# I need some advice, newbie here.

#### msarian

Joined Sep 25, 2017
12
Hello,

I need to create a circuit that will make a small speaker go off when the power source (24 Watts Max) is disabled and then a battery takes over. Speaker needs to sound no more than 15 seconds.

#### Sensacell

Joined Jun 19, 2012
2,685
What does "go off" mean?

#### msarian

Joined Sep 25, 2017
12
A DC speaker to sound intermittently for 15 seconds.

#### Alec_t

Joined Sep 17, 2013
11,813
By "DC speaker" do you mean an audible alarm?

#### msarian

Joined Sep 25, 2017
12

#### ScottWang

Joined Aug 23, 2012
6,982
V = Voltage , power supply
I = current, A, mA
How many volts of the circuit? (5V,9V,12V,24V)
How much current (mA) the circuit draw?

#### msarian

Joined Sep 25, 2017
12
Basically, all I want to do is create a device that I can plug into a USB charger (PC or car) and when the power is on, the small speaker (audible device) remains off. When the power goes out, I want the speaker to sound.

#### msarian

Joined Sep 25, 2017
12
V = Voltage , power supply
I = current, A, mA
How many volts of the circuit? (5V,9V,12V,24V)
How much current (mA) the circuit draw?

12V
(mA) unsure at this time.

#### ScottWang

Joined Aug 23, 2012
6,982
12V Power source disable → 12V Battery on → 0.1uF/25V → npn inverter → 15S 555 monostable multivibrator → 2 Hz 555 astable multivibrator → 12V Buzzer

>USB charger
Is this needs to use a dc-dc (5V to 12V) converter?

#### msarian

Joined Sep 25, 2017
12
12V Power source disable → 12V Battery on → 0.1uF/25V → npn inverter → 15S 555 monostable multivibrator → 2 Hz 555 astable multivibrator → 12V Buzzer

>USB charger
Is this needs to use a dc-dc (5V to 12V) converter?
Scott, i'm not sure if I need a converter. I do appreciate your response above, but I guess I have to understand how to tie it all together.
Like I said, I do not know much about electronics. I just have a great idea.

#### msarian

Joined Sep 25, 2017
12
12V Power source disable → 12V Battery on → 0.1uF/25V → npn inverter → 15S 555 monostable multivibrator → 2 Hz 555 astable multivibrator → 12V Buzzer

>USB charger
Is this needs to use a dc-dc (5V to 12V) converter?
What component would make the battery go on when the device receives no power from the source?

#### ScottWang

Joined Aug 23, 2012
6,982
If the battery is 12V then you will need a 5V to 12V converter.

The circuit below shown the trigger function and alarm(1 Hz), you can adjust the frequency of alarm, please refer to the link, I changed the bjt to N MOSFET for the power consumption of battery and using the cmos 555.

You still need to decide how to active the battery, use plug or switch, or connected all the time but use some voltage detector to detect?

555 Monostable Circuit Calculator.
555 Astable Circuit Calculator.

#### msarian

Joined Sep 25, 2017
12
Scott,

Thats what I'm trying to figure. The battery needs to become the source of power when this prototype gets unplugged from the 12V or USB plug.

The power source can also be a USB port in a vehicle. I want to say the output on that is 24 Watts.

#### Alec_t

Joined Sep 17, 2013
11,813
What is the "device" that is being powered by the source or battery?

#### msarian

Joined Sep 25, 2017
12
What is the "device" that is being powered by the source or battery?
The device is what I'm trying to build.

#### Alec_t

Joined Sep 17, 2013
11,813
when the power source (24 Watts Max) is disabled and then a battery takes over.
What is the battery voltage?
How does disabling the power source cause the battery to take over? Do you have a circuit for doing this automatically?
What is the battery powering?

#### msarian

Joined Sep 25, 2017
12
What is the battery voltage?
How does disabling the power source cause the battery to take over? Do you have a circuit for doing this automatically?
Hi Alec,

This device will be small. I imagine the battery will be a small lithium battery, maybe 3V.
I would like this device to be plugged into a USB port, either my PC/laptop or in the car.
When the device loses power, the battery needs to sound the mini speaker for 15 seconds and then shut off.
While there's power, the speaker needs to remain off.

#### msarian

Joined Sep 25, 2017
12

#### msarian

Joined Sep 25, 2017
12
If the battery is 12V then you will need a 5V to 12V converter.

The circuit below shown the trigger function and alarm(1 Hz), you can adjust the frequency of alarm, please refer to the link, I changed the bjt to N MOSFET for the power consumption of battery and using the cmos 555.

You still need to decide how to active the battery, use plug or switch, or connected all the time but use some voltage detector to detect?

View attachment 135807
555 Monostable Circuit Calculator.
555 Astable Circuit Calculator.
It almost has to be like tamper switch. When it detect no power from the source, it must switch on VIA the battery.