# How to convert 0.9VAC to 12VDC

#### joe0886

Joined Jul 27, 2015
14
I am wanting to hook up a 12V fire alarm horn and strobe to an alarm clock speaker circuit. When the alarm clock goes off, it outputs 0.9V to the alarm clock speaker. I want to use this 0.9VAC and turn it into 12VDC to power my fire alarm horn and strobe, which will activate when the alarm clock goes off.

I am new to electronics, but my issue is I cannot find an AC Step Up Transformer that goes down to this low of voltage on the primary side. I am also unable to find a Full Wave Bridge Rectifier, that would work with this low of voltage.

The horn strobe can operate on as low as 8VDC, and draws about 150mA. Any help or suggestions would be a great help as I am new at this, and learning as I go. I have considered an Arduino, which puts out 5VDC but it can only handle 40mA max output. Would some type of MOSFET work? I am not sure quite how these operate yet, but just a thought. Thanks again.

#### Alec_t

Joined Sep 17, 2013
11,169
Welcome to AAC!
The 0.9V can't be used as you described (not enough power) but could be used to switch a bipolar transistor which in turn could trigger an SCR or MOSFET circuit to switch the horn and strobe on. What will turn them off again?
You will need a 12V supply with enough current capability for the total load, plus some margin. The Arduino isn't needed unless you want it for some fancy control of the horn/strobe.

#### tcmtech

Joined Nov 4, 2013
2,867
YOur best option would be to find where the power is supplied or controlled from going to the radio or beeper part of the alarm clock system and tap into that.

Either that or spend the ~$6 -$8 it costs to buy a normal digital multi event outlet timer unit and use that to turn power on and off to the 12 volt stuff through a common 12 volt adapter power supply.

#### ScottWang

Joined Aug 23, 2012
6,884
You will need 12V power supply and idea as below:

Microphone Preamplifier → half rectifier → NPN bjt stage → PNP bjt stage → CD4013 once trigger switch(with reset) → B(NPN driver),C ← fire alarm horn and strobe +12V

#### AnalogKid

Joined Aug 1, 2013
8,389
As mentioned, there is not enough energy in the speaker drive signal to power the horn no matter how you convert it. It should be easy to do with a small control or switch circuit. If you can, read the speaker ratings on the back of the speaker magnet. It might say 8 ohms, 16 ohms, etc. abbreviated with the letter omega, and maybe a power rating such as 0.5 W. That and the 0.9 Vac will tell us what is available to drive an electronic switch. Can you build a small circuit on perf board?

ak

#### joe0886

Joined Jul 27, 2015
14
As mentioned, there is not enough energy in the speaker drive signal to power the horn no matter how you convert it. It should be easy to do with a small control or switch circuit. If you can, read the speaker ratings on the back of the speaker magnet. It might say 8 ohms, 16 ohms, etc. abbreviated with the letter omega, and maybe a power rating such as 0.5 W. That and the 0.9 Vac will tell us what is available to drive an electronic switch. Can you build a small circuit on perf board?

ak
I am able to build a small circuit on a perf board, not a problem. I appreciate everyone`s quick response and help with this matter. The back of the speaker says 8 Ohms 1 Watt. I can get a 12V power supply from a plug in power supply or battery. Just struggling with how to open and close that circuit, and how I would size the components. Thanks Again. My thought was to somehow use the 0.9VAC to close a switch(transistor?), which would then allow my 12V circuit to close, activating the horn strobe. When the alarm clock buzzer shuts off, it would then open the 12V circuit.

#### joe0886

Joined Jul 27, 2015
14
Welcome to AAC!
The 0.9V can't be used as you described (not enough power) but could be used to switch a bipolar transistor which in turn could trigger an SCR or MOSFET circuit to switch the horn and strobe on. What will turn them off again?
You will need a 12V supply with enough current capability for the total load, plus some margin. The Arduino isn't needed unless you want it for some fancy control of the horn/strobe.
When the alarm clock buzzer is shut off, or the snooze is hit, it opens the circuit for the buzzer(speaker). This would also (in theory) open a switch(MOSFET/transistor?) to open the 12V horn strobe circuit. I would be using a 12V battery for a supply, or a plug in 120V power supply with correct output voltage and current. My girlfriend doesn't get up with her three alarm clocks, this should surely wake her.

#### joe0886

Joined Jul 27, 2015
14
YOur best option would be to find where the power is supplied or controlled from going to the radio or beeper part of the alarm clock system and tap into that.

Either that or spend the ~$6 -$8 it costs to buy a normal digital multi event outlet timer unit and use that to turn power on and off to the 12 volt stuff through a common 12 volt adapter power supply.

