I have a door entry buzzer that isn't operating the way I'd like.

Right now, when the magnetic door contact is broken (door open) the buzzer continues to go off until it is reconnected (door closed) then the system's internal timer begins and keeps the buzzer active for a set time (adjustable down to 0 seconds).

What I'd like to happen is the buzzer begin when the door opens, and shut off after about 1 second, regardless of whether or not the door has closed.

I'm assuming I can put some sort of circuit between the system and the buzzer to achieve this. The current running to the buzzer is 24V 1.2A. Any ideas on how to do this would be greatly appreciated.

 

Is that 24V AC or DC?

If somehow the existing system thinks the door closes almost immediately after it opens, you would be happy with the buzzer continuing to buzz for a second or two, which the system can do now?

What is the existing "internal system"? Do you have a schematic or other details?

 

24V DC.

Yes, if the system could think the door closes immediately I could set the time delay with the variable resistor.

I don't have a schematic, but here's a picture of the internal system...

The wire on the bottom is from a 16VAC transformer plugged into the wall. The wire on the right is from the magnetic door sensor, which is connected to the COM and NC connectors on the sensor (leaving the NO connector unused). The wire on the top goes to the buzzer.

 

The current running to the buzzer is 24V 1.2A.

That seems strange. First of all, a buzzer shouldn't take anywhere near that much current, and second, I don't see any way that the 16VAC is being boosted to 24V. I see the rectifier module, but I don't see a transformer. Have you actually measured the voltage and current to the buzzer, or are you quoting the "specs" that are writtern somewhere?

In addition, what is written on the 8 pin IC in the middle of the PCB?

 

It's actually a mechanical bell ringer thing. The transformer plugged into the wall says...

Pri. 120VAC 60Hz 30W
Sec. 16.5VAC 20VA

But I measured just over 19VAC coming out of the transformer. The 8 pin IC says...

F EB07
LM
555CN

 

It's actually a mechanical bell ringer thing. The transformer plugged into the wall says...

Pri. 120VAC 60Hz 30W
Sec. 16.5VAC 20VA

But I measured just over 19VAC coming out of the transformer. The 8 pin IC says...

F EB07
LM
555CN

So, the IC is a timer that is controlling how long the bell rings after the door is closed; it apparently controls the relay in the upper right corner via Q1, and the relay switches the power to the bell. But back to the bell requirements of 24VDC at 1.2A; where did you get those numbers? Is that written on the bell?

Anyway, what could be done is to disconnect the wires from the door switch, and reroute them through a new timer circuit that works the way you want, and then route the output from the new circuit back into the connector where the wires were removed. There should be some place on the control board where you could pick off power for the new circuit. The whole thing might be contained in the existing enclosure.

The new circuit could be a NE555 one-shot that triggers for one second when the door opens, and then goes low. That way, the existing timer could continue to operate as it is. You could still adjust it to add to the one second if need be.

 

No, that's not written on the bell. That's just what I measured the PCB sending. As a matter of fact, the bell is kind of loud, haha. Not sure if less voltage would make it quieter? But that's a non-issue compared to making it a shorter ring.

 

No, that's not written on the bell. That's just what I measured the PCB sending. As a matter of fact, the bell is kind of loud, haha. Not sure if less voltage would make it quieter? But that's a non-issue compared to making it a shorter ring.

So you measured the current as well as the voltage? Not that it really matters if you can install the new timer circuit I described.

Can you measure the DC voltage between pins 1 and 8 of the 555 IC?

 

I'm out of time to work on it today, but I'll look into what you mentioned. I'm guessing something like this?...
http://www.instructables.com/id/Make-an-LED-Blaster/step2/Build-the-555-ic-one-shot-timer/

DC voltage between pins 1 and 8 is about 12V.

 

I'm out of time to work on it today, but I'll look into what you mentioned. I'm guessing something like this?...
http://www.instructables.com/id/Make-an-LED-Blaster/step2/Build-the-555-ic-one-shot-timer/

DC voltage between pins 1 and 8 is about 12V.

