Doorknob touch alarm

Touch Alarm Breadboard Circuit

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

Door 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.

Oscilliscope without Touch

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.

Oscilloscope with Touch

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.

Oscilloscope Node 9

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.

Alarm Tripped

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

  1. Using the intended buzzer for your hotel or dorm room. (This is a bit more impressive than the old sock trick.)
  2. 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.)
  3. 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

44 thoughts on “Doorknob touch alarm

  1. SLATE

    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.

  2. Jason

    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.

  3. Matthew

    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.

  4. Dave

    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.

    “Who’s there?”

  5. None

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

  6. harrison

    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?

  7. Wim

    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.

  8. Matthew

    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.

  9. Matthew

    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.

  10. vineela

    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.

  11. Dave Johnson


    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.

  12. raghvedra tatke

    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.

  13. vag

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

  14. joseph

    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!

  15. jakub

    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?

  16. Dave

    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

  17. Shashank


    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?


    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.)

  19. rayman

    i am doing this doorknob touch alarm as our school project…but i am having a problem with it after transferred the component to matrix board…we used 9V as a supply voltage…that c2 is it 0.0047F or 0.0047uF?..the problem is that the alarm triggered continously…we set the potentiometer to 12.9kohms already for the sensitivity but still it keeps on triggering…i need help asap =(

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  22. singay wangchuk

    I would like to try this guys. seems interesting, we have been instructed to find a topic for our mini project.
    so therefore, could anyone of you help me do this by emailing me the procedures and parts that i need for proceed my mini project for this semester.

  23. RK Murthy Challa

    It is fantastic and very useful circuit for every body . I noticed it too late.

    29-06-2012, 18.24

  24. Aimie

    I got project that must be submit before this april 2013. It is about door knob touch alarm. Ive already testing the component on the breadboard but the result is it is not success.when i connect it with 9v battery, the buzzer produce sound and the led lights up.Cannot stop. Now im doing my troubleshooting. Is anyone here know why is the sound keep on and the led cannot turn off???? Thank u

  25. Jt marcelino

    Hi, i am looking for a design project, and i think this one is perfect for my group as engineering students. It would be my pleasure id anyone of you could send me the procedures and the list of components. Thank you.

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