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Microcontroller based electric bell ringing.

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Full Member level 4
Sep 15, 2014
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Dear All,
I am switching 230v ac electric bell on particular alarm time by alarm clock circuit.But sometimes bell continously ringing does not stop.Here is a relay circuit.please reply.
(here i am using delay function to switch on & off bell)

Zip and post your complete code. Mention which MCU you are using.

When the bell rings continously, the relay is engaged?
If yes, then the alarm clock is the problem.
Otherwise, the snubber is the problem.

What is the power requirement of the bell?
What do you use as an alarm clock?
Replace PNP transistor with NPN.

Maybe MCU is hanging and WDT is not implemented in code.

Dear sir,this code and project working in Proteus without any error.above problem observed sometimes in actual board.but can become trouble in future .here I am using 230v ac electric bell as alarm bell according to application.
(What is wdt).

You have find if it is software problem or hardware problem.
when bell continuously ring you can measure the relay driver base emitter voltage, or you can add one led with a series current limiting resistor in parallel to relay to check.
Proteus simulation is not 100 % grantee the circuit real operation.

Dear sir . one thing I noted during continuous ringing rtc(time) runs normally.that means controller does not hang.

Is your mC decoupled? Have you got stable power supply that will not produce brown out when relay engages?
Post at least a photo of your pcb.

You could increase the resistance value of base resistor.

How much value of base voltage should be the ire to switch off bell in BC 557(PNP)transistor?

If VBE is less than 0.6 or 0.7 V then transistor enters cut-off region.

I use 104pf as decoupling capacitor.and transformer ,bridge rectifier in power supply.

mcu stops when bell rings.

I have designed alarm clock to ring 230v ac electric program I used delay function to energies relay.many times MCU stops when alarm bell ring.can it be solved by using intrupt in program?

Re: mcu stops when bell rings.

cannot solve this problem using interrupt.
already a forum thread and replies were given ,IMHO, for such question.

Re: mcu stops when bell rings.

i hope u hav connected protection on the reay to prevent the cpu from any loss, or u shud use the triac for switching.. reverse emf in relay can cause the cpu to be damaged or even force it to get hang sm times..

Re: mcu stops when bell rings.

It sounds like improper design of the electronic circuit.
Post the circuit's schematic.

Re: mcu stops when bell rings.

I have designed alarm clock to ring 230v ac electric program I used delay function to energies relay.many times MCU stops when alarm bell ring.can it be solved by using intrupt in program?

It is difficult to offer any meaningful advice without examining a detailed schematic of your design, including all power supply sections.

Also your description is rather vague, particular the phrase:

...many times the MCU stops when the alarm bell rings.

By "stops" are you implying the microcontroller resets itself, or simply pauses execution until the software delay routine is completed?

Utilizing a timer in conjunction with an interrupt and Interrupt Service Routine is often preferable to utilizing a software delay routine.

Software delay routines, by their inherent nature, prevent the microcontroller from executing other tasks until the software delay routine is completed.

However, if the microcontroller is, at times, resetting itself when a relatively high voltage actuator is energized, it suggests there is a hardware design issue, particular either back EMF or EMI related.

Posting a detailed schematic of your design would certainly help clarify potential issues, as a flyback diode, aka freewheeling, suppressor, clamp diode, across the inductive load is considered essential in designs with inductive loads and a complete snubber circuit maybe required in cases involving EMI. A careful review of your designs power supply section should also be performed.


When the i/p is low the base current will flow through the transistor.But when the i/p is high the base current will not stop because the controller i/p voltage at pin is less than 12v ,it should be 5v or 3.3v depend on your controller. Try to use npn transistor instead of pnp.

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