This is a simple, cheap, and effective 1750KHz toneburst circuit, essential
for "opening" older repeaters in the UK.
This circuit has the added advantage that it can be left powered as it has
an inbuilt 500 mS tone-burst timer. [i.e. when power is applied, the tone-burst
only sounds for 500mS, after than a mere few µAmps are needed on standby]. If you are using a common ground (0V) you may be able
to use just the output to the "hot" side of the TX input, check for
polarity, otherwise it can be wired permanently across the input in parallel
with your audio source.
The IC is a Motorola MCI 4001BCP or
The only important adjustment is RV1, this controls the output frequency.
There are several ways to adjust this, but assuming you don't have a frequency
counter, you can use trial and error by testing it on your target repeater.
Start with the trimmer in the centre position, most repeaters are very
tolerant! [unlike the repeater groups!]
Another tried and tested way to set up
the tone is by ear. Use a hand-held or a known good radio, and monitor it on a
scanner, with your tone-burst connected to your transmitter apply the power and
adjust for the same tone. RV2 adjusts the output level, in general, we have
found the lower the level - the better it works.
If you want to get a constant output from the LPWS tone-burst circuit for
testing etc. simply apply 12 volts via a 100K resistor to pin 6. This will
disable the internal timer.
This can be built on a board, or to save space, we have one actually built
entirely on the back of the MCI
4001BCP chip. Another space saving way, is to use an IC
socket, solder the components to the pins before plugging the chip in. After
testing, both RV1 & RV2 can be substituted for fixed resistors to save
Tried, tested & it works!
(well ours do anyway)