To visualize the signal level, LED indicators are widely used, built on the architecture of specialized microcircuits. They are used in a wide variety of devices: level indicators of the input signal of radio receiving equipment, level indication on a sound amplifier, testers for debugging circuits that use the pulse-frequency principle of load control.
All level meters are based on multi-stage comparators.
Comparator - a logic element that compares the parameters of two incoming signals.
The analyzed signal is supplied to one channel of the comparator, and the reference reference voltage to the second. If the amplitude of the first is higher than the reference voltage, a logical unit appears at the output, if it is lower, a logical zero.
The operation of the simplest comparator can be demonstrated on the K155LN1 microcircuit, the unit cluster of which is the "NOT" element.
Such a microcircuit is the simplest logical comparator. When the input voltage is from 0V to 2.4V (which corresponds to logical zero), the output is 2.7V, as soon as the input voltage exceeds 2.4V, the output signal drops to zero volts.
There are several microcircuits for level visualization. The most multifunctional circuits, in my opinion, allow you to create microcircuits on the lm39xx architecture. This line includes three microcircuits: lm3914, lm3915 and lm3916. The minimum decoupling easily allows you to create an LED sound level indicator with your own hands, even without deep knowledge of radio electronics.
They all represent a ten-range analyzer. They differ in the way they differentiate the input signal. For lm3914 it is 1V, for lm3915 - 3db, for lm3916 - 1db.
Sound level LED indicator on lm3915
Let's assemble a volume indicator on LEDs using comparators on lm3915.
Let's figure out how the circuit works.
The analyzed signal arrives at input 5, its amplitude should be 10V. To match the amplitude of the incoming signal, we need a transistor switch. The analyzed signal is fed to its base through a resistor voltage divider at R5.
Logical structure lm3915 The sound indicator on the lm3915 can operate in two indication modes - "point" and "column". In the first case, the LED corresponding to the current signal level lights up, in the second - all LEDs from zero to the current level. Switching of indication modes is carried out through the switch between the common wire and the input "9".
Non-standard application
The indicator using lm3914 can be used as a compact tester of small batteries and accumulators.
The supply voltage of such a circuit is from 5V to 12V. Conveniently powered by "Krona" or four AAA batteries.
Capacitor C1 - 50 μF 25V, pull-up resistor R1 - 1MΩ. R2, R3 - 4.7-5kΩ each. The measurement range for the 1B circuit with a graduation of 0.1V. R2 adjusts the measuring range, R3 - the LED current. If output 9 is turned off, the indication will be a "bar", but the power supply is quickly discharged.
