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Can a lightbulb be switched on using two mobile phones? (Make a call and the bulb switches on)

I need this for a kid's school project so the questions like should the bulb keep glowing after the call ends do not bother. Its for a kid's school project so should be as simple as it could be.

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    \$\begingroup\$ Do you want to be able to use the phone more than once? \$\endgroup\$
    – EM Fields
    Commented Oct 9, 2014 at 9:59
  • \$\begingroup\$ What sort of relay contact arrangement is required and what sort of load on the contacts is envisaged. These may not seem relevant questions but they are to me. \$\endgroup\$
    – Andy aka
    Commented Oct 9, 2014 at 10:08
  • \$\begingroup\$ Based on a few comments below if using a LED was OK instead of a bulb and it has a vibrator motor you just put a LED and series current limit resistor across the motor. Experimenting with different resistor values and what effect they have might be an interesting kid's experiment as long as you know how to explain it. \$\endgroup\$
    – PeterJ
    Commented Oct 9, 2014 at 10:55
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    \$\begingroup\$ Please don't use sms-like abbreviations. If you take care of the wording and style, you'll get better feedback. \$\endgroup\$
    – clabacchio
    Commented Oct 9, 2014 at 11:51

2 Answers 2

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I think something like the circuit below is good enough to switch your relay. I only drew the coil of the relay here.

Cropped Schematic

(Original)

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Use the "small power" signal from the speaker output to trigger something like a 555 Timer set up as a one-shot pulse of whatever time you want to hold open a relay, either by driving the relay directly if the small output of the timer IC is enough, or by using a high side MOSFET or BJT as a load switch from the 9V you have available.

The audio style signal will not really be useful for direct amplification to activate a relay switch, unless you like musical relays!

Your relay specifications are quite important for this, so make sure you give it the required current to hold it open. You may also not even need a relay, whatever your relay is doing could probably be done easier and straight to the point using a MOSFET load switch.

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  • \$\begingroup\$ Okay ... Let us make it more simple ... My neice needs a school project. An AC/ DC bulb is to be switched on using two remote mobile phones. Time is not a factor. To eliminate the musical relay effect, can we use the power used for vibrator motor in a mobile? Ckt shud be as easy as for a kid to handle. Like using a couple of relays, bjts etc. \$\endgroup\$ Commented Oct 9, 2014 at 10:18
  • \$\begingroup\$ how will the behaviour of these lights work, will they stay on forever after being triggered? will they need a timer? How are you going to access the "power" from the phones? \$\endgroup\$
    – KyranF
    Commented Oct 9, 2014 at 10:24
  • \$\begingroup\$ I want to use the phones as a trigger only, no if the bulb switches off after the call ends.. No problem. Power can be drawn from a 6v-12v battery or AC 230 v socket. I will get the bulb accordingly. \$\endgroup\$ Commented Oct 9, 2014 at 10:41
  • \$\begingroup\$ it's not easy, as using the call (ring tone, speaker output) or the phone vibration events are only at the start. You will need an external timing circuit to hold the lights on for a certain time. The other issue is getting access to the phone signals, you will have to destroy the phones to get access to the vibration motor connections or the speaker connections. \$\endgroup\$
    – KyranF
    Commented Oct 9, 2014 at 10:43
  • \$\begingroup\$ I will use a basic phone .. Not an iPhone ;),.. Yes, I will break one phone to get the trigger signal. Okay, so we need a timer circuit for sure. Noted on that. Do you have a better idea? Using DTMF wud involve arduino ... That's going to be a headache. Hmm.. Anything like a ready to use vibration sensor which senses the vibration of the phone and activates a timer/relay/DC bulb/motor... Anything ? \$\endgroup\$ Commented Oct 9, 2014 at 10:52

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