noload current

[Fahime]

Hi,
I have a doorbell running on the battery( 2 AA batteries=3volts). When the key is not hitted(when the bell doesn't work) a 3.5mA current is drawn of the battery. I'm wondering if this amount of current discharges the battery in a short time or not? I mean, is 3.5 mA a reasonable noload current or not?
Appreciated for your comments.

 

[dch222]

A rough estimate - AA alkaline cells are about 2000ma-hours so at 3.5mA would discharge in about 2000/3.5 = 571 hours (24 days).

Doesn't sound like long for a doorbell. Where is this current going I wonder?

 

[audioguru]

Don't forget that the battery will be dead in 24 days if the circuit isn't used. It will die sooner if the doorbell is rung (ringed?).

It is too bad that the doorbell project is in many threads. Its schematic needs to be attached again so we can see where the battery current is going. I can't remember if it has an amplifier that idles all the time.

 

[Sceadwian]

Why would a doorbell have a standby current?

 

[Fahime]

Why would a doorbell have a standby current?

I guess it's because of the amplifier that is always on. In fact it has an enable pin but I don't know how to control it(I mean by what component). So I set it as enable always. If you have any idea about controlling it please let me know.
Herewith is the shematic for it.

 

[ericgibbs]

hi Fahime,

Looking at the drawing, as you say the 'system' is always drawing current from the battery.

I would suggest the easy way, would be to move the two contacts on the push button so that they switch the battery onto the system, just link the melody i/c trigger pin directly to the supply rail.

Only problem is, the melody would only play while the button is pressed, but it would save the battery.

Im not sure if the melody i/c is designed to give a range of melodies and this change may fix the melody played???

EricG
EDIT:
Add R1,C1. Choose C1 to give duration to suit melody, R1 to limit charge current into C1. D1 low forward drop diode.

 

[audioguru]

I think the melody IC has a DC output voltage when it plays.

Then use a series resistor to the base of a transistor with a capacitor to ground as a filter. Its collector resistor will cause the audio amp to be in a low current Chip Disable mode at rest, then turn on when the melody plays.

 

[Sceadwian]

Right amplfier and tone generator have a current draw, but why are these circuits alwas provided with power? The door bell button should be used to directly switch the power line to the tone generator.

 

[kchriste]

Well, the tone generator needs power because it has a timer that allows the sound to continue even when the button is released. Otherwise you would just get a Ding with no Dong or just a D- if the user pushes and releases the button quickly.
AudioGuru's solution should work.

 

[audioguru]

The doorbell button is pushed for only a moment. The melody IC plays some pretty long tunes so it is "smart". It probably has an extremely low current drain when it is idling.

 

[Fahime]

I think the melody IC has a DC output voltage when it plays.

Then use a series resistor to the base of a transistor with a capacitor to ground as a filter. Its collector resistor will cause the audio amp to be in a low current Chip Disable mode at rest, then turn on when the melody plays.

Dear EricG and Audiguru,
Thanks for your suggetions. Actually I thought of those ways before but I thought they won't be the right solutions. After your comments I decided to pactice them but unfortunately the resulat was the thing I expected. I describe why:
In the first way (using a capacitor, a resistor and a diod) the capacitor discharges very quickly while I need it keep the charge about 7seconds. Although I may be editable. I practiced resistors from 1ohm to 100K.

The second way(using a transistor as Audiguru recommended). I interpreted you mean the base of the transistor is connected to the output of the melody IC (through RB) and the collector to the pin 1 of Amplifier(That is for enabling Ic). That is to say, the amplitude of output signal of the melody IC is not constant, it changes from 1.8 volts to 0. The frequency is about 0.2 ms. Please see the attached file.

Please see if me logic is true:
When the melody signal is in it's pick, the transistor should be saturated and the collector goes VCE(sat) that is close to zero. So the MC34119 will be enable, The times the signal is zero, the trasnsitor is cut off and Vc=Vcc and the amplifier is disabled. so when there is no signal the amplifier is disable too.

