OI01 is an overload indicator for line level circuits.
The detection level can be set so indication occurs before the audio is clipped, making it useful for recording applications.
An integrated timer ensures a clear indication of even very short peaks.
The two comparators U1D and U1E detects when the signal on any of I1..I4 reaches the threshold set by VR- and VR+ and discharges C1.
The circuit around U1B and U1C is a timer with a hold time of around 40 ms to insure a clear indication of even short peaks.
The four inputs I1..I4 should be driven from low-impedance sources as the input impedance varies with the level.
Any OP-AMP output can drive these inputs - optionally via a 100 Ω resistor to isolate the OP-AMP from capacitive loading by the wiring.
More inputs can be added by adding more input diodes. Put the diodes near the I+ and I- inputs.
If you decide to run long wires from the I+ and I- inputs, keep them clear of sensitive circuit nodes.
With the component values shown, the detection level is 18 dBu with ±15 V supplies ( 3 dB below clipping of a typical OP-AMP ).
The circuit around the two TL072 allows 2 of the inputs to have a lower detection threshold.
Another use of this circuit is to detect the actual clipping level of an inverting amplifier by measuring the signal on the inverting input.
Fig.2: Clipping detection on an inverting amplifier. R1 is 100 Ω to 1 kΩ mounted close to the amplifiers inverting input to reduce loading from the wiring.
Detection of voltages beyond the supplies is possible. See OI01A_Calc.ods in the files download.
All capacitors are ceramic types ( the 100 nF have 5 mm pin-spacing to ease the PCB lay-out ). The other components are non-critical.
Fig.3: Photo of mounted PCB.
Download OI01A design files.
I have boards available for this project. See the PCBs page.
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