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> Triggering An Output With A Very Small Voltage
rayder
Posted: April 23, 2012 04:04 am
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can anyone give me some help on figuring how to trigger a transistor with an input thats not likely to exceed 30mv (3.5mm line out)? done a bit of searching but unable to find anything of use sad.gif
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MacFromOK
Posted: April 23, 2012 04:16 am
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OP amp?


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rayder
Posted: April 23, 2012 06:17 am
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well ive tried with a TL074. got 5v hooked to vcc+, ground to vcc-, transistor base on the output. and i switched the input from - to + and the opposite was grounded but its not working
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CWB
Posted: April 23, 2012 12:22 pm
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it sounds like the input of the TL074 is not biased correctly ... this will throw a heavy bias on the driven transistor .
do you have a schematic of what you are doing ?

a "bootstrap" circuit is needed .
look at the left hand side of the circuit on the bottom of page 8 here :
http://www.datasheetcatalog.org/datasheet/...ronics/2297.pdf

do you have any other opamps available ?


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rayder
Posted: April 23, 2012 12:41 pm
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Ok i'm still relatively amateur when it comes to electronics. don't have a schematic, i just hooked everything up as indicated in the pin description diagram. no caps etc but i did use resistors between the output and the transistor base. i'm not too sure how the biasing works?
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CWB
Posted: April 23, 2012 12:48 pm
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when using a single sided supply (B+ and ground/-) you have to bias the input of the opamp to run the output at 1/2 of the supply voltage ...
elsewise , the output will be "slammed" either fully to "B+" or to "ground" .

are you amplifying DC or an AC/audio signal ?
a bit more of a description of what you want to do would be helpful .


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rayder
Posted: April 23, 2012 12:57 pm
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i'm putting together a camera remote interface for my dslr and i've already started the software side for my iphone as that is the simple part lol. basically, the dslr reads the voltage on pin 4 in a mini usb plug. an off the shelf manual remote uses a two stage push button and three resistors. one voltage tells the camera the remote is hooked up, one tells it to auto focus and the other tells it to take a picture. i want to output an audio signal from the left hand audio channel to trigger the auto focus via a transistor and the right hand channel to trigger the shutter also via a transistor.

oh and no other op-amps laying around. not even sure why i had that one laying around lol.
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CWB
Posted: April 23, 2012 01:41 pm
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hmmm ...

sounds like (i'll take the pun) you need an ac coupled audio amplifier and comparator circuit .
when feeding into a camera , you gotta be careful about what you do to avoid letting the magic smoke out of the parts inside the camera ...
and avoiding a costly repair .

here is a site that might be what you need to get you started :
http://www.hdrlabs.com/occ/hardware.html

a google search of "remote trigger dslr circuit" will provide much information ...
you just have to sift through it a bit .
you can bet that someone else has (built) a proven remote trigger circuit .
a little digging can help you avoid re-inventing the wheel .

mimicking the "half push focus recycle" would be something i would want to incorporate .
i do not like the constant "auto focus" function .
juuust about the time i have a shot set up , a little puff of wind rustles the leaves and then (whirrrr-chic-chic-chic-glick) the auto focus does it's thing .
---PMO--- dry.gif


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CWB
Posted: April 23, 2012 01:44 pm
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ps ...

i have to head out ... busy day ahead .
do some digging and post some links .
i'll be back later on and/or someone else can proffer help .


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rayder
Posted: April 23, 2012 02:56 pm
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So i did a little research on ac coupled op-amps and came up with the following circuit. Not sure how to pick the correct capacitance though. According to CircuitLab.com simulator, if the input is 15mv then the output should jump to 0.87v which should trigger the transistor. At the full 30mv input, output only jumps to 1.75v which is still fine for the transistor. Hopefully i'm at least on the right track with this one lol. Any criticism would be greatly appreciated guys. Cheers

