T
here are many applications for
a device of this type. Just some
of the things we thought of ‘off
the top of our heads’, include:
- Cutting power (or fuel) to a motor if
it exceeds a certain speed - Switching a fan on at low vehicle
speeds to provide improved cooling - Giving a warning to change gears
when the engine RPM is approach-
ing the tacho red line - Switching from long to short intake
runners at a particular engine RPM
to optimise power delivery - Switching off a pump if a low meter
records the water low is outside a
speciic range - Switching on an alarm if wind speed
exceeds a certain threshold - Applying a brake or feathering blades
on suitably equipped turbines.
Of course, there are countless other uses
- you’re probably thinking of others that
suit your particular application.
As long as it has, or can be itted with
a sensor, to provide a frequency which
varies with speed, temperature, low or
other parameter, you can use our new
Deluxe Frequency Switch. It can do all
of this and much more.
Setting the two frequencies
You need to set up two fre-
quencies, not one as you might
have initially thought.
Why do you need two fre-
quencies? We need to set two
frequencies because if the
signal from your chosen sensor
varies by even a small amount
at close to the switching
threshold, the relay would be
constantly chattering on and
off – not good at all. So we set
an upper frequency threshold
above which the sensor signal must
rise before the relay switches on. And
then we set a lower frequency threshold
below which the sensor signal must
drop before the relay is switched off.
You can set these two frequencies
close together or far apart.
Setting the frequencies is dead-easy
and there are several methods for do-
ing it. The irst method is to feed in
your wanted set frequency, say 500Hz,
from an oscillator or other source to
the sensor input and then press switch
S2. Then feed in the wanted lower fre-
quency, say 400Hz, and then press S1.
The second method is arguably even
easier. You just set one frequency, say
500Hz, and then use an on-board trim-
pot (VR1) to set the hysteresis. This
will effectively set the lower frequency
(down to a minimum of 250Hz in this
example) and you can tweak it at the
time of installation.
If you don’t have an oscillator you
could use the real signal that you intend
controlling the unit with, so long as
you can hold it steady at the required
frequency/frequencies for long enough
to press the switch(es).
Alternatively, if that’s too dificult,
you actually can get the microcontroller
to generate the wanted frequencies.
This second method is more involvedthan the irst and we will describe the
procedure later in this article.Detection time and delay
You can also conigure the unit with a
switching delay which is adjusted with
trimpot VR2 and can be set between
zero and 500ms (ie, half a second). This
ensures that if the signal frequency
only momentarily crosses one of the
thresholds, it will not cause the relay
to switch.
The input signal frequency must
remain at or beyond the threshold for
the entire delay time before any relay
switching will occur.
Each time the frequency crosses the
threshold, the delay time starts again.
If you prefer switching to happen
immediately then set the response time
to zero (ie, VR2 fully anticlockwise).LED indicators
To help in the set-up and installation
procedures, we have included indicator
LEDs to show when an input signal is
present and its frequency range:- LED2 lights for frequencies between
0.5Hz and 10Hz - LED3 lights between 10Hz and
100Hz - LED4 between 100Hz and 1kHz
- LED5 for frequencies between 1kHz
and 10kHz and all four LEDs
light if the frequency is above
10kHz.
Other LEDs show when
the set threshold frequency is
reached and whether the relay
is on or off.
Relay options
The relay is a double-pole
change-over (double throw)
type (ie, DPDT) which can
switch one or two loads, eachSwitch devices on or off according to the frequency of just about any sensor
signal up to 10kHz. So you can switch something on or off if a sensor signal
frequency goes above or drops below a figure which you can easily set.
Deluxe
Frequency Switch
by John Clarke
Specifications
Supply voltage ..................10-16V
Supply current ....................20mA (relay off); 60mA (relay on)
Signal frequency range ........1Hz to 10kHz
Signal amplitude ................>1.4V peak-to-peak
Threshold setting resolution .. 20Hz at 10kHz; 1Hz at 2.27kHz;
0.2Hz at 1kHz; 0.002Hz at 100Hz.
Hysteresis ........................0-50%
Switching delay .................signal period plus 0-500ms
Signal frequency bands: <10Hz, 10-100Hz, 100Hz-1kHz, 1-10kHz