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Help with audio tone circuit.

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mccoy_tm

New Member
Hi, I'm one of those people that know just enough to be dangerous so I apologize in advance for asking so many questions.

I'm trying to put together a circuit that will create an 1khz audio tone that will be pulsed on and off at the framerate of a running movie camera. The reason I'm doing this is because I want to have a signal that I can record onto a spare audio track so that I'll have a sync reference that I can use to match the recorded dialog with the filmed images.

I think the first part is easy. I was planning on using a 555 set up in monostabe mode to connect to the shutter contact switch in a movie camera. The switch closes momentarily, once for every frame of film that runs through the film gate so this would create a series of 20ms pulses that I could use to "gate" an oscillator on and off. (see attached image)

This is where I'm getting stuck. I found a few circuits for sine wave oscillators, one which I've included in my attached image. I built it and it make a nice tone but I've no idea how to attenuate the signal to bring it down to line level. Right now it's putting out a sine wave that measures 5 to 6 volts p-p where a line level signal is 1 volt p-p. I'm not crazy about the dual polarity battery setup either. I'm going to put all this in one little box and would love to power it with just one 9 volt battery. My questions are:

1. Any ideas how I might do this with one battery (another oscillator, maybe)?
2. How do I get a "line level" compatible signal and safely interface it with and audio recorder?
3. How do I modulate the audio tone with my 20ms pulses?

Thanks for any help,
Marshall
 

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marcbarker

New Member
Usually, it's a MIDI-interfaced SMPTE box for a TimeCode Stripe.

if you drive the base of an emitter-grounded NPN, via a (say 10K) resistor from the 555, this transistor can 'short out' the tone signal in time with the 555 output.

wire a resistor, say 47 K between the tone osc output and the transistor collector.

Gated tone is available at the transistor collector.


To make it single supply, throw away the twin-t osc and replace with a phase-shift osc, based on a single transistor.

To attenuate the signal, the 47 K might do that already
 
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Hero999

Banned
I don't see why you can't convert the twin t oscillator to single supply.

You could use a dual op-amp such as the LM1458 and configure the other half as a Schmitt trigger oscillator.

EDIT:
I didn't realise this is only running from 9V so use a lower so use a low voltage op-amp such as the LM358.
 
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marcbarker

New Member
I don't see why you can't convert the twin t oscillator to single supply.

You could use a dual op-amp such as the LM1458 and configure the other half as a Schmitt trigger oscillator.
Yes, it's a shame to throw it away, properly stabilised they're among the lowest THD osc you can get. The problem I have with it is the number of parts is high, and even higher to convert to SS.

The PSO alternative I suggest are based on a BC549, 3 caps and 4 resistors and runs well on SS.

I didn't understand why a schimtt trigger? Am I missing something?
 
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Hero999

Banned
He wants a pulse generator doesn't he?

That's why I suggested the Schmitt trigger oscillator.
 
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