The amplifier in class A I have made has a standby mode activated by the remote control of the preamp I also realized. A blue LED continuously lit indicates that the amp is working. To signal that the amp is in standby, I didn’t want a flashing ON/OFF led, but a led that turns on and off gradually. So I made a circuit based on NE555 to get this result.

In the Eagle schematics attached, the values given to R3, R4 and C3 produce a frequency of 0.3 Hz and a duty cycle of 51% with the 555 in astable mode. The transistor T4 inverts the output (3) of the 555 and the transistors T3 and T2 increase the output power.

The RC filter made up of R8 and C1 makes it possible to obtain the triangular signal necessary for the progressive switching on and off of the led. The latter is controlled by T1.

In the picture, the amp is at the bottom with the led on, the preamp is on top with its LCD graphic screen which displays the parameters (active source, volume level etc.) and 2 VU meters, all controlled by a micro-controller.

In the attached files, you can find the schematics, the printed circuit layout and a small video showing the progressive blinking of the led.