How to switch a relay from a low voltage AC source?

[sgtpepperaut]

hi there,

i am currently working on a project that involves a cellphone that puts out ~1.3 volts AC signal to the speaker/vibrator. I am trying to switch a 12 Volt standard automotive relay with whenever that vibrator/speaker goes off (ie when the phone gets called)

wondering if anyone has any info on what hardware to use to make this possible.

I was reading about transistors and MOSFET amplifiers but i am not quite sure that i can simply hook the ac source up to the base and have 12 volts at the collector putting out 12 volts at the emitter.

Any info is appreciated! thx

 

[Wade Hassler]

Are you sure that it's 1.3VAC?
The few cellphone vibrators I've seen were just DC motors with an unbalanced shaft.
It makes most sense that it would run from the battery voltage.

 

[Hero999]

Well it's doable but you'll need another power supply.

 

[sgtpepperaut]

hey. yes i was surprised as well! the IHF speaker and vibrator are a single piece and have two contact coming from the board. That source was definitely AC! ...for the speaker that would make most sense

i attached a pdf. checkout page 9 with the earpice/vibrator in one part. (i did not see any switching/regulating components in the earpiece by itself. not sure how its done.

 

[sgtpepperaut]

its going in my car where i have 12 VDC (car) and 5 VDC (USB power of the computer) readily available if that helps?

 

[Hero999]

What frequency is the AC?

Here's an idea.

 

[sgtpepperaut]

dont have an oscilloscope here but my best guess would be <50Hz for the vibrator (carrying the most power?) and > 400Hz for the audio part of the signal.

ok so you rectify the ac source. feed that into the base which draws current from the emitter allowing the relay circuit to run current and thus close the relay?

why is there a diode after the relay going back to the front?
is there a way with less components? all i really need is the relay to close once) for example without rectifying it?

 

[Reloadron]

Hero's suggestion is a good suggestion and should work using a minimum number of parts. The diode across the relay is to surpress the back EMF inductive kick off the relay coil when the coil is de energized.. Nice little circuit.

Ron

 

[crutschow]

Many automobile relays take 150 to 200mA, thus the transistor may not have enough base drive to close the relay with a 1kΩ base resistor from perhaps 2.6VDC. The base resistor should be more like 100Ω with the capacitor increased to 1000µF.

 

[sgtpepperaut]

allright thx for the info guys! ill def give this a try.

this is a standard NPN transistor right?

 

[sgtpepperaut]

automobile relays take 150 to 200mA

yeah i measured round 140mA...

 

[crutschow]

this is a standard NPN transistor right?

Yes. A 2N2222 or something similar.

 

[Hero999]

Then you need to add another transistor.

 

[sgtpepperaut]

NEVERMIND read next post! ok. i set up the circuit and the relay engages. However when the AC source is OFF the relay stays connected until i break teh connection.

i am using a 100 ohm resistor and a 1000µF capacitor

any suggestions what im doing wrong? or why this not working?

please don't laugh at the set up i had no breadboard haha

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[sgtpepperaut]

NERVERMIND had the transistor hooked up the wrong way around MEH!!!

THANKS GUYS!!! works like a charm....!

15 dollar prepaid phone +6 dollar in parts = TRULY REMOTE CAR STARTER HEHE

 

[crutschow]

How are you going to use this to remote start you car? You need to turn on the ignition, press the starter, and then detect when the engine starts to release the starter.

 

[sgtpepperaut]

How are you going to use this to remote start you car? You need to turn on the ignition, press the starter, and then detect when the engine starts to release the starter.

i already got a micro-controller and carpc hooked up. i have been doing it for months over wifi. just needed an impulse from the phone over GSM network

if you are curios:

Project Complete: Iphone Remote Control - Benzworld.org - Mercedes-Benz Discussion Forum

thx for the help!

 

[Hero999]

Good, I'm glad you've got it working, it looks like one transistor was fine after all.

It looks like you increased the capacitor to 1000µF, did you reduce the base resistor to 100R too?

My only concern would've been the 1µF capacitor not being large enough to couple the low frequency signal but it was obvious unfounded.

Are you going to now put it on a PCB or stripboard?