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On -off relay timer

MJJones

I have an idea if an audio sensing relay.   After this circuit detects audio it turns on a speaker amplifier for a curtain lenth of time and after that length of time has expired it will then turn off the amplifier. 

I think a 555 timer is in order here. Can someone please help me with this circuit. 

Thanks in advance. 

MJ

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  • 0

    I forgot to mention. Every time this circuit detects audio the timer will reset.  I think I want to set a timer for 2 hrs. 

  • 0 in reply to MJJones

    I think that 2 hours is a bit too long for a 555, you start getting into the realms of large capacitors and high-value resistors followed by unpredictable behaviour. It depends on how accurate you want the timer to be. I have made things involving long time-delays in the past and have used digital counter chips with mains-derived clocks.

    Having said that, I believe I have heard of a later version of the 555 that will do what you want without the inaccuracies. It might be worth looking at the RS or Farnell web sites for data sheets and example circuits based on the latest 555's.

    A point to watch out for, assuming you will be switching the output with a relay, is that the relay must be almost silent as it switches off, otherwise the audible *click* will reset the timer and turn the speaker on straight away. If the timer and the speaker are some distance apart then this may not be a problem. To avoid any such problem I would recommend the use of a solid-state relay as they have no moving parts. Except the electrons...   They will be a bit more expensive, but from my experience they are truly fit-and-forget so well worth it.

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  • 0 in reply to MJJones

    I think that 2 hours is a bit too long for a 555, you start getting into the realms of large capacitors and high-value resistors followed by unpredictable behaviour. It depends on how accurate you want the timer to be. I have made things involving long time-delays in the past and have used digital counter chips with mains-derived clocks.

    Having said that, I believe I have heard of a later version of the 555 that will do what you want without the inaccuracies. It might be worth looking at the RS or Farnell web sites for data sheets and example circuits based on the latest 555's.

    A point to watch out for, assuming you will be switching the output with a relay, is that the relay must be almost silent as it switches off, otherwise the audible *click* will reset the timer and turn the speaker on straight away. If the timer and the speaker are some distance apart then this may not be a problem. To avoid any such problem I would recommend the use of a solid-state relay as they have no moving parts. Except the electrons...   They will be a bit more expensive, but from my experience they are truly fit-and-forget so well worth it.

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