Research: Building Schrödingers box

Posted on December 4, 2011


The problem

Schrödinger's cat-dilemma: is it dead or alive?

We travel. We have a cat. The cat is old. The cat can not come with us in most of all cases. We currently have no-one to take care of the cat when we are gone. As the cat is old, she can die while we are gone.

We want to know if she is well.

Observing the cat by its movements

As long as the cat is alive, she will move through the house. If we can observe these movements, we can establish with certain certainty that she is alive. Movements are:

  1. To the kitchen to feed herself
  2. To the living room to sleep in her hammock or observe the pigeons on the roofs of the houses at the other side of the street
  3. To the bathroom to relieve herself


  1. Movement-detector – We need something to moniter the cats movements. The simplest solutions are to use a light bridge or infrared-sensor. We go more into detail about this later
  2. Wireless transmission of states – To avoid running cables through the house, I want to use wireless technology.
  3. IO board to connect to computer/server – When we receive the signals, we need some kind of technology to feed it into a computer
  4. Computer/server – To broadcast the state of the cat: dead/alive, we need a computer/server

Jumping towards the conclusion of this blog: Learning basic electronics

As I was researching several options including cheap electronics like 6 euro wireless doorbells and Infrared movement sensors, I found none really satisfied me.

Using wireless parts like XBee and Pololu Wixels to send the state of each sensor feels like overkill: like calling a cab to cross the street. Most parts in the chain can be solved by using very basic electronic components.

Additional blog posts: basic schematics to build sensors and do wireless IO

As my own knowledge of the basic electronics needed for this project is very limited, I will need to do some research and re-vitalize my knowledge: which will be posted in later posts.

Bottom line of these posts:

  1. Building your own sensors and electronic components – For solutions like Arduino, Pololu Wixels and the IOIO board for Android
  2. How does it work? – Basic descriptions of each part and how they work and work together


  1. General purpose components – Like transistors, capacitors and resistors: which are multi-purpose and which you can buy in any electronics shop
  2. Simple schematics – Which are easy to grasp and easy to build
  3. Simple explanations – Which will be easy to understand for people who never did any electronics

Basic setup of the Schrödinger Box

Basic setup of the wireless observers in the Schrödinger box

To know with some certainty if the cat is still alive, we need to observe either the cat or the environment.

We choose to observe the environment for certain state-changes, in this case movement, to use as the basis of our assumption that the cat is still alive.

Movement in house == “cat is alive”

In other words: if we can somehow sense that something is moving through the house while we are gone, we can assume with high probability that the cat is still alive.

Possible solutions

  1. Infrared movement-sensor – After testing with a low cost Infrared sensor, I discarded that solution. The 6 euro sensor simply is not reliable in its response. It is very sensitive to surrounding light, the additional light sensor interferes too much and the so-called PIR (Infrared sensor) seems to be only responding when I trigger it from very close distance
  2. Light bridge – The alternative is to use a light bridge. A light bridge consists of two parts: a light source, shining on a light-sensor. If the light hits the sensor, all is well. If the light-source is not detected, something is passing the bridge and “breaking” it, marked as “activity” in the illustration above. The schematics for a light bridge is very simple and easy to build
  3. Wireless doorbell – In the first prototype I chose two 6 euro wireless doorbells to broadcast the state of our activity-detector. Used separately, the doorbells function fine. When you activate the transmitter of one, the other remains silent. When you activate the transmitter of the other, the one remains silent. However, when connected to my experimental setup, both receivers start to influence each other, leading to both receivers being triggered when one doorbell is activated and not responding at all when the other is activated.
  4. 2 transistor FM transmitter – About 25 years ago I built a very simple FM transmitter, consisting of 2 transistors and a hand full of resistors and capacitors. The schematic is shown below

Light bridge, "activity" is sensed when the light source is no longer registered by the sensor

Advancing the possible solutions

  1. One receiver, many transmitters – To avoid the issues I had with the electronic doorbells, I decided to go for a “one receiver, many transmitters” solution. This solution has some disadvantages and limitations, but most are irrelevant for this project as we only have one cat. One receiver, many transmitters means that:
    1. All transmitters broadcast on the same frequency – As you only have one receiver, you can only tune that receiver to one radio channel an d receive one state at the time.
    2. You can not receive more than one signal – If more than one transmitter is activated at the same time, their signals will “compete” for reception. In most cases, the strongest signal will win. So only one of the two signals will be received
    3. You can not use this when you have simultaneous updates – Since only one signal will be registered and processed
  2. Using a different signal per sensor – To distinguish each sensor, we will be using a different tone / wave length per sensor. For instance:
    1. Sensor 1: 100 Hz – a low humming tone, representing Sensor 1
    2. Sensor 2: 1000 Hz – a higher pitched tone, representing Sensor 2
    3. Sensor 2: 2000 Hz – even higher
  3. Interference – The 2-transistor FM transmitter is quite basic and will “echo” its signal over multiple radio channels, apart from the channel we want and need to broadcast on.
    1. Only short bursts – As our signal will be sent for only a short amount of time (less than one second when the cat passes) and is quite weak, we can assume it will not spoil the radio-listening pleasure of our neighbors
  4. Wave / sound generator – To create the specific tone/signal per sensor, we need a wave/sound-generator.
  5. High and low-pass filters – To filter out specific the signals from each specific sensors, we need to filter out each specific tone/signal/wave length and feed that to a “switch” on the receiving side
  6. FM receiver – To receive the signals from our Sensors we need an FM receiver. As FM receivers are relatively complex (definitely more complex than FM transmitters) we use a cheap receiver from wherever we can find one
  7. Tuning the transmitters and the receiver – Each transmitter will be tuned to broadcast on the same channel. Using the individual tone/wave generators as input for each transmitter and using the speaker of the FM radio/receiver, we can easily test and fine-tune the transmitters by simply tuning the transmitter until we hear the beeping sound on our receiver

Block schema for simple wireless activity monitor

Block schema of our solution


  1. Activity registered by sensor – Cat breaks light bridge
  2. Power switch – “Activity” is sensed and used to turn on the “Power Switch” for the Wave Generator and Transmitter
  3. 100 Hz signal transmitted – Wave Generator starts generating 100HZ signal which is broadcast on FM by the Transmitter
  4. 100Hz signal received – The Receiver receives the signal and passes it to the Filters
  5. 100Hz signal filtered – The 100Hz filter filters out the 100Hz signal and creates a “pulse” which is sent to “pin 1” of the IO board
  6. Activity registered by IO board – The IO board registers the activity related to the sensor that registered activity
Simple 2-transistor FM transmittor, used to transmit state: “activity”