Transistor relay switch from line level AC signal - I Need Help!!

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barrelwaves

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Hello, the circuit shown is designed to recieve line level AC sinewave of 1kHz, this signal is then amplified by a gain of roughly 4 to around 2.5v, the output from the TLO71 opamp is sent through a diode removing the AC content and with a low pass filter creating a DC voltage for the transistor to open at 0.7v, this switch is to operate a 5v relay attached to the collector of the transistor. The simulation shows, with a pulsed input to represent the input level going up and down, the relay operating and switching on and off with the input, however, upon physically building this circuit it does nothing of the sort! I have even just isolated the opamp section and have found that it is producing a distorted square wave, just like if there was no feedback resistance and also as if the input is clipping? Starting to drive me a bit mad now so would anyone please be able to help? Is there a problem with the design i am missing? or if anyone has a good experience with the TLO71 opamp then please come forward!

Regards Paul Jones University of Derby, England
 

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There's a lot about the circuit that is wrong, for a start the opamp is connected to a single supply, but isn't wired as such - this will prevent the first part working. The component values after the rectifier look completely wrong, I would move the capacitor to the cathod e of the diode, and make the resistors a LOT higher.

Interesting you're at Derby Uni, it's not far from me - and my daughter did a couple of trial days there (through the National Association of Gifted and Talented Youth - NAGTY) - didn't rate it though, poorest of the four she's trialled at (sorry about that!).
 
Ok well thanks, the TLO71 can operate single rail and is hard wired just like the data sheet.
But thank you about the capacitor location will try that, and increase the load resistors.
 
I think you are rather too trusting about datasheets!, it probably assumes you know to fit the parts they don't show!.
 
hi,
Connection for a single rail supply will depend upon the Vac signal source.

Is the 50Hz sine wave going to swing about ground say, +/-700mV?

Do you want the relay to switch ON/OFF at a 25Hz rate?.

In the actual circuit does the relay fail to operate or latch ON ?

Have you calculated the OPA, Zout, when using high value feedback resistor values?

Whats the purpose of the 220R on the OPA output pin?
 
Hi,
The input sorry is 3kHz, and what i was aiming for was the AC signal to be turned into a DC signal affectively, which will latch the relay when the RMS voltage of the AC input is around 0.5V, so i can turn a relay on and off by altering the amplitude of the input sine wave.
Yer ignore that 220R i was trying something because there was voltage left at the output when i cut the input? bit strange!
The opamp output should be 2.5v but this has varied as i have tryed different methods. I found that the diode needs about 2v to operate efficiently.
 
barrelwaves said:
Would you like to share how the opamp should be connected for songle rail supply?
Click to expand...

You need to bias the opamp, and ac couple the input, the output, and the feedback. Here's a really nice graphic I stole from various posts by Audioguru!
 

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Right this is the updated version which i must say seems to be simulating a lot better, was this the right idea?
 

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hi paul,

Not quite right, yet.

You are trying to apply a +2.5v offset into the OPA non-inv input.
Whats the impedance of your Vac input source.?
If its low and dc coupled whats the Vdc offset, at the non-inv input going to be?

Also you say you want a gain of about *4, whats the gain of non-inv OPA?

The 10uF is on the wrong side of the diode.

As its a 3kHz sinewave source, why don't you use ALL ac coupling, as shown in Nigels, jpeg.[originally posted by audioguru]

My choice would be cct in the [90-180 deg] quadrant of the jpeg.
 
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Nigel Goodwin said:
Not mine! - I stole it from Audioguru!

But it's a very useful little diagram!.
Click to expand...

I will edit earlier post, with due credits to agu.

You had already said it was agu's drawing, so I knew you were not claiming the credit.
[hope the snows gone!]
 

Thanks for the feedback, will move the capacitor, it is simple acting as a high pass filter though?
Not sure what you mean about the gain of the non-in opa, as the gain is determained by the feedback loop is it not?
 
barrelwaves said:
Thanks for the feedback, will move the capacitor, it is simple acting as a high pass filter though?
Click to expand...

No, as a low-pass one - essentially converting the rectified output to smooth DC. Where it was it's just shorting the output of the opamp.

Not sure what you mean about the gain of the non-in opa, as the gain is determained by the feedback loop is it not?
Click to expand...

I'm not sure what he was referring to either?.
 
hi,

Quote:
I'm not sure what he was referring to either?.

The OP said he wanted a gain of *4.

Gain of a non inverting opa is: G= (Rf/Ri) +1

So, (400K/100K) + 1 = 5

QED.
 
Use a low voltage opamp like half of an old LM358 dual opamp. It works with a supply voltage down to 3V. Its output goes low to near 0V.

The TL071 has a recommended minimum supply voltage of 7V. Its output low voltage is about +1.2V.

Opamps oscillate if you short the output to ground with a capacitor.
 
New Idea!

Having a real problem trying to get the opamp to do what i want, so redesigned a full wave bridge rectifier circuit. have tested and does work switching relay on and off. Only problem is requires 1.3V AC to do this and line level may not go that high, depending on the soundcard but is more like 0.5-0.7v supply i need to operate. Could someone have a look please and tell me how to make it more sensitive?
 

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No reason to have a bridge rectifer (and it may cause grounding problems) a simple single rectifier will double your sensitivity. However, a simple opamp is dead easy to use, I posted a graphic with the four different configurations previously - but you choose to ignore all of them in favour of a non-working fifth method?.
 
As i said, the new design is tried and tested and working much much better. and really is the simple way of doing it, opamps tend to act a little different in real situations and funny enough not very well for mine. The final design of the opamp version was moderated and accepted as the "way to do it", with your schematics and all the others incorporated correctly. But in this case i have found the simple full wave rectifier to work much better. Because it works you see.
 
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