[time-nuts] help

Nick Sayer nsayer at kfu.com
Mon May 2 15:24:43 UTC 2016


To flesh this out a bit more, on a Raspberry Pi, it would be easy to make a cron job that would pulse a GPIO pin high. They really *want* you to use Python (thus the name), but this is easy to do in just a shell script. First, do this to set things up:

#! /bin/sh

GPIO_PIN=9 # pick whatever one you like

echo $GPIO_PIN > /sys/class/gpio/export
echo out > /sys/class/gpio/gpio${GPIO_PIN}/direction
echo 0 > /sys/class/gpio/gpio${GPIO_PIN}/value

Next, run this script out of cron:

#! /bin/sh

GPIO_PIN=9
echo 1 > /sys/class/gpio/gpio${GPIO_PIN}/value
sleep 1
echo 0 > /sys/class/gpio/gpio${GPIO_PIN}/value

That will make a positive going pulse with the leading edge synchronized to cron (for sufficiently vague definitions of “synchronized”).

As for the hardware side, take the GPIO pin and connect a 10k resistor between it and the base of a 2N4401 transistor. Connect the emitter to ground and the collector is a classic “open collector” switching output. Think of it like a switch connection to ground. When it’s on, there is a low impedance path to ground. When it’s off, it’s high impedance. You can use it to work a relay (be sure to add a flyback diode across the relay coil) or directly to switch any load that doesn’t exceed the abilities of the transistor.

If you want to be a little safer, you can use an opto-isolator instead. Connect the GPIO pin to a 150 Ω resistor and then to the anode of the LED in an optoisolator. Connect the cathode to ground. The optoisolator itself can be either a phototransistor type or a driver triac type (the latter would be used to drive a power triac to switch AC loads on and off).


> On May 1, 2016, at 6:30 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
> 
>> But are you sure you want SMPTE... Do you have a source already?
>> 
>> 
> You don't need GPS or SMPTE if you have an Internet connection.  The
> computer can use a set of NTP servers from the "pool" to get time.  The
> result is good enough that the seed of sound delay resulting from your
> random distance to the bell will be the largest source of error.
> 
> If you convert timing errors to distance at the speed of sound.   You would
> need the GPS only if you car about bell to ear distances of about one foot,
> give or take
> 
> So for this use case the OP does not need a GPS or even a SMTPE connection
> just a WiFi link to the internet would be more than enough for controlling
> a horn blast from a light house
> 
> 
> -- 
> 
> Chris Albertson
> Redondo Beach, California
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