LM 3915 Sound Level Meter Circuit

This is a one chip sound level meter that can be use for displaying sound level of an amplifier or simply the sound level from a microphone.The heart of the circuit is IC LM 3915 Audio level IC.Even though it is a stand alone IC , a peak detector based on Transistor BC 558 and diode 1N4001 is also included for better performance.

Supply voltage can be from 3V to 20V.The input is set for audio line voltage (1V peak to peak) and has a max input voltage of 1.3V. To make the circuit use a moving dot display instead of bar graph display,Pin 9 can be should be disconnected from +V.

Part List

C1 2.2uF 25V Electrolytic Capacitor
C2, C3 0.1uF Ceramic Disc Capacitor
R1, R3 1K 1/4W Resistor
R2 10K 1/4W Resistor
R4 100K 1/4W Resistor
R5 1M 1/4W Resistor
D1 1N4001 Silicon Diode
Q1 BC 558 PNP Transistor
LED1-LED10 Standard LED or LED Array
U1 LM3915 Audio Level IC
MISC Board, Wire, Socket For U1

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Laser Level Detector

Rotating laser levels, which are very handy  for setting objects in a room or garden at the  same height, are available at prices of a few  dozen pounds. At relatively large distances  and for outdoor use, the light from the rotating laser beam is often not easy to see, and  the laser beam detector described here can  be useful in such situations. The detector  works well at distances up to 50 metres (150  feet) and consists entirely of standard components. The detector is housed in a plastic case  that can be fixed to an object (such as a post  or a beam). It has three LEDs and a beeper that  indicate whether the object should be raised  or lowered. 

LEDs with a transparent package and integrated lens (round surface) are used as sensors. The top and bottom detection zones  each have five LEDs and two opamps (IC1a &  IC1b or IC1c & IC1d), which drive the ‘Move  Up’ and ‘Move Down’ indicator LEDs. The middle sensor LED drives the ‘OK’ indicator LED  via two opamps (IC2a & IC2b).  The rising edges of the opamp output signals  trigger three separate monostable multivibrators (type CD4047). If desired, the circuit  shown inside the dashed outline (one gate of a  CD4044 quad RS latch) can be used in place of  each of the monostable multivibrators. In this  case the output signal has the opposite polar-ity, so the BS170 N-channel MOSFET must be  replaced by a P-channel type. 

The monostable time of the middle retriggerable MMV should be longer than the rotation period of the laser (e.g. with a 2 rpm laser  it should be longer than 500 ms) so that the  beeper will emit a continuous tone. Most  rotating laser levels have variable speed, so  this can also be achieved by adjusting the peed if necessary. The monostable times of  the upper and lower MMVs are dimensioned  to generate clearly distinguishable short and  long beeps, respectively. The three MOSFETs  (T1, T2 and T3) are configured as a wired-OR  gate to drive the shared beeper. The fourth  MOSFET (T4) drives the ‘OK’ LED. 

The circuit can be housed in an enclosure  together with three penlight cells.

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Build a Hi Low level Comparator Wiring diagram Schematic

Build a Hi-Low level Comparator Circuit Diagram, The voltage to be compared is fed through diode Dl and D2 to the voltage dividers Rl and R5 where the low and high limits are present. When the voltage level of an input signal exceeds the high threshold limit set with potentiometer Rl, the diode Dl becomes forward biased and the increased voltage on the inputs of the op amp drives it into positive saturation. Similarly, a decrease of the input voltage at the op amp inputs turns the op amp to positive saturation. Potentiometer R3 is used for zeroing the op amp in the off state.

Hi-Low level Comparator Circuit Diagram

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