What's the simplest audio line-level signal sensor (on/off) - relay/transistor/triac?

[flx]

Hello, everyone.
This question is so simple that it may sound complicated to all you knowledgeable folks here; please consider it's coming from a total newbie.

What is available:
- A vibration motor, runs at anything between 1.5V DC and 4.5V DC, < 100 mA
- An audio source, line-level (actually headphone level @ low volume)
- A power source providing minimum 3 Volts (a 3.7V battery pack)

What is to be accomplished:
- Spin the motor when there's any audio coming in (above noise level)

Basically, this "project" needs to close the circuit between the battery and the vibrator, and keep it closed for as long as there's audio at the input. Think of this as replacement of a speaker/buzzer with a vibrator: instead of hearing a beep, feel a vibration. Please imagine something as simple as possible... could it be a transistor / diode / relay / triac or any combination thereof? What values? (sorry if my suggestions seem silly - remember my noobness)

Many, many thanks in advance!
**broken link removed**

 

[Papabravo]

A precision rectifier, peak detector, and a comparator. Two IC's and a handfull of discrete components.

 

[flx]

Thanks, Papabravo. But, uhm... isn't there an even simpler way to do it? I know that if I had - say - DC current instead of audio signal, I could use a high sensitivity (low voltage) relay, maybe a solid-state one. So the only problem remains how to transform the AC audio signal into a *roughly* DC to poke that relay.

 

[Externet]

Look at the task from another angle:

To energize a relay, a tiny 5V one, with say a 250 Ω coil, will need 0.1watt.

5V ÷ 250 = 20mA
5V x 0.02A = 0.1 W

If your audio signal can provide such power at the levels you want the relay to click on, and stay off under such level, fine, you can connect a relay at the audio as the 'simpler' way you want.

(Don't need DC; relays do work with AC at their coils too)

For sources weaker than 5V and 20 milliamperes you need amplification to meet the relay demands. That is what Papabravo tells you with detection and comparator circuits.

So, get the most sensitive -least power demanding AC relay you can find- and hook it to speaker wiring. Adjust the volume to get the action you want.
For a 'line level' the relay alone will hardly work. Depends on the audio source impedance.
Simpler than that... don't know.

Miguel

 

[flx]

Hmmm...

Thanks, Externet. If found this little relay on RadioShack's webpage and I was wondering if it'd be triggered by an audio signal as low as the one going through a cellphone handsfree. I could even yell if necessary!

• Type: OMR-112H
• Application: UL E822922
• Rated coil voltage: 5 VDC (at 20XXX°C)
• Operating voltage: 3.5 VDC (at 20XXX°C)
• Release voltage: 0.5 VDC (at 20XXX°C)
• Maximum applicable voltage: 8 VDC (at 20XXX°C)

 

[Externet]

I do not think that one will work with the audio level of a cell phone, but try.

Look for a reed relay, 3V coil , with the largest coil resistance you can find (>200Ω?) and there is a chance. If there is no coil resistance data, ignore as a choice.

Play with your home stereo speaker audio, check at which volumes (power) a relay will behave the way you want.
Miguel

 

[flx]

But what's the voltage of such audio signal anyway? Think of normal volume (not loud, but not soft either) listening via any garden variety earbuds. I have no means of measuring this output.

The relay I posted the link to is actually a reed relay. It's the only thing available at RadioShack... no 3V in their product line. I don't have any idea where else to go in Chicago.

On the other hand, let's not forget about the power source available: almost 4 Volts and the vibrator spins *anywhere* above 1V @ <100mA. And obviously it doesn't need perfect DC, so a relay may not even be needed. A simple circuit that would allow the passage of *some* juice from the battery to the motor would work fine. There's A LOT of leeway available from 4V to 1V, so any losses are NOT a problem. Plus, the current being fed to the motor doesn't even have to be perfectly continuous; as long as it's not AC, the vibrator spins happily.

Please help...

 

[Externet]

The means of measuring that audio level is with a multimeter; most work fine past 20KHz (AC) and will give you an average reading in audio Volts. Can be half a volt, 1 volt, 4 Volts full blast... just measure, won't bite you because it's audio instead of 60Hz AC.

Try your RadioShack relay; at your home stereo first, measuring the voltage at which behaves the way you want. If your cell phone audio can sustain that same level with your relay connected, well, done.

Another way is to use a transistor (Darlington?) ; saturating it when audio peaks feed the base and will pass DC at that audio 'pace' . As the load is a motor, should work fine, the motor won't start/stop in a millisecond, keeps spinning by momentum until real low signal (silence) cannot sustain transistor conduction. Try this, perhaps works without refinement:

Cao--------1KΩ-------B

+4V---------M--------C

Gnd-------------------E

Cao=Cellphone audio out
B=base
C=collector
E=emitter
M=your tiny motor
+4V and Gnd = your 4V battery

Go play with it!
Miguel