There has been some interest, quite a lot of views in my network slate, since it got featured on Hackaday last week.

The networking is very primitive, it just sends a string of text via a wifi module to a specific computer  IP address, and nothing else. I don’t really have the network programming skills to do more than that.  So it’s time go back to the basics and make a simple modern timecode slate.

What’s in a slate?

It has to be a EBU/SMPTE timecode reader, so it can read somebody else’s timecode

It has to lock onto incoming timecode, and internally start a locally generated clock that matches the external timecode.

It must not drift (too much).  If the internal clock of the slate is accurate to 1 part in 1 million, 24 hours later it should be accurate to about 80 milliseconds,(Wolfram Alpha) or about 2 picture frames,  so on an 8 hour day, that’s about 1/2 a frame.

It has to display the timecode, when asked. One frustrating  thing about timecode, is that it is technically valid only at the end of each picture frame, so when displaying timecode you must add 1 frame to the last decoded numbers.  It’s quite easy to do, and takes about 15 microseconds of cpu time every frame.

Power. A slate needs electrical power. Modern  cheap microprocessors can run off about 3.3 volts but do not need a lot of power, a high brightness LED needs about 2.3V to glimmer and a couple of amperes when in light sabre mode,  so a pack of 4 AA cells should do and a basic switch mode (i.e. reasonably efficient) power regulator for the LEDs.

Interface to the outside world, buttons, switches, magnets and connectors.

Connectors

Timecode  is normally an unbalanced signal in a non studio environment so a BNC or 1/4″ jack is often used

Switches

A power switch would be handy

You need to set the timecode flavour, 24, 25 30 fps etc. Normally this is done with a binary coded rotary switch.

A Jam switch, tells the micro to lock onto incoming timecode, after looking at the flavour switch.

A brightness potentiometer to set the LED intensity when a Slate is due. This can be replaced with an Ambient Light Sensor if needed.

A clap switch, this can be  either a classic reed switch, a bit fragile, but been around for decades, or a more modern Hall Effect sensor, very small and reliable.

The clap switch controls the whole operation of the slate.

When the clapper is open it tells the cpu to wake up the LED power supply from an energy saving  sleep mode and display the timecode. In my slate it restores power to the LED driver IC’s, which were found to consume too much power when there was nothing to display.

The instant the clap is sensed, by a magnet in the moving clap arm, switching on a hall effect sensor in the lower clap arm, the display numbers are frozen and the LEDs go to full brightness for 4 frames, then they blank and  the whole slate goes to sleep mode, just incrementing an internal frame counter until the clap arm is opened again. Other things can be displayed, mainly of interest to the operator, such as battery voltage.

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