Schematic Rise

Saturday, November 8, 2014

Voice Bandwidth Filter

This circuit passes frequencies in the 300Hz - 3.1kHz range, as present in human speech. The circuit consists of cascaded high-pass and low-pass filters, which together form a complete band-pass filter. One half of a TL072 dual op amp (IC1a) together with two capacitors and two resistors make up a second-order Sallen-Key high-pass filter. With the values shown, the cut-off frequency (3dB point) is around 300Hz. As the op amp is powered from a single supply rail, two 10kO resistors and a 10µF decoupling capacitor are used to bias the input (pin 5) to one-half supply rail voltage.

Circuit diagram:

Voice Bandwidth Filter Circuit Diagram

The output of IC1a is fed into the second half of the op amp (IC1b), also configured as a Sallen-Key filter. However, this time a low-pass function is performed, with a cut-off frequency of about 3.1kHz. The filter component values were chosen for Butterworth response characteristics, providing maximum pass-band flatness. Overall voltage gain in the pass-band is unity (0dB), with maximum input signal level before clipping being approximately 3.5V RMS. The 560O resistor at IC1bs output provides short-circuit protection.

Alarm using your own Voice

This alarm plays your prerecorded voice message. It is built around the readily available quartz clock. Take the buzzer out of the quartz clock and connect its positive terminal to pin 1 and negative terminal to pin 2 of optocoupler IC MCT2E (IC2). Pin 4 of IC2 is grounded and pin 5 is connected to trigger pin 2 of monostable multivibrator IC 555 (IC3) as shown in Fig. 2.

Fig. 1: Voice recording schema

Fig. 1 shows the schema for recording your voice message. When you press switch S2, it plays the meassage. The control schema shown in Fig. 2 avoids the need for pressing switch S2 and thereby sounding the voice alarm automatically at the preset time.

Connect points A and B of the recording schema to the corresponding points A and B of the control schema. After making the connections, press record switch S1 to record your 6-second voice message through condenser microphone. Set the desired alarm time in quartz clock. At the time of alarm, buzzer terminals provide voltage to the internal LED of optocoupler IC2. This results in conduction of internal transistor of IC2, and its collector voltage at pin 5 drops to trigger IC3. The output of IC3 goes high for approximately 6 seconds. During this period, the prerecorded message is heard continuously. The message repeats every 6 seconds. The sound is loud enough in a room.

Fig. 2: Voice control schema

The schema operates off 3 volts and it can be easily fitted in a small box and fixed on the back side of the alarm quartz clock.

Sourced by: EFY Author: Naga Babu Araya

Voice Scrambler

With this schema you can modify how your voice sounds by changing the pitch of your voice. This schema can be connected to a phone and with a duplicate schema on the end of the phone line, you can have a scrambled voice communication. The way the schema works is as follows: If we cut the schema in half at the T2 transformer and include the LM324 on the left side, you will see that the LM324 portion of the schema is a tone oscillator which shifts the frequency of all input signals to a new higher frequency. When the voice and the tone oscillator mix frequencies the voice is not recognized. The voice signal is then inputted to the second stage which again shifts the voice signal again. I recommend that the first stage be tuned to a frequency that is 100hz lower then the second stage.

Schematic Rise

Saturday, November 8, 2014

Voice Bandwidth Filter

Saturday, August 30, 2014

Alarm using your own Voice

Thursday, August 14, 2014

Voice Scrambler