Doorknob touch alarm

Posted by Matthew Tue, 25 Apr 2006 10:14:00 GMT

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

Doorknob touch alarm 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

Comments

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Jason said about 15 hours later:

You think a relay or SSR in place of the buzzer could easily be implemented with this design? Then this circuit could turn off/on lamps or other 120Vac items.
Matthew said about 16 hours later:

I’m sure it is possible. The circuit will function with a VCC from 3 to 15 volts. All you need to do is match the VCC up to the relay or SSR. I’m not sure how much current they require, but you should be able to connect it directly to Q/Q_bar or add a buffer.
cddborn said 17 days later:

I could see this connected to a recording of a dog barking.
Dave said 17 days later:

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?”
None said 17 days later:

How does this circuit function as term as the alarm getting triggered does it close a connection? Open a connection? Sense hotness in hand?
Wim said 18 days later:

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.
Matthew said 19 days later:

I would refain from calling it capacitive touch sensing.

Here is what happens in a nutshell:
1. A ‘square wave’ is generated by the oscillator.
2. The wave travels down two seperate paths. Generally, the top channel (connected to the doorknob) charges faster than the bottom channel.
3. When you touch the doorknob, some of the current is grounded out by your body. This causes the top channel to charge slower.
4. 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.
Matthew said 20 days later:

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.
harrison said 23 days later:

what manufacturer are the IC’s and what is the full part number?
harrison said 23 days later:

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?
Dave Johnson said 2 months later:

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.
vineela said 2 months later:

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.
Marcos said 2 months later:

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
Jobette said 5 months later:

this sounds cool. i want to try it too. Can anyone send me the procedure on how to make this project??? thank you!
abu zaid haris said 5 months later:

i m astudent of ee. i hv been given this project . plz. help me
Phyu Sin said 5 months later:

Dunno how to do…Gosh
Mon Mon said 5 months later:

I will help you. But give me a break
joseph said 6 months later:

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!
jakub said 6 months later:

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?
Matthew said 6 months later:

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.
raghvedra tatke said 7 months later:

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.
harry said 7 months later:

van we also use this in the pcb?
vag said 7 months later:

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!!!
SLATE said 8 months later:

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.
Mike said 8 months later:

Why does C2, not have a unit on the schematic, is it uF?
jethro said 10 months later:

can u send me procedures and the list of components and where to buy them?its brilliant,whwew!
Dave said 10 months later:

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
himani said 10 months later:

i m unable to find what i want. there is lack of information. so i m not satisfied with urinformation.
mia said over 2 years later:
hey! i`m doing this sort of an investigative project. need help. :) anyone care to educate me more? thanks!
Shashank said over 2 years later:

Hi

I wonder if this small circuit can add protection to my Motorcycle. Is it possible to use it with motorcycle, as motorcycle has more metal area than knob.

Also is there any substitute mosfet for ZVNL110A? can i use BS170?
johanna said over 2 years later:

where can i find the parts list of this project? im so interested with this particular project!
Matthew said over 2 years later:

The parts can be found at a local electronics components shop. You can also try to find the appropriate components online at http://www.mouser.com or http://www.digikey.com
jbridge@makelengineering.com said over 3 years later:

Hey! That buzzer you used in this project would be perfect for something Im desigining. Could you e-mail the specifics on it? (i.e. who u ordered it from, the name of it, model. Im getting the impression its s star mmb-01 buzzer, but am having a hell of a time finding anything on the web.)
Mehbub said over 3 years later:

What is alternative to ZVNL110A? will any Mosfet work?
piyushmlk@yahoo.co.in said over 3 years later:

why can’t it be used for metal doors
teja said over 3 years later:

hey its very nice.I have to do this as project.Please help me
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can you give more

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robert said over 3 years later:

where is the foil pattern?
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Thanks for posting the graphic design, I would like to read more ..

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andrew said over 3 years later:

Thanks for providing us that schema.
andrew said over 3 years later:

So thank you again:)
j_vague@msn.com said over 3 years later:

I plan to build this circuit to play with and to compare it to other touch circuits. I understand all of the schematic except one part. Are there two ZVNL110A’s or only one? It is shown in two places, but both places are identified as Q1, and I can only identify one in the photo.
Derma Tend said over 3 years later:

hey the two battery is not enough for the chemical mixing process…… may be it need some more power..

thanks
Topmidi said over 3 years later:

You can use BS107 instead of ZVNL110A. The circuit works very good with this MOSFET.