Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Door open too long alarm

Status
Not open for further replies.
Hi.
I have a reed switch i would like to add on my door that if its left open too long (variable time adjust) it will trigger a siren or buzzer.
I was looking at a 555 timer but not sure how to put it all together.
If someone would assist.

Yours
 
These are typically called "door prop alarms" and get quite complex. When I did some research, I basically found 2 companies that offered similar products with ebay having decent prices.

12 DC, 24 DC or 24 AC are common supplies. They are typically combined with electric strikes and an alarm system.

"door" in your case is somewhat undefined. I would look at the operation of commercial units before embarking on your simplistic one. Define "too long" Long timer delays are difficult to do with the 555.

You already have a few choices when the door is let open too long: 1) It cancels, 2) It stay in alarm until manually reset.

I WAS considering one for a freezer.

You could look at www.picaxe.com.
 
thanks for the input.
I was hoping to put the reed switch on the door and hook to a circuit. then when the door is open the timer starts for set time (possibly 3 min or less) then if still open after that time and alarm noise will sound until the door is closed.
 
Hi Kiss. That circuit is ideal to what i need, the only problem is i dont have any place to buy the 4541 chip from unless i want to wait via ebay for 1 week. I need this working for Tuesday night.
Is their a way to do a 555 one similar for 3 mins as i have heaps of 555 chips kicking around. thank you
 
Here's a datasheet on the 555: http://www.ti.com/lit/ds/symlink/lm555.pdf

Probably gonna be tough, if you have to do it from your parts box. Look at the app note for the LED.

I think RESET would have to be connected to a monostable with edge detection. (i.e. door closed). Door open would provide the trigger pulse.

Then again, you need edge detection to determine if the timer times out.

A FF would do the alarm on/off type stuff. if you have power all the time you could use a DPDT relay in the standard self-latching mode. One contact is connected so that it holds the coil on. The alarm pulse that is in parallel to the relay and the NC pushbutton that turns off power. A standard start/stop station.

These, hhttp://www.fairchildsemi.com/datasheets/74/74VHC123A.pdf or similar chips have the NICE ability to do positive or negative edge detection.

Some info here: **broken link removed**

There's always issues with squaring up the signals (Schmidt trigger) so the edge detector works. There are logic level translation issues; for instance CMOS is mixed with bi-polar logic. The new logic families fix that.

The 555 is 1.1 * R * C and at like 2 M and 2 uf, it's like 4 seconds. When you start adding electrolytic caps in the timing circuit, leakage and tolerances get to be an issue. Polyester caps are available in +-5%. Some electrolytics are +80%, -20%.

You didn't mention if your switch is Closed or open when the door is open, so that could require inversion.

Power-up issues have always been a problem. So, with stuff dependent on the state at power up, some sort of reset circuit is normally required.

I liked the now obsolete LM3905 timer.

So basically, I see something like.

Switch conditioning (Schmidt trigger)
Edge detection (I'll pick positive, for now) which triggers the timing interval
Edge detection (negative (closed door)) ; resets with a pulse (another monostable)

If the timer gets to the end (another edge detector), the alarm FF (relay latch) stays on.

Two resets you have to worry about: 1) power on and 2) alarm reset.
SOMETIMES an RC circuit works, but an RC followed by a Schmidt trigger is usually better. I purchased a single board computer that would not reset reliably with a linear supply but worked fine with a SMPS. I modified the power up reset circuit.

Edge inputs typically need to be square edges.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top