A Mute Solution

A project log for Customizable 20W Amplifier

4 Input, 3 Way Tone Control and Customizable LCD Display

smartroadsmartroad 06/21/2020 at 08:470 Comments

In my last log entry I mentioned about the need to mute the amplifier while the unit switched on as you need to give the amp chip about 1/4 second to boot. This meant that the amp would play anything on the input at near full volume until the controller could turn the volume down. The controller doesn't have any IO left to control the mute pin on the amplifier so I needed another way to hold the mute pin low for at least 1/4 second.

I wanted it to be as simple as possible so started with the idea of a simple RC timer to hold the pin low. While this worked, the issue came from the inability to quickly discharge the capacitor once power was removed. This would have meant that if the power was applied before the capacitor has discharged it would start again at full volume. The capacitor does discharge, just painfully slowly!

After a lot of investigation I came to the conclusion that I would have to go a bit more complicated. On that post user Ken Yap said about using a small basic microcontroller to hold the pin low, but that felt a bit of a misuse of a microcontroller, although I did appreciate the suggestion :D

That though made me think, there is a chip that is A) Cheap and B) designed for this exact purpose. The venerable 555 timer!

The above circuit diagram shows the 555 in astable mode. Just for anyone who doesn't know about the 555, astable mode means the 555 will start with pin 3 (Q) output high and after a specific time with go low and stay there until power is reset. R1 and C1 provide a brief pulse to trigger the timer when power is applied. R2 and C2 are the main timer, providing about 1 second of delay. The transistor is used to invert the output so the junction of R4 and the collector of the transistor is low when Q is high and vice versa. The diode is used to block the high voltage so the "mute" pin is only pulled low as there is a resistor on the Adafruit board pulling it high and I didn't want to risk injecting 12V onto the 3V rail!