# Looking for a simple circuit design...really

#### steveu812

Jul 17, 2013
4
I'm not an EE, but have soldered simple circuits to repair LCD monitors and am well versed in auto repair and modifications.

I want to make a device that takes earphone jack output and flashes a light instead. In other words plugs into the earphone jack of my radar detector and flashes an LED in place of sound that would come from the speaker. Doesn't have to be synchronized or anything fancy. I realize that I'm losing quite a bit of info. in the translation, but I find that in most instances it only goes off at a necessary points anyway.

I would have 12vdc (actually between 12 and 15 volts to be safe) available to power the LED if necessary.

Thanks

#### Wabajig

Apr 14, 2012
75
I'm not sure if the voltage is high enough out of the jack to run an LED. Can you measure the output with a scope, measure peak to peak?

#### steveu812

Jul 17, 2013
4
I can easily get between 12 and 15 VDC to power the LED from a separate wire from and accessory circuit on the car. Maybe another way of describing what I'm looking for is a normally open relay that I can trip with the power from the audio jack closing the LED circuit.

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
The voltage and current available from a headphone jack are quite low. You will not be able to power an LED directly from it, nor activate a relay with it. You need an amplification stage to increase the audio level, and a signal detector to convert the AC into a clean on/off signal to drive the LED.

Here's a suitable design. This circuitry can be powered from your accessory circuit.

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#### steveu812

Jul 17, 2013
4
Awesome! Now as much of a noob as I am with this, I would expect to see 4 external connections a 12V +/- and the audio +/- This I see three external inputs and the +12V is clear. Is 0V a shared ground?

Thanks.

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
Yes. The "0V" connection connects to the negative of the battery supply (which is the vehicle chassis, I assume), and the screen of the audio cable that goes to the headphone socket. The centre wire of the audio cable, which goes to the "tip" of the headphone plug, is the "IN" signal.

#### BobK

Jan 5, 2010
7,682
Awesome! Now as much of a noob as I am with this, I would expect to see 4 external connections a 12V +/- and the audio +/- This I see three external inputs and the +12V is clear. Is 0V a shared ground?

Thanks.
The ground connection (sleeve) from the audio out jack should be connected to ground in the circuit.

Bob

#### steveu812

Jul 17, 2013
4
OK, great - thanks for the help.

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