Now that I added a transistor(PN2222A) and 2 resistors(Rb and Rc), see the second attached file, my calculations are as below:
regarding with the datasheet: HFE(min)=35
The saturation condition: VB<VC
VC=VCE(sat)=0.3
==> VB<0.3
VB=Vi - Rb x Ib <0.3 ==> RbxIb> 1.8-0.3 ==> RbxIb>1.5 (1)

VC=VCC- Rc x Ic ==>Rc x IC=3-0.3=2.7 (2)
Ic <= Hfe x Ib =35 x Ib (3)
from 1,2,3 ==> Rb/35Rc > 1.5/2.7 ==> Rb>20Rc
I choosed Rc= 100 and Rb=2.2K
The result was not good. Because the amplifier amplifyes just some parts of the signal and it destroys the output.
Please assist me to solve this problem or another solution.
Thanks

 

[audioguru]

The disable pin of the MC34119 audio amp is at least 50k ohms. So the RC for the transistor could be 10k and the RB could also be 10k. Try it.

 

[Oznog]

You didn't add a regulator not listed in the schematic, did you?

Some +5v regs have a high idle current, enough to seriously drain the battery over time. Others are lower by multiple orders of magnitude.

Microcontrollers are so much better at this. They have low-current sleep modes so they draw essentially nothing until a button is pressed to wake them up, they do a task, and go to sleep again.

 

[Fahime]

You didn't add a regulator not listed in the schematic, did you?.

I don't have a regulator. Why should I have it? I need a 3Vdc supply and I have 2 AA batteries in series.

 

[Fahime]

The disable pin of the MC34119 audio amp is at least 50k ohms. So the RC for the transistor could be 10k and the RB could also be 10k. Try it.

I tried it too. But the matter is the output voice. It is probably due to the switching time of transistor. As I said before the amplifier is not always on during recieving signal from M602. So it destroys the signal. I think using a transistor is not the solution. What do you think?
I've thought of a timer but it's expensive and need room. Also I'm not sure if a timer has the idling problem also!?

 

[RODALCO]

Run it of a low voltage transformer 3 or 5 volts ac and rectify it to dc.

 

[audioguru]

The fluctuating AC from the melody IC must drop below the 0.65V turn on threshold of the base of the transistor, causing the transistor to disable the audio amp IC.

The base resistor for the transistor could be two series resistors wih a filtering capacitor between them to ground. Then the capacitor's value can be selected so that the transistor turns on fast enough but turns off slower so that it stays on during the melody.

I have never seen (heard?) a battery-powered doorbell. Do you live in a tent in the desert?

 

[Fahime]

I have never seen (heard?) a battery-powered doorbell. Do you live in a tent in the desert?

Good question! I'm intrested in knowing your doorbell's supplies. If you are using city power, so u need a transformer. The transformer is expensive so people (in my country) use dangerous transformerless doorbells that also is broken very soon. So what do u prefer? A dangerous doorbell or a battery-powered one?
Anyway, Thanks alot for your assist in saving people's life.

 

[Nigel Goodwin]

A transformer is far cheaper than using batteries - if people around you are stupid enough to use tranformerless supplies that's their fault, not someone making a perfectly correct (and safe) suggestion.

Don't people knock on doors where you live? (not that we know where that is, seeing as you haven't entered your location) - I've never had a doorbell!.

 

[Sceadwian]

Audioguru, you've never seen/heard of a battery powered doorbell? You can pick them up in almost every hardware store in the U.S. The one's I've seen are small RF modules that trigger a receiver unit inside so you don't have to re-wire if you're adding a doorbell. I've also seen battery powered receiver modules for the hard of hearing which have loud buzzers and/or flashing lights on them. They can take the unit into whatever room they're in to be notified when the doorbell chimes or the phone rings.