Superpositioned: Doorknob touch alarm

Doorknob touch alarm

Matthew — Tue, 25 Apr 2006 03:14:00 -0700

I recently found a doorknob touch alarm schematic while browsing Discover Circuits’ archives. The project was originally intended as a present for my brother’s dorm room, but a bad capacitor and the lack of a proper oscilloscope caused delays. It has not made it off the breadboard, and it probably will not until his next semester. The circuit contains a few basic elements, an flip-flop based oscillator, a set of delays, a flip-flop as a sensor, and the audible alarm.

The schematic

Download the schematic

The oscillator

The first section of the circuit is an oscillator based on a flip-flop. Clock and D are both grounded while Reset is tied high. Hence, the output Q will only be high if Set (node 6) is high. When the output is low, the transistor Q1* is cutoff. This allows *node 6 to be charged with a delay relating to the system of impedances R1*, *R2, R3*, and *C3. Once the voltage at node 6 triggers Set, the output changes to high and Q1* is opened. *Node 6 then discharges out through the capacitor. Once node 6 is low enough, Set is no longer triggered and the output is automatically reset (because R is tied high) to low and the process is repeated.

The screen capture below shows node 6 charging and discharging as the blue trace. The yellow trace is the output at node 1. You can see that the output turns high when node 6 reaches the switching threshold of the flip-flop (about 1.8 volts). Right afterwards it spikes up due to feedback through C2*, but quickly starts discharging. The oscillator switches off when *node 6 returns below the 1.8volt switching voltage. Feedback through C2* draws *node 6 to ground before the process repeats itself.

In order to change the period of oscillation, adjust the value at C3*. If you would like to make the pulses longer, adjust *C2. The circuit works best right where it is at, though.

The delay and ‘sensor’

The output of the oscillator is divided down two paths. The time constants of the two delays are nearly equal and can be adjusted with the sensitivity potentiometer. The path to node 11 is the Clock input of the flip-flop, and the path to node 9 determines if there is an alarm or not.

In the capture above, node 9 high than the the clock. Hence, the flip-flop stays high when the leading clock edge triggers it to lock. When the doorknob is touched, your body absorbs some of the charge and node 9 charges slower. This can be seen in the capture below. When the clock edge rises, node 9 is not high yet and low value is locked into the flip-flop.

The alarm

The designer uses an audible buzzer in order to relay the alarm. This is also my intent for the circuit, but I use a LED in my photos because you cannot see sound. They are both attached to the inverting output of the second op-amp (Q-bar) because it is high when the alarm is triggered.

There is an endless number of uses for this circuit, but I will just name a few crazy ideas:

Using the intended buzzer for your hotel or dorm room. (This is a bit more impressive than the old sock trick.)
Connecting the output to a relay that triggers the doorbell for you house. Just make sure to put it in parallel with your standard doorbell switch. That way you can still hear the Fed-Ex man. (This one has a major cool factor when someone opens your door.)
Tying the output into a security or home automation system. You could have the lights turn on as soon as you touch the door handle to scare the dog away from laying on the door.

Photo Gallery

"Doorknob touch alarm" by himani

Thu, 15 Mar 2007 04:18:47 -0700

i m unable to find what i want. there is lack of information. so i m not satisfied with urinformation.

"Doorknob touch alarm" by Dave

Tue, 13 Mar 2007 05:33:54 -0700

hi would like to know what component the ZVNL110A (Q1) is? and the flip flop chip as all the pins from 1-14 are used apart from 2 and 13 does this mean 2 and 13 are just floating no connected to anything? thank you

"Doorknob touch alarm" by jethro

Thu, 08 Mar 2007 20:06:23 -0800

can u send me procedures and the list of components and where to buy them?its brilliant,whwew!

"Doorknob touch alarm" by Mike

Tue, 16 Jan 2007 19:30:28 -0800

Why does C2, not have a unit on the schematic, is it uF?

"Doorknob touch alarm" by SLATE

Sun, 31 Dec 2006 23:33:07 -0800

Why do you need a parts list? If you cant figure them out, you probably shouldn’t be building it.

The parts are on the schematic. I don’t even know how to read a schematic, yet I’m smart enough to figure out the parts are right on it. You just have to know what the symbols mean. A quick Google will tell you what the different symbols mean.

Hell, just looking at it, I just figured out that ) | is a capacitor and ^v^v^ is a resistor.

"Doorknob touch alarm" by vag

Fri, 08 Dec 2006 01:30:33 -0800

can somebody please send to my email ad the parts list and procedure of this project (vicvag1314@yahoo.com). i need it asap. Tnx u very much!!!