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VenomBallistics
Posted: April 23, 2012 06:09 pm
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swap R2 and R3 around.
you might want to ditch C1 if you intend to hold it is a state for any duration.
change the values of R4 and R1 to something equal like 2.2k or 3.3k and place 22u caps across them, remove this from the + input and use the center point as a signal ground.
add about 470K resistor to the + input to the center ground and it should start to breathe
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tekwiz
Posted: April 23, 2012 06:37 pm
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QUOTE (rayder @ April 23, 2012 05:56 am)
So i did a little research on ac coupled op-amps and came up with the following circuit. Not sure how to pick the correct capacitance though. According to CircuitLab.com simulator, if the input is 15mv then the output should jump to 0.87v which should trigger the transistor. At the full 30mv input, output only jumps to 1.75v which is still fine for the transistor. Hopefully i'm at least on the right track with this one lol. Any criticism would be greatly appreciated guys. Cheers

user posted image

You will need a common ground between the USB & the rest of the circuit. It's not shown in the diagram.
If you use a pot instead of fixed resistors for your reference voltage, it will allow you a bit of a sensitivity adjustment, by allowing you to move your trigger point.
Your feedback resistor may be low in value, too. You want to bias for maximum gain from the opamp, as this will insure a 'snap' action as the circuit is triggererd. You don't want it to be linear in response.
Note also that you may need some filtering on the input, because chances are your iphone has a Class D output amplifier, & the output of these is high frequency...more like a modulated RF signal than a pure audio signal. This must be taken into account.


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rayder
Posted: April 23, 2012 10:57 pm
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QUOTE (VenomBallistics @ April 24, 2012 03:09 am)
swap R2 and R3 around.
you might want to ditch C1 if you intend to hold it is a state for any duration.
change the values of R4 and R1 to something equal like 2.2k or 3.3k and place 22u caps across them, remove this from the + input and use the center point as a signal ground.
add about 470K resistor to the + input to the center ground and it should start to breathe

swapping R2 and R3 and changing R1 and R4's values gives an undesired result. output jumps to almost 70v. but i'm not sure how to "place 22u caps across them"? and your last line boggles me. i don't understand what you mean.

@tekwiz
the ground between the usb and the circuit will be connected, just didn't show much of the camera side of things in the schematic thats all. i'm not connecting the audio jack ground as well am i? as for the gain, i worked out that the gain should be 40-50x so that even at half power, it will still trigger. the trigger sound is going to be the same every time, its not being triggered by music.

oh and i have no idea how to go about the filtering on the input line cause thats beyond my knoweledge and as such i also don't know why i need to. any explanation would be awesome. cheers
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VenomBallistics
Posted: April 24, 2012 03:52 am
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QUOTE (rayder @ April 23, 2012 10:57 pm)
QUOTE (VenomBallistics @ April 24, 2012 03:09 am)
swap R2 and R3 around.
you might want to ditch C1 if you intend to hold it is a state for any duration.
change the values of R4 and R1 to something equal like 2.2k or 3.3k and place 22u caps across them, remove this from the + input and use the center point as a signal ground.
add about 470K resistor to the + input to the center ground and it should start to breathe

swapping R2 and R3 and changing R1 and R4's values gives an undesired result. output jumps to almost 70v. but i'm not sure how to "place 22u caps across them"? and your last line boggles me. i don't understand what you mean.

@tekwiz
the ground between the usb and the circuit will be connected, just didn't show much of the camera side of things in the schematic thats all. i'm not connecting the audio jack ground as well am i? as for the gain, i worked out that the gain should be 40-50x so that even at half power, it will still trigger. the trigger sound is going to be the same every time, its not being triggered by music.

oh and i have no idea how to go about the filtering on the input line cause thats beyond my knoweledge and as such i also don't know why i need to. any explanation would be awesome. cheers

it cannot jump to 70V unless your feeding it 70V or greater which would smoke the op amp .
you did your NFB loop backwards trying to reduce gain which just does not work in the non inverting scheme shown.

the last part you didnt understand is in respect to how these things work. theres no magic here, they can only produce an AC output because they have a + and a - rail to switch between in respect to ground. Its a three point setup. the caps placed in parallel to the symmetrical voltage divider is what we need to do to BS op amps into working in a single ended power environment. you do not need to feed power into your + input as you have, you need only give it a route to ground
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rayder
Posted: April 28, 2012 09:16 am
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can i use an lm386 to amplify the signal and then use a diode to rectify the signal to make it dc?
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