Peak audio level indicators can be used in tape recorders, amplifiers, mixers, and other radio equip­ment to provide a visual overload warning, and unlike slower res­ponding VU meters, they produce a proper response to fast tran­sients.

Circuit in lines (Click to enlarge)

The circuit is based on the inexpensive 723C device which, although primarily intended for use as a voltage regulator, can be adapted to work well in many other applications. The 723C has a highly stable 7V (nominal) reference voltage avail­able at pin 6.

This is coupled to the inverting input of an operational amplifier (which is also part of the 723C device) via an attenuator, R4,5. This gives a stable reference potential of a little over 2V at the inverting input. The input signal is coupled by way of sensitivity control RV1 to a common emitter amplifier based on Q1 and fed to the non-inverting input of the operational amplifier by C2 and R6.

Under quiescent condition or with a negative going signal at Q1 collector, the non-inverting input will be at a lower potential than the inverting one, and the output of the amplifier will be low.

If a positive going signal reaches a high enough amplitude, though, the non-inverting input will reach a higher potential than the invert­ing one causing the output to go high. D1 is then switched on with a current that is determined by the output of-the amplifier and which is largely independent of the supply voltage.

Discrete resistor R8 actually sets the output current. The specified value gives a nominal 20mA LED current. R7 provides positive feedback which ensures that D1 is either fully on or off. It also tends to hold D1 in the on state for slightly longer than would otherwise be the case, thus giving a clearer indication of a brief overload.

The unit can be adjusted to res­pond to input levels down to about 100mV rms, which should be more than adequate for all nor­mal requirements. RV1 is adjusted for the lowest sensitivity that causes D1 to come on with an in­put signal level equal to the lowest overload level. Quiescent current consumption is about 4mA.