Peak level indicator: The indicator shows by means of two LEDs when the signal level in either channel of a stereo audio system exceeds a preset value.
In the diagram. ICI a and IC I b function as a comparator. A reference voltage of 0-1 1 V is applied to their non-inverting inputs by P1 and P2 respectively. Resistor R3 ensures that the reference voltage cannot exceed the common mode range of the opamps.
The signals from the two channels are recti-, fled (half-wave) by Di and D2 and the resulting direct voltage is applied to the inverting inputs of the opamps.ance is hi Since the input imped-only 200- 30gh, the drop across the diodes is 0 mV. Resistors and R2 serve to limit the input current if the drive levels exceed the common mode range. When the peak value of the input sig drops across the diode. the output of the relevant opamp changes states (goes low), whereupon the associated LED lights. The circuit may be used at frequencies up to 20 kHz (allowing for variations in peak levels of ±0 25 dB). The indicator draws a current of only 0.25 mA when the LEDs are out and of 24 mA when both LEDs light. This drain may be reduced by the use of high-efficiency LEDs (which draw only 2-3 mA). The value of R4 and R5 should then be increased to 3.3 k Ω.