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dark_hawk:
Hi,
This is schematic is used to make an audio amplifier/ power board go into standby mode when there is no sound detected for an x period of time. Reason for this is that the unit has very bad thermal design and the power board gets really hot even if there is no sound being produced.
All credits of the design go to the original designer TGAH08 whom I can’t find anywhere and his website is down for 3 years now.


His description of the circuit:


--- Quote ---The auto standby circuit is built around the LM324 quad operational amplifier.
It has six wires to connect, +9v Red, -8v White, ground Black, standby control in Grey and out Yellow (the existing pcb track linking these two points, needs cutting) and an audio pickup Black screened.

The audio signal is picked up from the output of the opamp feeding the sub channel amplifiers. I used this signal as all the seven audio inputs bass signals are summed together and are sent to the bass along with the dedicated bass signal. This way if any of the eight channels have an audio signal the circuit will keep the unit switched on.

The audio input is buffered by the first opamp so as not to load the audio signal in anyway. The second opamp has a gain of about 200 and boosts the signal to a usable level. The third opamp is an active rectifier with a peak hold, this gives a DC voltage out when audio is present, and this holds TR1 on which keeps capacitor C5 discharged.

With no audio signal, TR1 switches off and C5 starts to charge up via R11. With the values given the CR time is 484 second or 8mins (CR time is to 0.7 of supply voltage), this voltage is connected to one input of the fourth opamp. This opamp is set up as a comparator and it’s other input is set at about 0.7 of supply voltage by R7 and R8, about 6.2v. 

If at any time an audio signal is picked up, TR1 will be turned on and rapidly discharge C5 via R13 (100R) and the timing period will restart. If C5 reaches 6.2v the fourth opamps output will switch high and pull the control line to 5v via R4 and D1, this will switch the sub to standby in an orderly way.  At any time an audio signal will ‘wake up’ the sub by switching this line back to 0v.

If the sub is switched to standby using the control pod the I/O control ics output will switch to +5v and this will, via R3 and D2, switch on TR1 discharging C5 and the timer will be held at zero. This will ensure an instant switch on the next time the standby button is used to switch the sub on.
--- End quote ---

What I want to do is use this design to force the unit into standby when not in use, but I don’t want it to wake the unit up when sound is detected again. I want to switch it on manually.
How do I do that?

Thanks

bob91343:
Add feedback around amplifier 4 to make it latch.

dark_hawk:
Thanks for the reply.

I'm sorry, but how do I do that?

EPAIII:
How? Add a resistor from pin 1 (output) to pin 3 (non-inverting input). That will provide positive feedback. You will need to experiment with the value, but 10K or 47K would be a good starting point and work down from there.

A NC, PB switch in series with this resistor should provide the manual reset you want.

PS: I am only speaking to your question, below. I did not look for other ways to provide the action you want and there may be a better one.




--- Quote from: dark_hawk on October 12, 2021, 10:22:52 pm ---Thanks for the reply.

I'm sorry, but how do I do that?

--- End quote ---

eblc1388:

--- Quote from: dark_hawk on October 12, 2021, 10:22:52 pm ---I'm sorry, but how do I do that?

--- End quote ---

I offer a different approach to this problem. Instead of latching the fourth opamp, I used its output signal to block the 'wake up' feature so any audio signal arriving later will not reset the fourth opamp. A more detailed description follows:

If at any time an audio signal is picked up, TR1 will be turned on and rapidly discharge C5 via R13 (100R) and the timing period will restart. If C5 reaches 6.2v the fourth opamps output will switch high and pull the control line to 5v via R4 and D1, this will switch the sub to standby in an orderly way. Also the "wake up" function via audio signal is disabled(bypassed to GND) using TR2 via R18 & R19. (The resistor R18 is there to enable user to reset the logic using the standby switch on the sub.)  Any audio signal after this timeout period will not ‘wake up’ the sub anymore. The output of the fourth Opamp will remains high.

If the sub is then manually switched to standby using the control pod the I/O control ics output will switch to +5v and this will, via R3 and D2, switch on TR1 discharging C5. This will force the fourth Opamp output to go low and remove the blocking of audio 'wake up' feature by TR2. Therefore, if the sub goes into standby via timeout, user can reset the logic by switching the sub manually to standby and then back to service.

You can use any small signal NPN transistor for TR2.

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