Peak Reading Audio Level Meter

This simple schema will indicate peak audio response on an analogue meter, similar to a tape recorders meter. The schema uses an opamp as a non inverting amplifier, but with one addition - a diode in the feedback loop. The schema has a fast response time and slow decay time to indicate peak readings. The 1N4148 diode provides half wave rectification of the input signal, the dc output being smoothed by the 22u capacitor. the capacitor will charge to the peak value of the input waveform, and then discharge via the meter and 18k resistor. I used a meter with a FSD of 150uA, but any meter with a FSD in the range 50-250uA may be used. The discharge time is around a quarter of a second. Increase the 22uF cap for a longer discharge time, or omit altogether to make an instantaneous reading level meter.

This schema will only work with a MOSFET type op-amp, bipolar types i.e. 741 and J-FET op-amps such as LF351 will not work in this schema.

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Sound Level Meter Wiring diagram Schematic

The NE604`s signal-strength indicator section is used, based on an internal logarithmic converter. This enables a lineardecibel scale so that the moving-coil meter (shown in the diagram) can be replaced by a digital instrument. The signal source is assumed to be an electret microphone that converts ambient noise into an electrical signal. Because this type of microphone normally contains a buffer stage, R7, R8, and C13 have been included to provide the supply voltage for this stage. The NE604 delivers an output current (at pin 5) of 0 to 50, which causes a potential difference across R2 + R3 of 0 to 5 V. 

 Sound-Level Meter Circuit Diagram

The input and output signal range is equivalent to a sound range of 70 dB. lb compensate for the effects of temperature changes, the required resistance of 100 KOhmhm is formed by two resistors (R2 and R3) and a diode (Dl). Any ripple remaining on the output voltage is removed by R4/C9/C10 before the output is buffered by IC2. The indicating instrument, here a moving-coil meter, is connected to the output (pin 6) of IC2 via a series resistance, R^ + . 

The preset is adjusted to give full-scale deflection (FSD) for an output voltage of 4 V. Calibrating the meter is a little tricky, unless you have access to an already calibrated instrument. Otherwise, if you know the efficiency of your loudspeaker, that is, how many decibels for 1W at 1 m, you can use that as reference. The scale of the meter can then be marked with the (approximate) value. In any case, the meter deflection must at all times be seen as an indication, not as an absolute value: it was not thought to be worthwhile to add a filter to the schema to enable absolute measurements to be made.

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Infra red Level Detector

This schema is useful in liquids level or proximity detection. It operates detecting the distance from the target by reflection of an infra-red beam. It can safely detect the level of a liquid in a tank without any contact with the liquid itself. The devices range can be set from a couple of cm. to about 50 cm. by means of a trimmer.Range can vary, depending on infra-red transmitting and receiving LEDs used and is mostly affected by the color of the reflecting surface. Black surfaces lower greatly the devices sensitivity.








Parts:

R1_____________10K 1/4W Resistor
R2,R5,R6,R9_____1K 1/4W Resistors
R3_____________33R 1/4W Resistor
R4,R8___________1M 1/4W Resistors
R7_____________10K Trimmer Cermet
R10____________22K 1/4W Resistor


C1,C4___________1µF 63V Electrolytic or Polyester Capacitors
C2_____________47pF 63V Ceramic Capacitor
C3,C5,C6______100µF 25V Electrolytic Capacitors

D1_____________Infra-red LED
D2_____________Infra-red Photo Diode (see Notes)
D3,D4________1N4148 75V 150mA Diode
D5______________LED (Any color and size)
D6,D7________1N4002 100V 1A Diodes

Q1____________BC327 45V 800mA PNP Transistor

IC1_____________555 Timer IC
IC2___________LM358 Low Power Dual Op-amp
IC3____________7812 12V 1A Positive voltage regulator IC

RL1____________Relay with SPDT 2A @ 220V switch
Coil Voltage 12V. Coil resistance 200-300 Ohm

J1_____________Two ways output socket





IC1 forms an oscillator driving the infra-red LED by means of 0.8mSec. pulses at 120Hz frequency and about 300mA peak current. D1 & D2 are placed facing the target on the same line, a couple of centimeters apart, on a short breadboard strip. D2 picks-up the infra-red beam generated by D1 and reflected by the surface placed in front of it. The signal is amplified by IC2A and peak detected by D4 & C4. Diode D3, with R5 & R6, compensates for the forward diode drop of D4. A DC voltage proportional to the distance of the reflecting object and D1 & D2 feeds the inverting input of the voltage comparator IC2B. This comparator switches on and off the LED and the optional relay via Q1, comparing its input voltage to the reference voltage at its non-inverting input set by the Trimmer R7.


Notes:

* Power supply must be regulated (hence the use of IC3) for precise reference voltage. The schema can be fed by a commercial wall plug-in adapter, having a DC output voltage in the range 12-24V.
* Current drawing: LED off 40mA; LED and Relay on 70mA @ 12V DC supply.
* R10, C6, Q1, D6, D7, RL1 and J1 can be omitted if relay operation is not required.
* The infra-red Photo Diode D2, should be of the type incorporating an optical sunlight filter: these components appear in black plastic cases. Some of them resemble TO92 transistors: in this case, please note that the sensitive surface is the curved, not the flat one.
* Avoid sun or artificial light hitting directly D1 & D2.
* Usually D1-D2 optimum distance lies in the range 1.5-3 cm.

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