IFD:Printing Acoustic Interfaces/acoustic sensing circuits

transformer microphone and 555 timer oscillator (outdated circuits)

A Radio Signal Receiver source

To get rid of our big magnets, we build an electro magnet with the help a a sending coil that will build up a magnetic field. When the current though this sending coil changes, it will induce a magnetic field, that we can use to induce a current in the second (receiving) coil. Two coils that share their magnetic flux via the air are called an "air core transformer". Both coils will be printed on paper, but on different sheets, leaving a gap filled with air. If the sheets are close together the second coil induces more current and lesser when it is further away. It is important, that the currents change constantly, because only changing currents can induce magnetic fields. We will use a high frequency changing current (50Khz) on the sending coil, that we will receive on the other coil. When we change the distance between the coils, the actual amplitude (volume) of this high frequency tone increases or decreases. To get the actual volume information of this high frequency tone, we use the half wave rectifier. This circuit is commonly used in radio signal receivers, where the amplitude of the the frequency of a radio station is changing with the actual transmitted sound wave. This is called Amplitude Modulation (AM Radio).

Sender: Square Wave Generator Circuit

555 Timer Chip Pinout source

To send the high frequency current, we will use the 555 Timer chip configured as a square wave generator. This circuit has little components and is comparably easy to build. It uses only one capacitor and one resistor to set the frequency of the square wave. You can find the circuit in the link below. Be sure to match the pinout in the circuit according to the figure on the left.

555 Timer Oscillator

Receiver: Half Wave Rectifier Circuit

When we change the distance between the coils in our paper built air core transformer, the actual amplitude (volume) of the high frequency tone increases or decreases. To get the actual volume information of this high frequency tone, we use the half wave rectifier. The circuit consists of a diode, a resistor and a capacitor. The diode lets only the positive current travel through it (in the direction implied by the arrow) and blocks the negative current, running against its direction. This leaves us with the positive half of the sending waveform. The final thing we need to do is average this positive half wave to get the average amplitude of this half wave. This averaging is achieved by the capacitor and the resistor. When the capacitor is charged, and there is no voltage on the right side of the diode, a discharge current will flow through the resistor to ground. The amount of discharge current is controlled by the resistor. The bigger the resistance the less discharge current will flow. We need to adapt the value of this resistor to have the capacitor discharge just a little bit, before the next positive half wave arrives at the right side of the diode. Then, if we receive less current in the second coil, the average charge on the capacitor decreases, following the actual acoustic waveform between our two sheets of paper.

Half wave rectifier and Transformer Coils

Full Circuit

TL072 Version

TL074 Version

One Sending and two Receiving Circuits

Printed Capacitive (Condenser) Microphone

Charge Mode Amplifier for Capacitive Microphone

275kHz Receiver

309 kHz Receiver