The overload indicator consists of a window comparator that measures the magnitude of an a.f signal. Two of the opamps contained in a TL072 is supplied with a reference voltage by potential divider R1-R2-R3-P1. The outputs of the opamps drive T1 via diodes D1 and D2 (that function as the half-waved rectifier), which in turn actuates D3. Network R5-R6-C2 ensures that the LED lights even during short signal peaks.Capacitor C2 is charged fairly rapidly via D1 (or D2) and R5, after which it discharges slowly via R6, R9 and the base-emitter junction of T1. Capacitor C1 also contributes to the longer lighting of the LED.
When the travel of the signal at the input is high enough, IC1a is toggled by the positive half period of the signal and IC1b by the negative halves. In this way, a peak above the maximum level will be indicated even when the signal is asymmetrical.
Because of the symmetrical power supply and design of the indicator, the reference voltage for both opamps can be set with one potentiometer.
The circuit draws a current of 5-6 mA when the LED id off. When an overload peak is indicated, the LED draws an additional 20 mA. With values as shown, the reference voltage can be set roughly between 0.9 V and 5.5 V.
The circuit can be connected to the output of a power amplifier, but potential divider R7-R8 then needs to adapted and protected by diodes to supply lines.
Parts list:
Resistors:
R1, R3, R8, R9 = 10 k(ohm)
R2 = 1.8 k(ohm)
R4 = 1.5 k(ohm)
R5 = 1 ohm
R6, R7 = 100 k(ohm)
P1 = 10 k(ohm) preset
Capacitor:
C1 = 10 uF, 35 V
C2 = 1 uF, 35 V
C3, C4 = 100 uF
Semiconductor:
D1,D2 = 1N4148
D3 = LED, red
T1 = BC547B
Integrated circuits:
IC1 = TL072 or TL082