I like the digital event timer idea too. Do you know if you can program these to open/close the circuit at set times, for lets say a time period of a month? As in day one: on at 2pm, day two: on at 4pm, etc. Kind of like a lighting control timer, that could be used to program parking lot lights to go on and off at set times.

#### joe0886

Joined Jul 27, 2015
14
I like the digital event timer idea too. Do you know if you can program these to open/close the circuit at set times, for lets say a time period of a month? As in day one: on at 2pm, day two: on at 4pm, etc. Kind of like a lighting control timer, that could be used to program parking lot lights to go on and off at set times.

I have dug into it more and found my power supply on my CD7613 Chip. I get 5.7VAC right off the chip, when the alarm buzzer is sounding. How could I best utilize this?

#### AnalogKid

Joined Aug 1, 2013
8,389
The 7613 datasheet indicates a max audio output of 0.25W into 8 ohms, or 1.414 Vrms or 2 V peak. If you are measuring on the chip's pin 12, that pin has a DC offset we can't use. But if both the radio and the alarm horn have fully isolated power supplies, we can connect the transistor switch input directly across the speaker. Note that the 7613 does not have an alarm tone output option or any clock functions; it is a radio only, and makes audio, noise, or both.

0.25 W into 8 ohms equates to 176 mArms, more than enough to drive a transistor switch to sink 150 mA.

An opamp or comparator will give a cleaner on/off transition, but here is the basic idea:

Speaker > coupling capacitor > catch diode > filter capacitor > discharge resistor > base current limit resistor > 2N4401 > alarm horn.

The 2N4401 is a common general purpose transistor rated for 500 mA collector current. This should be ok for your 150 mA alarm horn, but a small power transistor like a TIP29 or TIP31 will have more margin.

ak

#### joe0886

Joined Jul 27, 2015
14
The 7613 datasheet indicates a max audio output of 0.25W into 8 ohms, or 1.414 Vrms or 2 V peak. If you are measuring on the chip's pin 12, that pin has a DC offset we can't use. But if both the radio and the alarm horn have fully isolated power supplies, we can connect the transistor switch input directly across the speaker. Note that the 7613 does not have an alarm tone output option or any clock functions; it is a radio only, and makes audio, noise, or both.

0.25 W into 8 ohms equates to 176 mArms, more than enough to drive a transistor switch to sink 150 mA.

An opamp or comparator will give a cleaner on/off transition, but here is the basic idea:

Speaker > coupling capacitor > catch diode > filter capacitor > discharge resistor > base current limit resistor > 2N4401 > alarm horn.

The 2N4401 is a common general purpose transistor rated for 500 mA collector current. This should be ok for your 150 mA alarm horn, but a small power transistor like a TIP29 or TIP31 will have more margin.

ak
You mentioned that if the radio and alarm horn had isolated power supplies, we could use the speaker power supply to connect to the transistor switch input. When you said alarm horn, I assume you are talking about the "fire alarm horn/strobe". And not the alarm clock speaker. Am I correct? If so, then yes, I will have a separate 12v power supply to power the "fire alarm horn/strobe". I will be looking at the transistors you mentioned. How do I proceed with sizing the other components that you mentioned in the circuit? I will look into that on my own over the coming days. I greatly appreciate your help. In essence the alarm clock speaker power circuit opens and closes the transistor, which opens and closes my 12VDC power supply. Am I correct? Thanks again.

#### AnalogKid

Joined Aug 1, 2013
8,389
You are correct. We take the audio waveform going to the speaker, rectify and filter it to make a DC voltage, that voltage turns on the driver transistor, that transistor switches the low side of the load (alarm, horn, strobe, whatever) to ground. The high side of the load goes to its own power source. Without the filter capacitor, the rectified audio would switch the transistor on and off very rapidly, and the load might not like this. With the filter capacitor, the transistor stays on continuously through the audio envelope, but there is a delay between when the audio signal ends and when the transistor switch turns off, probably around 1 second. There are ways to shorten that turn-off delay with a more complex circuit, but this application probably doesn't care.

ak

#### joe0886

Joined Jul 27, 2015
14
You are correct. We take the audio waveform going to the speaker, rectify and filter it to make a DC voltage, that voltage turns on the driver transistor, that transistor switches the low side of the load (alarm, horn, strobe, whatever) to ground. The high side of the load goes to its own power source. Without the filter capacitor, the rectified audio would switch the transistor on and off very rapidly, and the load might not like this. With the filter capacitor, the transistor stays on continuously through the audio envelope, but there is a delay between when the audio signal ends and when the transistor switch turns off, probably around 1 second. There are ways to shorten that turn-off delay with a more complex circuit, but this application probably doesn't care.

ak
Since it is a audio waveform, is this why it read 5.7VAC on the meter, and 3.3VDC when I switched the meter to DC? Thanks for the quick reply. That all makes sense. The delay is not a problem. Do you have any good references I could look at to learn how to find the correct rectifier and filter capacitor?