Yes, that's one version of the circuit, although I wouldn't use the CMOS version. You should be able to pick up that same 12V to power the new timer. Post again when you are ready. Good luck.

 

Ok, looks like this is what I need...
http://clarkson-uk.com/555-timer/operation/frames4.html
Like you suggested, I'll pick up power from the main unit. I'll be splicing this in between the door and the main unit. The connections on the door's magnetic switch are hooked up to NC, but I'm guessing I'll want NO for this to work properly? I'll be running to pick up the materials in just a bit.

I'm guessing this would work if I only use 8 of the pins?
http://www.radioshack.com/product/index.jsp?productId=2062597

 

Use a momentary contact magnetic switch.

 

Ok, looks like this is what I need...
http://clarkson-uk.com/555-timer/operation/frames4.html
Like you suggested, I'll pick up power from the main unit. I'll be splicing this in between the door and the main unit. The connections on the door's magnetic switch are hooked up to NC, but I'm guessing I'll want NO for this to work properly? I'll be running to pick up the materials in just a bit.

I'm guessing this would work if I only use 8 of the pins?
http://www.radioshack.com/product/index.jsp?productId=2062597

Yes, but why not get this one?
http://www.radioshack.com/product/index.jsp?productId=2062596

And bear in mind that I believe this will work, but I haven't tried it, and it may require some "tinkering." There could be an easier way. For example, maybe ramancini8 will explain his/her idea.

 

Ah, thanks tracecom. I didn't see that one when I was searching.

ramancini8, I'm open to that idea, but after some googling, I'm not sure... isn't that what I already have? Maybe you could point me toward what you're talking about. Thanks.

 

Ok, I'm stumped. I got the stuff and hooked it up, double checked it. But it just keeps the bell continually going. I unhook the power source running to my contraption, as well as the trigger wires, but same result. idk if this means anything, but the power across the door trigger input (on the main unit) is 12V with nothing hooked up, and about 0.7V with my board hooked in. Am I missing something obvious?

 

Ok, I'm stumped. I got the stuff and hooked it up, double checked it. But it just keeps the bell continually going. I unhook the power source running to my contraption, as well as the trigger wires, but same result. idk if this means anything, but the power across the door trigger input (on the main unit) is 12V with nothing hooked up, and about 0.7V with my board hooked in. Am I missing something obvious?

What you should do is get your new one-shot working on its own before you try to connect it to the door bell. You can use a 9V battery for temporary power. Step by step.

What values did you use for Ra and C?

 

I have another 100k for Ra, and 4.7uF for C.

I have it hooked up to 12V and I get 12V at terminals 2, 4, and 8. I'm getting 8V at terminal 5. And 0.05V at terminal 3. None of these values change (even for a moment) when connecting the trigger. What else should I do to troubleshoot?

 

Here's the circuit I use for a NE555 one-shot.

The 100k resistor and 4.7 μF cap you are using for timing will produce a pulse of just over 1/2 second. Increase the cap to 10 μF if you want a full second.

Bypass the .01 μF cap that you have connected to pin 2, and try it. If that works, you will need to add a couple of 10k resistors, one to +12 v on both sides of the .01 cap. (See my schematic.) Then try it again.

If that fails, post a close up photo of your assembly, and maybe someone can spot a wiring error.

You also need a .1 μF cap across the power pins (1 & 8) of the NE555 to suppress noise on the power.

 

Success on the one-shot using this!...
http://www.finkbuilt.com/static/images/articles/timer4.gif

I'm getting power off the main unit and when the trigger inputs are touched, the output is 12V for about half a second. It's perfect except that when hooked up to the input on the main unit, the main unit sees it as a closed circuit so the bell continually rings (I'm assuming because there's always about 0.05V running through the output).

I tried hooking up the output directly to the bell, but I get nothing from the bell. Do you think it actually requires 16V?

I'm just guessing here, but would it be possible to use a transistor as a switch so that the connection to the main unit's input is left open until it rises above 1V or so?

 

Is the door switch open or closed while the door is open?