Visual Zero Beat Indicator

Light-emitting diodes connected with reverse polarity provide a visual indication of zero-beat frequency. Each LED is on for only half cycle of the input. When the input frequency is more than 1 kilohertz away from the zero-beat frequency, both LEDs appear to be on all the time. As the input frequency comes within about 20 hertz of zero beat, the LEDs will flicker until zero beat is reached. Both LEDs glow or flicker until zero beat is reached, when they go out.

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Visible Voltage Indicator

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Ten Step Voltage Level Indicator

This ten-step adjustable analog level detector is capable of sinking up to 40 milliamperes at each output. The voltage range at the input pin should range from 0 to 2 volts. Circuits of this type are useful as liquid-level indicators, pressure indicators, and temperature indicators. They may also be used with a set of active filters to provide a visual indication of harmonic content of audio signals.

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Sound Sensor

By using a microphone, high-gain amplifier (Fig. b), and detector-relay driver (Fig. a) a sound-detecting alarm system can be constructed. If you want a latching setup, make the dotted connections to the relay shown in Fig. a.

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Five Step Voltage Level Indicator

This circuit provides a visual indication of the input analog voltage level. It has a high input impedance at pin 8 and open-collector outputs capable of sinking up to 40 milliamperes. It is suitable for driving a linear array of 5 LEDs to indicate the level is 5 steps. The voltage at the analog input should be in the range of zero to approximately one volt and should never exceed eight volts.

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Beat Frequency Indicator

This circuit uses LEDs to display the beat frequency of two-tone oscillators. Only one LED is on at a time, and the apparent rotation of the dot is an exact indication of the best frequency. When f1 is greater than f2, a dot of light rotates clockwise; when fl is less than f2, the dot rotates counterclockwise; and when f1 equals f2, there is no rotation.

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AC/DC Indicator

By using two switching transistors and two LEDs, this circuit can distinguish low-level ac and dc signals. If the red LED illuminates, the signal is positive dc. If the yellow LED lights, the signal is negative dc. If the signal is ac, both LEDs will light.

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