Over my years working as an Automation Systems Integrator, I occasionally get projects that are different from the normal industrial process and manufacturing systems, such as developing the controls for a unique museum exhibit to simulate a bat’s echolocation. This project was for the Children’s Center of Sofia, in Sofia, Bulgaria, which prompted the name “The Bulgarian Bat Detector.”

In a near dark environment, the museum visitors rotate a viewfinder toward a display depicting an environment for bats. A couple of bats located in the display are the targets for the detector. As the viewfinder approaches the bat’s position, the sound level of an audio track increases in volume with a maximum level when pointed directly at a bat. The volume level decreases as the viewfinder leaves the bat’s position.

The control solution required that the bats are actually detected, which meant that the controls must compare the direction of the viewfinder to the location of the bats in the display. The volume of the audio track is set based on the position of the viewfinder relative to the bats with volume increasing when approaching the bat and decreasing when leaving.

The biggest hurdle encountered was how to control the audio volume without excessive development and testing while also keeping costs low. As an experienced automation systems integrator, I did not have a volume control solution in my bag of tricks, and it quickly became apparent that some type of hybrid solution was needed. The position detection and process logic would be handled with an AutomationDirect CLICK® PLC, the audio playback would use a commercially available digital audio repeater, and the volume adjustment solution would need to be built using electronic components.

The viewfinder’s position is determined using a touchless Hall Effect encoder attached to the rotating shaft of the viewfinder to determine the rotational position of the magnet. The encoder output of 0.25V to 4.75V DC represents 0 to 360 degrees. This output range is perfectly suited for the CLICK’s built in 0V to 5V analog input. The PLC also has a built-in scaling configuration for the analog input providing the position’s 0-to-360-degree value.

For the audio playback, I used the QuikWave™ EM38A, which is often used by museum exhibit developers. It allows MP3 audio files to be played in a variety of ways. For this application we used a playback script to run a single audio file in a continuous loop.

Audio Volume Solution

One of the more challenging parts of the project involved how to control the audio volume from a PLC output. My customer, Andrew Tomasulo of Connect Interactives, came up with a solution that used Light Dependent Resistors (LDR), which consists of an LED and a photoresistor as a single device. The intensity of the LED regulates the resistance of the photoresistor. Because the only connection between the LED and the photoresistor is light, the photoresistor is an isolated and passive device. The PLC’s 0-5V analog output signal is used to vary the device’s resistance.

In order to use the LDR solution for volume control, two LDR units are set up as a voltage divider. Two separate 0-5V analog outputs from the PLC provided opposite voltages to the devices (0-5V and 5-0V). The resistance of the LDR’s is from 1,200 ohms down to 60 ohms, which provides a full volume control range.

The setup adjustment control uses four potentiometers to set the target locations for the bats in the display. The audio output volume increases and drops within +/- 10 degree range of the target. A 5V power supply voltage regulator (LM7805) was added to the circuit board to power the potentiometers. The outputs of the potentiometers are sent to an analog input module of the CLICK PLC.

Packaging the System

All of the components were packed into a 12- x 10- x 5-inch non-metallic enclosure. Field connections are made using Micro (M12) connections and the power cord uses a European Schuko plug (remember this unit is going to Bulgaria). The custom circuit board was hand assembled and mounted on stand-offs. Almost all of the components came from AutomationDirect except the audio playback unit, custom circuit board, potentiometers, and the power cord.

This was a fun project with its share of challenges but in the end it all worked out (and no bats were harmed in the process). I’m thinking of adding a trip to Sofia, Bulgaria, to my bucket list so I can see and hear the unit in operation.

For more information: 

Rees Engineering

AutomationDirect

CLICK PLC

QuikWave Audio Player