Electronic film clapperboard

[soundsuper]

I often work on live sporting events for tv and am looking to build a sound/video sync checker for use in the field, sometimes under very bright skies like in India. The design would be say up to 8 bright white LED's arranged in a circle on each face of a square box. The complete rotation cycle perhaps varies between 2-5 seconds with each LED turning on and off in turn. The 4 sets of LEDs driven from the same oscillator. When the '12 o'clock' LED is illuminated, an audio pulse is triggered for the duration of the LED illumination and heard through an inbuilt loudspeaker with perhaps a line level direct output via an appropriate connector. The audio signal/sample should have a steep initial transient akin to two bits of wood being hit!
The reason for having the LEDs on 4 sides is that it is useful to check the individual camera/recording/playback paths all in one go.
Any help with the circuit design would be much appreciated. Many thanks.
Soundsuper, UK

 

[RadioRon]

This sounds fairly straightforward. What degree of precision is acceptable between the different events? I'm thinking that a microcontroller is a good solution to this application, but this implies that there will be time delays on the order of perhaps one to ten microseconds. If this time error is trivial, then a microprocessor is perhaps the easiest solution. I like the Atmel AVR series since the parts are inexpensive, you can make your own programmer quite easily, and software for development is all available for free. So, give us your best guess on how accurately the sound pulse should line up with the LED illumination. What frequency do you want the audio sample to be? Would you like a pure tone, or perhaps something more complex?

To get things started, consider using an AVR development board made by Olimex, or Sparkfun or one of the many others supplying such things. They are widely available with many options, and usually less than 30pounds in cost. This gets you started very quickly. While you are at it, these suppliers often sell simple programmer cables that allow download of code from your PC to the board for about 10pound more, so buy that too. Let us know your level of programming skill. If you know the C programming language, then you are well along already. If not, but you know how to program in BASIC there is a good solution there too. Another choice is to go with an Arduino or such platform instead, which is easier to program.

interfacing a micro to turn on and off LEDs is quite simple. for lower power (ie 20mA) LEDs, you can wire each led directly through a resistor to the microcontroller. For higher levels of current, a buffer transistor will be required to give some current gain, one transistor for each bank of LEDs. I prefer using an N channel FET between the LED cathode and ground as these are cheap, very good and reliable. You still need the current limiting resistor in series with the LED too. If you want to run fairly high power LEDs (like 1 watt), then we should use a constant current source or specialty IC designed to drive an LED. So please decide how bright the LED has to be first. I can say that using a black background allows a white LED of only medium power to be visible for a long way.

The microcontroller can easily generate a simple audio tone directly from one of its i/o pins, but an amplifier would be required to drive a speaker. You should also use an op-amp to buffer this audio out to the line output, so we are talking about two amplifiers, one for driving an 8 ohm speaker and an op amp for driving the line.

 

[soundsuper]

Dear Radio Ron
Many thanks for your thoughts and suggestions. Microseconds processing delay is not a problem. In TV we work in the order of (PAL) video frame delay in increments of 40ms. The sound pulse should be say up to 20ms within the 12 o'clock LED trigger. I have no experience of programming or using microprocessors but am happy to give it a go! The audio signal/sample should have a steep initial transient akin to two bits of wood being hit!

 

[RadioRon]

For a beginner, I recommend that you google Arduino tutorial and read those to get a feel for what is involved. You will have to prepare yourself to learn. This implies effort and perhaps a bit of mental pain but it will be worth it. Once you know how to program microcontrollers, the world of projects will expand for you. Another good beginners language is Basic and you can use Basic with Atmel AVR type microcontrollers by using BASCOM by MCS electronics. I use it myself and find it is very good. It is a free download from MCS and it works with many popular programmers. Maybe you should start by learning a bit of Basic language, it is a good beginners language.

Do some reading first before you decide on what hardware to buy. You don't need a fancy or complex development board for your project, so you should not expect to spend more than US$40 for a development board, perhaps $30 for a programmer and $0 on programming software. You can do it much cheaper than this by making things yourself, but a beginner might get frustrated with minor problems, so a development board already built is a better start.

 

[ronv]

Sync

I think I know what you are after, but how about a picture to make sure.

I'm more of a hardware guy so I think I would do it with a counter. One count for each LED. How about a POP! for the sound. Similar to when you plug in a microphone to an amplifier. One big transient.

 

[soundsuper]

Thanks - I shall have a look.

 

[soundsuper]

Hi Ronv.
Thanks for taking the time with the drawing. That's basically what I had in mind although I think I might mount the speaker inside one of the rings of LED's. Not too sure about a 'pop' sound - perhaps a 'click' though.

 

[ronv]

OK. I think we can make the sound like that of plugging a mic into an open amplifier using the rising edge of the LED. I'll try to get a circuit today.

 

[ronv]

Sync

Here is one I think will work.

The 555 is just a clock (~.5 seconds)
The 4017 is a decimal counter that gets reset at 8.
The 2804 is a darlington driver to power up the 4017 to drive the LEDs.
The FET is to drive the speaker with the rising edge of count 0.
I used bright red LEDs so they could be run in series from 12 volts. Is red OK?
For clarity (and because I'm lazy) I just drew 2 of the 8 sets of LEDs.
Let me know what you think.

 

[soundsuper]

Hi Ronv
You are a star and many thanks for taking the time to look at this. What a great forum of expertise. I look forward to building this over Christmas. Many thanks to RadioRon too and will certainly learn more about programming. BTW, is everyone on this forum called Ron -

 

[ronv]

Keep in touch and let us know how it comes out.