a NC switch the fact that the door is normally closed pushing on it makes it actually a NO switch. You may have to put a capacitor in series with the switch to just give the trigger a pulse when the switch closes instead of a continuous closure, though I am not sure about that. A simulation or a rough prototype would let you experiment. The circuit itself is a standard off the shelf design and will work as I have used it successfully several times in my projects. You will probably have to rectify and regulate the voltage from the transformer to 12 volts for the timer circuit but you could also use the power from the transformer to continue to power the buzzer with a small relay, controlled by the 555,  to turn it on and off. The project you propose is very doable and I agree with you it would be much more professional than a continuous buzzer.

John

Another way to go would be to detect the sounding of the buzzer would be to attach the series combination of a resistor, a half-wave bridge rectifier and the B-E junction of an NPN (E attaches to BOS) transistor.  The collector is connected to TOS through a resistor.  This node goes low when the buzzer sounds, except during the zero-crossings.  An opto-isolator could be used instead here, BTW, shunt the input LED with a reversed signal diode.  So, these zero crossings occur at a rate 2x the line fq.  We can count-and-compare for our duty pulse, ultimately connecting to a relay as John describes.

John's way works:  Using the 555 in the datasheet way is considered unstylish in some design circles, but that's no nevermind.

Hi Simon, I have set up a mock up of an improved circuit which allows the door switch to close and remain closed. The components that I have selected for C1 and R1 give a time of approximately 5 seconds. This circuit only turns on an LED but it could just as easily turn on a low current relay which could turn your buzzer on and off. This circuit will only ring the buzzer once for 5 seconds each time the door is opened no matter how long it is held open. It will reactivate only if the door is closed and reopened.

John

Thanks, I appreciate it. I'll fiddle with your design and see what I can simulate as far as relays go, that way I can give the schematic to the maintenance guy to get his OK on safety.

Hi Simon, You probably already know this but it is important to put a diode accross the relay coil reverse biased to the power so that any inductive voltage spikes from the relay coil can not damage your other components. Good luck on your project.

John

No I didn't know that. I'll have to look up what that means.