Amplifier turns on and off

Thread Starter

martinsson93

Joined Nov 4, 2021
71
Hi!

I have bought a headphone amplifier which I have made small changes to. The two things I've done is:

- disconnect the on-off button by soldering a cable between the poles
- adding longer wires to the diode so that the led could be attached a bit from the amplifier

What have happend though is that now when I turn the power on, 5 V (which is requires), the amplifier starts to turn on and off all the time.
It's on for longer than it's off (it's only off for a second or so, and then on for some seconds).

What could this be due to? The power comes from a power bank.

Thank you very much!
 

Yaakov

Joined Jan 27, 2019
5,502
This is completely expected If the switch is used to turn the device both on and off. If short press turns it on and long press turns it off, then turning on and off as you describe means it’s working perfectly.

This sort of power management is more common than not today. You will either have to see if you can get it to default to on and switch the power supply on and off, or build a one-shot that connects the button terminals for a short time after power is applied.

The former is probably not possible with your device so the latter is the answer. You can use a relatively simple circuit featuring a 555 IC to do this. It’s easy to build, and you can tweak the timing so it is reliable.
 

Thread Starter

martinsson93

Joined Nov 4, 2021
71
Thank you Yaakov! This might be the case.

Can't really remember if it turns off by pushing it a bit longer, but it sounds like this might be the cause.

So ... I'm a beginner and don't know anything about one-shot circuits with 555 IC. Is it simple enough for you to help me draw something and explain to me how it should be connected?

I'm going to start search for more information and see if I can understand more about it.
 

Yaakov

Joined Jan 27, 2019
5,502
There are many tutorials online concerning the 555. It’s a very old and venerable IC with a lot of fans.

Here is a calculator to help choose the resistor and capacitor values you will need to get the timing you want:

https://www.allaboutcircuits.com/tools/555-timer-monostable-circuit/

You will also need to use a transistor to actually switch the device on when triggered by the 555 output.

There are a lot of people here (I’m looking at you @crutschow among others) that love to whip up simulations to show the circuit operating and that might help you understand it better.
 

Thread Starter

martinsson93

Joined Nov 4, 2021
71
Great help Yaakov!

I'm very confused right now as too what I have to do, but hopefully I will understand better when I have read more about it.

How do I know what timing I want for example?
 

Thread Starter

martinsson93

Joined Nov 4, 2021
71
Just by reading about it very quickly, do I only need a resistor, capacitor and the 555 IC for the circuit?

And of course the transistor that you meansioned (which I don't really understand .... yet)
 

BobTPH

Joined Jun 5, 2013
4,753
Depending in how the button was wired, it might be possible to simply replace it with a capacitor to get the effect you want. Try placing a 100nF caoacitor where the switch was.

Bob
 

Yaakov

Joined Jan 27, 2019
5,502
Depending in how the button was wired, it might be possible to simply replace it with a capacitor to get the effect you want. Try placing a 100nF caoacitor where the switch was.

Bob
This might work, indeed. But @BobTPH would you explain to the TS the theory of it?
 

Thread Starter

martinsson93

Joined Nov 4, 2021
71
I haven't desoldered the button though. Only connected a wire between its two poles, to try to disconnect it.

Can I add a capacitor there without removing the button?

Thank you for helping me out guys, can't say it enough!
 

Yaakov

Joined Jan 27, 2019
5,502
I haven't desoldered the button though. Only connected a wire between its two poles, to try to disconnect it.

Can I add a capacitor there without removing the button?

Thank you for helping me out guys, can't say it enough!
Since the switch is NO (Normally Open) it can stay without having any effect. The idea of the capacitor is that current will (appear to*) flow through it until it charges up, then it will stop. So, when the power is first applied, the cap will conduct across the switch acting like a short but as it charges it will become less and less conductive until is appears to be an open circuit. It might take some time for the cap to discharge after power is removed before it can do its trick again.

*for the pedants in the audience
 

BobTPH

Joined Jun 5, 2013
4,753
Yep, that is the theory. If it doesn’t come back on after removing snd reapplying power, a resistor in parallel might help, but it has to be large, 100K or 1M.

Bob
 

Yaakov

Joined Jan 27, 2019
5,502
Yep, that is the theory. If it doesn’t come back on after removing snd reapplying power, a resistor in parallel might help, but it has to be large, 100K or 1M.

Bob
The bleeder resistor has to be a large enough value that it doesn’t conduct sufficiently to turun on the amp, but the bigger it is the longer it will take for the cap to work as desired since the discharge rate will be inversely proportional to the resistance.

You can try bridging the switch with candidate resistors starting with low(ish) values until it stops working as a switch. At that point it can be placed across the capacitor without interfering and when the charging voltage is removed (the power supply is turned off) the cap will discharge through the resistor getting it read for another round.

The time it takes for this to happen can be calculated as the RC Constant which is some simple math. If you end up needing the 555 one-shot, the same RC circuit (a capacitor discharging through a resistor) will be an essential element. It’s how the 555 gets its timing.

None of this, individually, is very hard to understand at all. If you approach it stepwise, and give yourself time to understand it, you will find it logical and predicable—and a lot of fun.
 

LowQCab

Joined Nov 6, 2012
1,918

Attachments

Thread Starter

martinsson93

Joined Nov 4, 2021
71
Hi guys, I have been trying to connect a capacitor to the switch, and it solves my problem but instead my old trouble came back.

The thing is that I want to disconnect the switch (on-off button) for the amplifier and instead connect the amplifier directly to a power supply of 5 V.
Before I hade to both connect the power supply cable and then push the on-off button on the amplifier, but I want everything to run just by adding a switch to the power supply which turns everything on.

When I now connected a capacitor to the on-off button it runs without problem but I still have to push the on-off button for the amplifier to start.

Do you understand what I'm aiming for?

Thank you!
 

Yaakov

Joined Jan 27, 2019
5,502
Yes. But the cap should have been the solution (if it had worked).
How long do you have to press the switch under normal conditions for the amplifier to turn on?
 

Yaakov

Joined Jan 27, 2019
5,502
Well, it looks like you might have to take the 555 one shot approach. But while it is not as simple as the capacitor solution would have been, it's not terribly hard, either.
 

Thread Starter

martinsson93

Joined Nov 4, 2021
71
I see.
I just bought 555 IC timers, just to have them and try to connect them to the push button in some kind of circuit.

Can anyone of you describe to me how I'm suppose to think regarding the output pulse width? I want the amplifier to be on as long as it has power to it.

Thank you!
 
Top