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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

  • 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.

  • 0

    Conceptually you need a mic followed by an amplifier to SET an S-R flip flop which drives a relay to turn on your audio amplifier. The flip flop also would enable a CD4060 oscillator/counter. An appropriate counter output (2 hours) would RESET the flip flop (and the counter).

    Or you could just use a Pi Pico and a little firmware. 

  • 0

    If you take a look at this site:

    https://www.talkingelectronics.com/projects/100%20IC%20Circuits/1-100_IC-Ccts.html

    (and follow the links at the top for 50 555 circuits, and the two-hundred transistor circuits) you'll find a load of building-blocks that you can experiment with and combine to make what you want. 

    There are sound-triggered circuit examples there, and there are relay switching and delay examples too, some using 555, some using a 4060 IC which you may well need as Doug mentions.

    At the very least, it should give you more than enough ideas to start experimenting and prototyping.

    Please share your final circuit to help others.