When automating special effects, the most important part is making sure that they are well timed.  For live events such as a haunted house, the best way to get the timing right is to use sensors to detect where your guests are and when they reach various locations. So for this Halloween, I am doing a series of projects that demonstrate different sensor that can be used to automate a haunted house.

In this project, I am going to show you how to make a simple proximity sensor.

## Project Steps

### Materials

Here are the materials and tools that you will need to make a simple proximity sensor:

Materials

Infrared LED

Infrared Phototransistor

33 ohm Resistor

10 kohm Resistor

Black Heat Shrink Tubing (the same diameter as the phototransistor)

Perf Board

Tools:

Soldering Iron

### How an Infrared Proximity Sensor Works

To make a simple proximity sensor, all you need is a light emitter and a light detector. The light emitter is constantly on. Whenever that light hits a nearby object, some of the light is reflected back to the detector. The closer the object is, the more light will be reflected. By measuring the output of the light detector, you can get a rough approximation of how close the object it.

This isn’t exact because the amount of reflected light also depends on the physical properties of the reflecting surface (sometimes called its reflectivity). However, it works well enough to use to activate special effects in a haunted house.

I chose to use infrared light for this sensor because it is invisible to the human eye. So your guests won’t notice it. Also it won’t experience as much interference from the lighting in the room.

### The Sensor

The IR LED is connected to the 5V supply with a 33 ohm series resistor. To find the appropriate resistor value for the LED use this formula:

Resistor Value = (Supply Voltage – LED Voltage) / LED Current.

In my circuit, the supply voltage is 5V. The LED is rated for 1.3V at 150 mA. This gave a resistor value of 24.7 ohm. The closest resistor that I had above this value was a 33 ohm resistor. So I used that.

The emitter of the phototransistor is connected to ground. The collector is wired to a 10 kohm resistor that is connected to 5V. An additional wire is connected to the collector to act as an output pin for the sensor. I soldered the sensor components together on a small piece of perf board. I added three pins to connect the sensor to the signal processor.

To ensure that only reflected light is detected by the phototransistor, it is important to add a light barrier between the LED and the phototransistor. The most effective way to do this is to put a black piece of heat shrink tubing around the phototransistor. This also helps to make the sensor more directional.

### Signal Processing With an Arduino

In most cases you will use a microcontroller such as an Arduino to monitor the signal from the sensor. To do this, connect the ground terminal from the sensor to the GND pin on the Arduino. Connect the 5V wire from the sensor to either the 5V pin or a digital output pin set to HIGH. Then connect the signal wire from the sensor to an analog input pin on the Arduino.

The AnalogRead function measures the voltage of the signal coming from the sensor. When the voltage drops below a set threshold, the Arduino activates the your special effects. Here is an quick example of the code that you could use.

// Example Code

int InputPin = 0; // analog pin 0 is the input pin

int OutputPin = 1; // digital pin 1 is the output pin

int val = 0; // variable to store the value read

void setup()

{

pinMode(OutputPin, OUTPUT); // sets the digital pin as output

}

void loop()

{

digitalWrite(OutputPin, LOW); // sets the output pin initially to LOW

if (val < 800) // if sensor value is below threshold set output HIGH

{

digitalWrite(OutputPin, HIGH); // sets output pin HIGH to activate special effects

delay(1000); // waits for a second

}

}

### Signal Processing With an OP Amp

If you don’t have an Arduino, you can use an op-amp (operational amplifier) to monitor the signal from the proximity sensor. All you need is a 741 op-amp IC and a potentiometer (variable resistor). Connect the 5V supply to one side of the potentiometer and to V+ pin on the IC. Connect the ground wire to the other side of the potentiometer and to the V- pin on the IC. Then connect the non-inverting input (marked as “+” to the center lead of the potentiometer. Lastly connect the inverting input (marked as “-“) to the signal wire from the sensor.

This configuration of the op-amp functions as an adjustable comparator. The potentiometer sets the reference voltage for the sensor. Then whenever the signal from the sensor drops below this reference voltage the output goes HIGH.

Depending on what kind of circuit you want to activate, you may want the output to be a LOW signal instead. You can do this by switching the wires connected to the op-amp’s input pins. So if you connect the sensor to the non-inverting input and connect the potentiometer to the inverting input, the output will be LOW whenever the sensor is activated.