10 Sound Pushbutton
A lot of arcade game players will instinctively shout commands at the screen when the action gets exciting. There are so many times when it would be more fun to control game movements by vocal commands than by pushing buttons. This device, the sound pushbutton, lets you do just that, since making a sound will have the same effect as pressing a paddle button. A potentiometer provides sensitivity control with a wide range of adjustment, so low-level background noise will not set the device off. The construction is simple and the materials are inexpensive and easy to obtain.
The sound pushbutton is encased in a small plastic box. The device consists of a microphone element, a pot, and a circuit board with one chip, three capacitors, one LED, and 14 resistors. It is connected to the computer with a 3-wire cable and a simple plug/socket.
To begin, cut a piece of circuit board to fit inside the box. Cut the board by scoring both sides deeply with an X-acto knife and breaking it over the edge of a table. File off the corners of the board so that it doesn't hit the screw posts in the box. Mount the board on the bottom of the box with four small bolts. You will need spacers or extra nuts to raise the board off the metal bottom of the box.
Next position the microphone on the center of one end of the circuit board. Mount the element by looping wire over it and soldering the wire to the board. Carefully determine where the microphone will be located near the end of the plastic box and drill a 1/4-inch hole to let sound inside to reach the element.
The sensitivity pot is mounted through the top of the box on the end away from the microphone. You may have to add a 3/8-inch flat washer to mount the pot on the thin plastic. Now drill a hole for the cable in the end of the box just beneath the pot. Mount the LED on long leads so that its base is about 1/4-inch above the circuit board. Find the spot on the box just above the LED and drill a 3/16-inch hole so that it can be seen. As a finishing touch, you might want to add rubber feet to the bottom of the box so it will not scratch your desktop.
Figure 10-2 is an explanatory schematic which shows the appearance of the sound signal at five points along the circuit path to the computer. The key to this circuit is the LM3900 chip, a quad operational amplifier that operates on +5 volts.
Operational amplifiers (op-amps) are basic and popular building blocks for analog circuits. The standard op-amp has two analog inputs (one plus, one minus) and a single analog output. It takes the difference between the incoming signals and multiplies the difference by an enormous amount (the gain) to produce the output. The gain may be as much as 100,000 to 1. To achieve control over such large gains, a set of input and feedback resistors is used with the op-amp.
In the first section of the circuit (figure 10-2), resistor R3 is the feedback resistor and R1 and R2 are the input resistors. The gain of this stage is R3 divided by R1 (100K divided by 3.9K = 26). The small signal coming from the microphone, shown in the first waveform drawing, is multiplied by 26 to become the second waveform. Resistor R2 adds a zero voltage to the input from the microphone, helping to prevent oscillations and improve temperature stability.
Most op-amps use plus and minus power supplies to provide a full range of possible outputs. The LM3900 uses only the +5 supply, so its output is limited to the range of 0 to 5 volts. When you are designing a circuit with the LM3900 you have to make certain that your intended output will fall in this range.
In the first stage of the circuit the input from the microphone is amplified. The second stage provides additional gain, and this gain is controlled by the sensitivity pot. This stage uses the LM3900's limited output range to amplify half the waveform so that weak signals will be close to 0 volts and strong signals will be amplified to +5 volts.
The third stage of the circuit provides additional gain and filters out the high frequency portion of the signal with capacitor C3. This filtering makes the signal more like the steady pushing of a button and eliminates bounce and electrical noise. The LED is also driven by this stage.
The last stage compares the signal with half of the +5 volts provided by the voltage divider (resistors R12 and R13). The output of this stage is then either 0 or +5 and is therefore a proper digital signal.
WIRING THE CIRCUIT
Remove the circuit board from the box and add a 14-pin DIP socket to the components (the microphone and LED) already on it. Now add the wires, resistors, and capacitors, marking off each on a copy of figure 10-3 as they are installed.
Since there is no mechanical strain on this device, the pot you buy does not have to be an expensive one. If you purchase the pot given in the parts list, you will have to shorten the shaft and add a knob. The cable requires only three wires. Telephone cable with four wires works well for this unit and gives you an extra wire to double up the ground.
The best plug for this unit is a simple plug/socket similar to the more complex one described in chapter 3. For this one, do not bend or cut short any of the socket pins. You will almost always be using the sound pushbutton with another controller, and you want all the controller functions to pass through the plug/socket.
When you have finished the wiring, examine your work closely for solder bridges and shorted wires. Plug the LM3900 into the socket in the board and check to make certain there is at least 50 ohms between pin 1 and pin 8 of the plug. We got a reading of 1.3K when we tested the prototype. Mount the circuit board on the bottom and close the box.
Turn off your computer. Plug the new sound pushbutton into the game I/O connector and turn it on again. If the computer doesn't start up exactly as usual, shut it down immediately and recheck all your work on the new unit.
Now run the Controller Checkout program from the software chapter and watch the screen to determine if the 0 pushbutton responds to sounds. Try various settings of the sensitivity control while making sounds of differing volume.
Turn the computer off, plug a controller you know is in good working order into the back of the sound pushbutton plug, and rerun the Controller Checkout program. Test all the functions of the sound pushbutton and the controller. Now you can try it out with your favorite games. Instead of wearing out your fingers, you can yell at the screen and get a response.
The output of the sound pushbutton is very close to a standard digital signal and should be acceptable to any computer system that has at least one pushbutton input and that can provide a small amount of +5 voltage. You will, of course, need a different plug, and the plug pins may have different numbers than the ones shown for the Apple. See the section on other computers in the Electronics Tutorial for more information on how to adapt this controller to other computers.
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