"Doorknob touch alarm" by harry

Thu, 07 Dec 2006 23:50:35 -0800

van we also use this in the pcb?

"Doorknob touch alarm" by raghvedra tatke

Fri, 24 Nov 2006 04:24:34 -0800

i,m satisfied by the information. now i can make door knob touch alarm very easily.alarm is very effective it even wakes me up in the morning.

"Doorknob touch alarm" by Matthew

Tue, 07 Nov 2006 15:32:29 -0800

As soon as you take your hand off the door knob, the alarm stops. I am sure you could easily add to the circuit to modify its behavior.

"Doorknob touch alarm" by jakub

Tue, 07 Nov 2006 13:49:52 -0800

When the finger is touched to the door and the alarm goes off, does the alarm stay latched on? or will the alarm stop when the hand is taken off from the door knob? so will i be able to turn the buzzer on and off as i touch the knob and take my hand off the knob?

"Doorknob touch alarm" by joseph

Thu, 02 Nov 2006 02:27:38 -0800

ei i need an written output ‘bout “how does a touch door knob works?”

can u help me out!

i need it asap tnk u vry much!

"Doorknob touch alarm" by Mon Mon

Mon, 16 Oct 2006 03:38:24 -0700

I will help you. But give me a break

"Doorknob touch alarm" by Phyu Sin

Mon, 16 Oct 2006 03:37:11 -0700

Dunno how to do…Gosh

"Doorknob touch alarm" by abu zaid haris

Mon, 09 Oct 2006 10:22:20 -0700

i m astudent of ee. i hv been given this project . plz. help me

"Doorknob touch alarm" by Jobette

Sun, 08 Oct 2006 00:26:13 -0700

this sounds cool. i want to try it too. Can anyone send me the procedure on how to make this project??? thank you!

"Doorknob touch alarm" by Marcos

Fri, 21 Jul 2006 17:10:13 -0700

ME and my group are having a senior project and we wanted your permission to do the project. I would like you to send me a email with the part kit list and every step how to do the project. my email is carbajalcr7@aol.com or tony_nguyen2020@yahoo.com. Thank you

"Doorknob touch alarm" by vineela

Thu, 20 Jul 2006 09:40:55 -0700

i am studying eng(eie).we are given this project.can u plz give me someguidelines how to start the project and where to get the components.

"Doorknob touch alarm" by Dave Johnson

Sat, 08 Jul 2006 09:22:21 -0700

Matthew,

I’d like your permission to link to this project from my Discover Circuits website. I think people would like more details on how my circuit works. Thanks for taking the time to build it, test it and document it. I’m pleased to see that someone finds it useful.

"Doorknob touch alarm" by harrison

Thu, 18 May 2006 19:17:04 -0700

and why do the resistors say “2.2 m” and not 2.2 ma? what is an m? also,voltage rating for capacitors, and where to find all the parts?

"Doorknob touch alarm" by harrison

Thu, 18 May 2006 19:01:56 -0700

what manufacturer are the IC’s and what is the full part number?

"Doorknob touch alarm" by Matthew

Mon, 15 May 2006 21:52:03 -0700

My last comment is somewhat misleading. This is a capacitive touch sensor by definition. However, touch sensors typically contain more components and operate at much higher frequencies.

"Doorknob touch alarm" by Matthew

Sun, 14 May 2006 07:02:45 -0700

I would refain from calling it capacitive touch sensing.

Here is what happens in a nutshell:

A ‘square wave’ is generated by the oscillator.
The wave travels down two seperate paths. Generally, the top channel (connected to the doorknob) charges faster than the bottom channel.
When you touch the doorknob, some of the current is grounded out by your body. This causes the top channel to charge slower.
The second flip-flop is designed to detect which channel charges faster.

Your body’s capacitance does the work. Therefore, this is the simplest form of capactive touch sensing. Good quality capacitive touch sensors are much more complicated.

"Doorknob touch alarm" by Wim

Sat, 13 May 2006 22:41:13 -0700

None: It’s capacitive, I think. Touching the doorknob adds capacitace to one leg of the circuit, making it take longer to charge up, and the second flipflop is basically comparing the charging times of the two legs.

"Doorknob touch alarm" by None

Fri, 12 May 2006 18:10:10 -0700

How does this circuit function as term as the alarm getting triggered does it close a connection? Open a connection? Sense hotness in hand?

"Doorknob touch alarm" by Dave

Fri, 12 May 2006 14:53:11 -0700

Nice! And yeah, there could be a lot of different applications…

I had a bedroom doorknob “alarm” of sorts, as a teen, myself. It was a wire from the 2nd anode terminal of my old B&W TV set to the doorknob.

“Yelp!!!” “Who’s there?”