Introduction: Create an Light-sensor Analog Squawk Circuit!

Picture of Create an Light-sensor Analog Squawk Circuit!

This is a fun project to scare your cat, dog, partner or neighbor. It uses photocell, or light, sensors as resistors in an analog circuit to create weird sounds. Any sensor can be used, but photocells are cheap and easy to use.

Basically, you have the output of an opamp (Operational Amplifier) going into a sound system: in this case, a 1/4 inch jack into a mixer going to a speaker. Its only mono, but the skies the limit in terms of post-processing... Check out the included recording, for a glimpse of squawking fun :_)

SIDE NOTE: for circuit pros; dont use decoupling caps on the voltage regulator!

Step 1: Needful Things

Picture of Needful Things
  1. the 7414 opamp (sometimes called a Hex inverting Schmitt trigger)
  2. 9 volt battery
  3. a 9v battery clip
  4. 508 Voltage Regulator
  5. 1/4 inch TRS headphone jack
  6. An LED
  7. a 220 ohm resister
  8. THREE 10uF Capacitors, FIVE 0.01uF Capacitors (number 104), another cap of any size (experiment?).At least TWO light sensors
  9. various wires
  10. any kind of light-creating source (like a torch)soldering wire
  11. TWO potentiometers & TWO switches

You will also need a soldering iron, a multimeter and breadboard. See the second pic on how I clipped two wires to the multimeter so it can be used for testing on the breadboard.

NOTE: the images I included use extra jumper wires spread around the board so all connections can be seen easily. Normally you wouldn't need them and actually shouldn't, since they can add noise to the whole circuit.

Step 2: Soldering the Battery Clip and Setting Up the Power Supply

Picture of Soldering the Battery Clip and Setting Up the Power Supply

Firstly, I like to solder two jumper wires to the battery clip, so that I can stick them into the breadboard to check connections.

Second, set up the regulator. According to the opamp's datasheet, it should have a supply of around 5 volts. But our battery is 9 volts. You can use either an 805 voltage regulator, or a low dropout 2940 one, which basically uses less power than the 805. Either one will work.

Pin 1 should go to the raw 9 volts.
Pin 2 is Ground.
Pin 3 -s the 5 volts to supply the whole board.

This is all well and good, but this setup is always on. We will want to set up a switch, so that the baord can be turned on and off. Stick a switch in between the +9 volt battery clip and Pin 1 of the Regulator.

Next, lets set up a power LED, so we know if it is turned on or off. LED diodes are polarized, and we all know what will happen is we get the pins mixed up, dont we..... See the image to see how it can be breadboarded.

Step 3: Setting Up the Opamp

Picture of Setting Up the Opamp

now lets give the opamp power and ground. Pin 14 should be given +5 volts, and Pin 7 Ground.

Next, Pins 1 and 3 should be connected to +5V and ground via a 0.01uF capacitor (number 104). In the image, I used red jumper wires for the two caps going to +5V, just for clarity.

Tech note: Essentially, the opamp takes input voltages on Pins 1, 3, 5, 9, 11 and/or 13 and spits out a digital signal on corresponding Pins 2, 4, 6, 8, 10 and 12. That gives a total of SIX sensors that can be used on it for extras squawking goodness.

Step 4: Add the 1/4 Inch Jack & Other Caps

Picture of Add the 1/4 Inch Jack & Other Caps

Now we need to solder wires onto the 1/4 inch jack. It can take a bit of time for the metal on the jack to heat up, but soon your solder should melt onto it no trouble.

Attach the ground wire (the one soldered that is connected to the inside metal tubing) to the board's ground and the other wire to the Ground side wire of the 47uF cap.

Now connect the other positive leg of the 47uF cap to the Ground legs of two 10uF caps.

DO NOT get the polarization wrong on these caps! The side with the big strip of black should always be heading towards Ground.

The other positive legs of the two 10uf caps should be connected to the opamp's Pins 2 & 4. I used extra jumper wires spread around the board so all connections can be seen easily on the image. Normally you wouldnt need them.

Step 5: Add the Photocell Sensors

Picture of Add the Photocell Sensors

So that the amount of light present will produce a small amount of resistance, which can be used as control data coming into the opamp via the input pins. They come in all sorts of sizes, so feel free to swap different ones around to get different sounds!

Connect one photocell to the opamp's Pins 1 & 2 and another photocell to the opamp's Pins 3 & 4.

From now on, you should be able to hear some squarks from the circuit, if you plug in a pair of 1/4 inch headphones into the circuit's jack and shine some light on the photocells.

Feel free to swap out the caps for other values, to change the pitch(!)es.

Above is a recording I made of this circuit, but added three more photocells on the opamp's other input/output Pins (see last image). I plugged the circuit's output through my mixer and recorded the signal in Logic.

Jam on!!!


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