Selecting a rechargable battery for Speaker-Alarm Circuit

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ali8

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Hello,

I am designing simple Alarm circuit that consists of

1) rechargeable battery (not decided)
2) Microcontroller (PIC16F84, datasheet, Power:2mA*5V=10mW (@4MHz))
3) 4-digit 7-Segment Display (**broken link removed**, Power:40mW)
4) 8Ω, 5W speaker (datasheet, Power:5W)

The circuit has two modes: Normal mode (ignore) and Alarm mode.
Total power consumption for ONE HOUR of operation: 10m+40m+5 = 5.05W.
I need a battery that will give me 2 hours of operation before recharging.

Will a 3.3V, 3AH battery work?

- - - Updated - - -

Another question. If I use the 3.3V battery, then I need a voltage regulator/converter because the speaker and the 7-segment operate on 5V. I do not need any amplification at the moment, just the 5W speaker. Can you recommend a voltage regulator/converter?
 
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"Total power consumption for ONE HOUR of operation: 10m+40m+5 = 5.05W.
I need a battery that will give me 2 hours of operation before alarming. "

Device use 5W/h.

What you mean 2h of operation before alarming ?
 
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    ali8

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I must admit its little strange how you calc power consumption of device, but it does not matter.

3,3V 3Ah will be ok for your needs and for needed duration.

Try to make circuit - to use parts which can work on lower voltage as 3,3V that will simplify design, also try to find some strong and loud piezo speaker on lower voltage.

Boosting from 3,3V to 5V can be done of course, but that process will drain more battery. DC/DC converter efficiency is very important.

In your semi specification what you give, only that speaker is heavy consumer.
 

Thanks for your reply.

Actually, I just looked in digikey.com for some speakers and found that a normal 5W speaker works in 5V. It would be nice if I can find a cheap 8ohm 5W speaker that works on 3.3V or something similar. Do you have any suggestions?

Also, what is the "normal" way of calculating power requirements, since you say mine is weird?
 

How not to design a product.
Select the wrong parts then ask what kind of battery you need.
Select another voltage and ask what kind of voltage regulator you need.

ali8 said:
Will a 3.3V, 3AH battery work?
...
Can you recommend a voltage regulator/converter?
Click to expand...

How to design a product?


1. Specify Human interface requirements.
I want to design a clock with a digital display, switch inputs and alarm output
- inputs : ac power dc battery, mode switches, function switches, power cord, charger port?
- outputs: 4 digit display, audible alarm
- functions:////......\\\\
- budget: ?
- R&D time ???
- complexity level ???

alternatives considered.
- putting a watch into a calculator display
- etc etc

Goals and Criteria for design.
- learning experience ?
- learning to fail
- learning by example?
- over-ambitious ill-thoughten experiment in applied electronics?
 

Thanks for your reply.

Actually, I just looked in digikey.com for some speakers and found that a normal 5W speaker works in 5V. It would be nice if I can find a cheap 8ohm 5W speaker that works on 3.3V or something similar. Do you have any suggestions?

Also, what is the "normal" way of calculating power requirements, since you say mine is weird?
 

With a power supply that is only 3.3V, a low voltage audio power amplifier will have an output of only about 2.3V peak-to-peak if it is efficient.
2.3V p-p is 0.81V RMS. The calculation for power is [RMS voltage squared]/speaker impedance so the output power is only 0.082W which is almost NOTHING!

To have 5W into 8 ohms then the power supply for the ordinary amplifier must be about 20V!

A bridged amplifier has two amplifiers operating with opposite phases. Each amplifier drives one of the two wires of a speaker. Then the signal voltage swing across the speaker is almost doubled which also almost doubles the current. Then a power supply of only 12V can produce 5W into an 8 ohm speaker.
 

Sorry SunnySkyGuy, the second repeated post was an error. I am considering your post points and will get back here again soon. Thanks.
 

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