#### joe0886

Joined Jul 27, 2015
14
You are correct. We take the audio waveform going to the speaker, rectify and filter it to make a DC voltage, that voltage turns on the driver transistor, that transistor switches the low side of the load (alarm, horn, strobe, whatever) to ground. The high side of the load goes to its own power source. Without the filter capacitor, the rectified audio would switch the transistor on and off very rapidly, and the load might not like this. With the filter capacitor, the transistor stays on continuously through the audio envelope, but there is a delay between when the audio signal ends and when the transistor switch turns off, probably around 1 second. There are ways to shorten that turn-off delay with a more complex circuit, but this application probably doesn't care.

ak
When I am able to I will get you the specs on the 12VDC power source I will be using. Either a battery or 120v AC plug in DC power supply. Thanks much.

#### tcmtech

Joined Nov 4, 2013
2,867
I like the digital event timer idea too. Do you know if you can program these to open/close the circuit at set times, for lets say a time period of a month? As in day one: on at 2pm, day two: on at 4pm, etc. Kind of like a lighting control timer, that could be used to program parking lot lights to go on and off at set times.
Depends on how fancy of unit you get. The basic ones give you at least 20 on/off events per week that can be anywhere from one minute long to 23 hours 59 minutes long.

Personally if it was me I would just make your GF's not getting up on time as traumatic as possible so she would be afraid to over sleep. Buckets of ice water seem to work pretty good on sleeping people. So do air horns in the face and I hear a taser on minimum setting to the foot does wonders as well.

#### joe0886

Joined Jul 27, 2015
14
Depends on how fancy of unit you get. The basic ones give you at least 20 on/off events per week that can be anywhere from one minute long to 23 hours 59 minutes long.

Personally if it was me I would just make your GF's not getting up on time as traumatic as possible so she would be afraid to over sleep. Buckets of ice water seem to work pretty good on sleeping people. So do air horns in the face and I hear a taser on minimum setting to the foot does wonders as well.
LOL. I have been tempted to wake her with a bucket of ice water, but don't feel like cleaning up the mess.

#### joe0886

Joined Jul 27, 2015
14
When I am able to I will get you the specs on the 12VDC power source I will be using. Either a battery or 120v AC plug in DC power supply. Thanks much.
Also, this may adjust the components needed. My main power from the chip, before it goes through any capacitors or anything is 5.7VAC. By the time it gets to the speaker it is down to 0.9VAC. Just wanted you to know that too. Thanks.

#### tcmtech

Joined Nov 4, 2013
2,867
LOL. I have been tempted to wake her with a bucket of ice water, but don't feel like cleaning up the mess.
You could take her toys away and make her go to bed at a decent hour. That worked well with my wife.

She is one of those people who are for whatever reason is convinced that that she can get by on 4 - 6 hours a sleep a night (Everyone who has to work or deal with her every day strongly thinks otherwise) which means she would stay up until midnight to 2 am playing around on her laptop, iPad and or iPhone then have to get up at 6 am to go to work.

Once I started insisting that she shut that crap off and go to bed by 10 PM or I would shut our internet and WiFi off at night she would actually get around 8 hours of sleep getting up at 6 am became far easier and just by coincidence people started to find her to be far more productive and tolerable to be around in the day as well.

Go figure.

#### joe0886

Joined Jul 27, 2015
14
You could take her toys away and make her go to bed at a decent hour. That worked well with my wife.

She is one of those people who are for whatever reason is convinced that that she can get by on 4 - 6 hours a sleep a night (Everyone who has to work or deal with her every day strongly thinks otherwise) which means she would stay up until midnight to 2 am playing around on her laptop, iPad and or iPhone then have to get up at 6 am to go to work.

Once I started insisting that she shut that crap off and go to bed by 10 PM or I would shut our internet and WiFi off at night she would actually get around 8 hours of sleep getting up at 6 am became far easier and just by coincidence people started to find her to be far more productive and tolerable to be around in the day as well.

Go figure.
Very interesting. 4-6 hours per night just doesn't cut it. Shutting off the internet is a good idea. She likes to "nap"-more like sleeping for several hours when she gets home from work in the afternoon, causing her to stay up late, even though she gets up at 5am for work. I already told her I am putting the fire alarm horn and strobe above her bed. If the flashing strobe doesn't wake her, surely the 90+ decibel alarm will.