inserting the IC. For the test points, we
used two PC stakes, one for TP GND and
the other for TP3. We left the remaining
test points as bare pads so a multimeter
probe can be inserted.
Install the two 2-way pin headers
for JP1 and JP2 and then follow with
the capacitors.
The electrolytic types must be fitted
with the polarity shown (long lead to
pad marked plus; the stripe indicates
the negative side) and note that the
10 F NP capacitor is non polarised and
so can be installed either way around.
Next, mount transistors Q1 and Q2
and also REG1. Take care not to mix
them up because they come in identi-
cal packages.
Trimpots VR1 and VR2 are next to
be fitted. They may be marked as ‘103’
instead of 10kΩ. Orient VR1 with the
adjusting screw as shown.
CON1 to CON3 can now be installed.
CON1 and CON2 are 2-way types and
CON3 comprises two 3-way screw
connectors dovetailed together. Fit all
connectors with the wire entry to the
edge of the PCB.
Finally, the LEDs and relay RLY1 can
be installed. We placed the LEDs close
to the PCB, but they can be mounted
higher or mounted off the PCB if you
wish, connected with flying leads.
Although presented as a bare PCB,
the unit fits in a UB3 Jiffy box. In this
case, attach the PCB to the base of the
box using spacers. First, mark out and
drill 3mm holes for each of the corner
mounting holes.
You will also need to drill holes at
each end of the box for cable glands. A
gland at one end is used for the power
and signal wires while a gland at the
other end allows the relay contacts to
be wired up as required.
Set up
You have several options for setting the
unit up. You can set it up before instal-
lation using an oscillator or the actual
signal source (if it can be held steady
enough) when you install it.
- Oscillator method
Power the unit up with a 12V power
supply wired to CON1. Connect the
oscillator to CON2. Set the signal ampli-
tude to 2V peak-to-peak or 0.7V RMS.
Set the oscillator to your desired
upper threshold frequency (eg, 500Hz)
and press S2. Then reduce the oscillator
to set the lower threshold (eg, 400Hz)
then press S1. That’s all there is to it.
If you want to set a single threshold
frequency (ie, the upper threshold) and
use the hysteresis setting, fit a link to
JP2. Then adjust trimpot VR1 for the
required hysteresis (percentage) while
you monitor the voltage at TP1. Then
set the oscillator for the desired fre-
quency and press S2.
Alternatively, if you want to set
a single threshold frequency at the
lower threshold and use the hysteresis
setting for the upper threshold, fit a
link to JP2. Then adjust trimpot VR1
for the required hysteresis while you
monitor the voltage at TP1. Then set
the oscillator for the lower threshold
frequency and press S1.
Table 1 shows example relation-
ships between the voltage at TP1 and
the percentage hysteresis. For exam-
ple, if you set VR1 to give 1.25V at
TP1, the hysteresis will be 20% and the
resulting lower threshold frequency
will be 20% lower than the frequency
you set with switch S2.
Note that you can also set the unit
with only one threshold frequency and
that will mean the relay will latch on
when the signal goes above the thresh-
old and will stay on until the power
is turned off.
To set just a single threshold fre-
quency, set the oscillator to the desired
frequency and then press S2. Then
disconnect the signal from CON2
and wait until the signal LEDs all are
off. Then press S1 to set the lower
frequency to zero.
No link is required at JP1 if you want
the relay to switch on as the frequency
rises above the threshold set by S2 (andturns off when the frequency drops
below that set by S1).
Alternatively, install JP1 if you want
the relay to switch on as the frequency
falls below the threshold set by S1 (and
turn off when the frequency rises above
the threshold set by S2).2) Frequency meter method
The advantage of this approach is that
you don’t need an oscillator but you
will need a frequency meter or oscillo-
scope to measure the frequency at TP3.
To get into this mode, connect your
frequency meter or DMM between TP3
and GND.
Switch off power, hold down both S1
and S2 and then switch on the power.
The micro then produces a 100Hz
signal at TP3.
To adjust this default frequency to
obtain your desired upper threshold,
press S1 and S2 until it reaches your
target. S2 increases frequency, while
S1 decreases frequency.
Short presses of the switches will
alter the frequency at a slow rate. For
faster changes, hold the switch down
and the rate will change to a faster rate
after two seconds. Continue to depress
the switch for another two seconds and
the frequency will change at an even
faster rate.
This allows you to run through the
entire frequency range in less thanFig.2: component layout for the Deluxe Frequency Switch with a matching photo
below. We suggest using an IC socket for IC1 – and make sure when you place the
connectors, their wiring access holes all point to the outside of the PCB.