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Old 8th February 2011, 04:59 PM   #11
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Basically you need to build a simple Op-Amp amplifier with VERY HIGH gain. This will input the Audio Signal and amplify the 100mV or so up to 9V PLUS. Then all you need to do is rectify the output and feed it into an INTEGRATOR (Basically an RC Circuit) and use the DC output to power ON a relay.

You need nothing special as the quality of the audio into the relay IS NOT an issue. A simple 741 will do the job.
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Old 8th February 2011, 08:15 PM   #12
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Note that the OP has stated a limited knowledge of electronics. We're not going to help him much with verbal circuit descriptions. If there is a dual voltage supply available for the opamp, Andy's concept would work fine. If you only have a single rail, the ESP circuit is about as simple as you can find ready to go. D1 is the rectifier, C4 and R11 form the integrator Andy mentioned.

Realistically, you don't need a relay, just replace the relay with a 10K resistor and use a ZVN3310 for Q1.Connect the Hypex PSU standby pin to the junction of the resistor and Q1. When C4 is charged Q1 turns on and pulls the output voltage to almost 0.

I only mentioned the NE5532 because they are inexpensive, in my parts bin and I had a data sheet handy. Yes, any old opamp will do.

Even with a gain of 100 as shown in Rod's circuit, I find that the system takes a while to trip on at low signal levels. The capacitor takes a finite amount of energy to charge, and even with the comparator going to 12V there is a turn on delay.

Don't forget that you'll need a 12V supply - a 9VAC transformer (wall warts work, too), a bridge made with 1n4007s and a 100 uf 16 v cap will suffice. Be sure to provide a fuse in that transformer's primary. If you still want a relay it can be something cheap, you don't need a huge mains rated relay. A 12v DIP relay would work fine. (DS2Y type or cheaper)

Last edited by BobEllis; 8th February 2011 at 08:27 PM.
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Old 8th February 2011, 08:58 PM   #13
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Here's Rod's circuit modified as I suggested for relay free operation. The optional LED can be any color. It will be on when the circuit pulls the signal to ground. The value of R12 determines the brightness of the LED It can go down to 1K with 1/4W resistors to get 10 mA in a red LED. If you don't want an LED, just connect the resistor to Q1. Hypex doesn't specify the current to be supplied to the standby pin, but I assume it is less than 1 mA.

The LED current will be approximately (12-Vled)/R12. Power in R12 is the current multiplied by 12-Vled. If you get a dissipation over .125W, use a 1/2W resistor for R12. Vled is roughly 1.2V for red, 2V for green and 5V for blue.

I also added a psu - all diodes are 1n4007 or you can use a ready made 1A bridge.
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Old 8th February 2011, 10:22 PM   #14
jmbulg is offline jmbulg  Belgium
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Thanks to all of you, I'm getting slowly the picture, eg the C4-R11 part is simply the delay via a discharging RC circuit; in my naive implementation I had assumed off-the-shelf delayed relays exist, incorporating part of the circuit.

I could probably be able to implement the circuit (not very 'nicely', but I have enough place and it is not in the audio path), my biggest problem will probably chasing the components (not knowing which one can be replaced by similar ones). As a novice I obviously do not have a lot of them in stock ...

In all cases, I will make provision of a switch which allows to disable the whole autosensing and provide zero volt the the hypex ...

I wonder why hypex does not provide such an autosensing option on their SMPS ? (hopefully they are reading us ;-) )
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Old 8th February 2011, 10:57 PM   #15
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If you use the LED you can see that the circuit operates as intended before hooking up to your PSU (suggestion use a red LED and 1-2K for R12). All resistors can be 1/4 watt, cheapest industrial grade. Any dual opamp will do. NE5532, TL072, 1448, whatever is cheapest. The diodes can be 1n400x including D1. I buy 1n4007 in large quantities and use them all over the place, but any of the series will do. The electrolytic caps can be anything rated for at least 16V. If you want your amp to stay on longer after the circuit detects silence, add more 100 uf caps in parallel with C4. Each one will approximately double the time it takes to turn off. The smallest 9VAC transformer is god enough. Bigger won't hurt, except in your wallet. None of the parts are critical, and Rod chose standard values so they should be easy to find. Try Farnell.

This is a down and dirty utility circuit. A good chance to hone your skills building a circuit that is tolerant of less than perfect implementation. We're here if you run into trouble.
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Old 9th February 2011, 07:26 AM   #16
Mooly is offline Mooly  United Kingdom
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I used a very similar idea to Rod Elliots many years ago in a headphone amp,
Advice for hearing impaired setup on a new television?

The Opamp I used was a TLO62 for both low power and to allow R1/2/3 etc to be in the 10meg ohm region. That eliminates any question of whether the loading of the circuit is detrimental to the audio. The low power meant it could be powered from a nicad, with the main supply taking over when audio detected. As for detecting audio, a frequency response of around 100 to 1khz is fine so roll off etc isn't a problem.
I added a CMOS 555 timer between the opamp output and the FET (VN10KM) driving not a relay but an opto triac giving a defined delay of 5 mins. The FET was still needed as the 555 could drop out in normal silences, the FET providing a "carryover".

As Bob say, this kind of circuit has endless possibilities. It's nice if possible to design for ultra low power and to try and incorporate any scheme into using the existing PSU's available.
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