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I am working on a security project in which in case of any emergency my smartphone triggers an alarm or sound. This sound is in the form of audio by phone smartphones. When I connect earphone jacks, the audio is routed to the earphone jack, which means the sound plays only on the earphone and not with the speakers.

The voice from my smartphone is low and the alarm tone is very calm. I cannot change the ringtone from smartphone due to some device restrictions.

I want to make a circuit with MOSFETs and transistors which keeps the transistors off when the device is silent and triggers the MOSFETs when the device makes some sound.

According to my research, the output from the aux jack in smartphone is analog and the resistance also differentiates when the phone is silent and when it is making sound.

I tried this circuit with an IRFZ44N MOSFET which is a wire break alarm. I think modifying it can get the work done:

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These are some measurement values when the device is silent and playing alarm.

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Resistance on 1k range

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Please help me with it or suggest some other way.

JRE
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Ashutosh7i
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  • What AC voltage do you get when the alarm plays? Why is gate connected to source via a wire loop? – Andy aka Dec 17 '22 at 13:27
  • i dont have DSO and my multimeter too don't support ac measurement lower than 200v range so how do i measure it?, also the circuit is of a wire break alarm, i asked can i and how i can use this circuit for my purpose? – Ashutosh7i Dec 17 '22 at 13:54
  • @Andyaka it is around 2 volts when playing and 0 volts idle – Ashutosh7i Dec 17 '22 at 14:32
  • You could probably use a diode and capacitor as an AC detector then, use a comparator to drive the MOSFET. Or maybe this circuit I designed: https://electronics.stackexchange.com/questions/65891/difficulty-with-common-emitter-amplifier/65938 – Andy aka Dec 17 '22 at 14:36
  • @Andyaka is this the circuit you are talking about? https://i.stack.imgur.com/dHJT3.png would replacing the mic with audio input and led with a buzzer get my job done? – Ashutosh7i Dec 17 '22 at 15:57
  • It's worth a try @ash . R3 might need to be made bigger (desensitizing the circuit against trivially small signal levels i.e. false triggers) and, you might need to run at 12 volts if that's all you have got. I don't think 5 volts will work. I did design it back in 1986!! – Andy aka Dec 17 '22 at 16:55
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    the bottom two pictures show an incorrect way to measure resistance ... the DVM should be set to `ohms range`, not to a `continuity test` ... it is unknown if the display shows the correct resistance in the setting that you chose – jsotola Dec 17 '22 at 18:14
  • I am always concerned when people attempt to design a circuit without proper equipment, especially when it is something that might be critical, such as a "security project". I have several cheap DMMs (~$20) that can measure AC millivolts as well as frequency and duty cycle, and I have a cheap (~$30) scope that would be adequate for this job. It's also important to know how to use test equipment. – PStechPaul Dec 17 '22 at 20:29

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The datasheet for the IRFZ44 shows that its Vgs must be 10V for it to fully turn on and pass 31A. But you do not need it to pass 31A. Its threshold voltage is from 2V to 4V for it to conduct only 0.25mA which is almost nothing.

Your phone output might be 0.5V on the peaks of the ringer. So an IRFZ44 Will Not Work.

An IRLZ44 (see the L?) is more sensitive and needs an input from 4V for it to pass 21A that you do not need. Or its threshold voltage is 1V to 2V for it to conduct only a little so it also Will Not Work Unless the signal from the phone is amplified.

Why were you measuring DC when the audio signal is AC? Your cheap meter measures only 200VAC or 400VAC and probably only very low electricity frequencies.

You need a Mosfet that is much more sensitive and a voltmeter that can read low level audio frequencies.

Justme
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Audioguru
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  • so can you suggest me a circuit which can get my work done? i can use anything else than a mosfet too – Ashutosh7i Dec 17 '22 at 16:53
  • Mosfets with a very low threshold voltage are available but I do not know their part number or who sells them. A preamp circuit needs to be designed and made if you use an ordinary Mosfet. The Mosfet will conduct only during the peaks of the audio frequency which will appear dimmed. a rectifier and filter capacitor must be added to make it appear bright. – Audioguru Dec 17 '22 at 18:26