This is only a preview of the June 2007 issue of Silicon Chip. You can view 35 of the 104 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "20W Class-A Amplifier Module; Pt.2":
Items relevant to "A Knock Detector For The Programmable Ignition":
Items relevant to "Versatile 4-Input Mixer With Tone Controls":
Items relevant to "Fun With The New PICAXE 14-M":
Items relevant to "Frequency-Activated Switch For Cars":
Purchase a printed copy of this issue for $10.00. |
Frequency
Switch
By John Clarke & Julian Edgar
A cheap, adjustable design that lets you switch devices on
and off according to speed.
Main Features
• Adjustable switching level be-
tween 10Hz and 500Hz input
• Dual double-pole changeover 5A
relay contacts
• Switches on rising or falling
frequency
• Adjustable hysteresis
• Easy to build
T
HERE ARE MANY automotive
performance applications where
you want to turn something on or off
on the basis of road or engine speed.
A shift light is a good example – you
want a high intensity LED to illuminate just before the red-line, to warn
you that it’s time to snatch the next
gear. Or maybe you want a variablelength intake manifold to change from
long to short runners at non-standard
revs. Or perhaps you want to operate a
device on the basis of road speed – eg,
switch on an intercooler fan when the
car is moving only slowly or sound an
over-speed warning when you’re going
too fast, for example.
This Frequency Switch can do all
of those things – and more.
It also has adjustable hysteresis
(that’s the difference between the
switch-on and switch-off frequencies)
and comes with both a LED and a relay. The relay is there so that you can
switch big loads, while the LED can
be mounted on the dash so that you
can see the switch operation. Alternatively, the LED could be used purely
as an indicator – eg, as an over-speed
warning or as a shift light.
Construction
The Frequency Switch can be used to trigger a shift light – an indication that
engine revs are getting close to the red-line and it’s time to change up a gear.
82 Silicon Chip
The Frequency Switch is a simple
kit to build but you should make one
decision before you start work. Will
you be using it to detect a frequency
that is rising to the trip point or falling
to the trip point?
The unit can be configured to work
with either type of signal but if you
know which way you’re going, you
won’t have to make changes later on.
The detection of a rising frequency will
be the more common application – for
example, triggering a shift-light when
engine revs reach a high speed.
However, if you want something
switched as frequency decreases to
a certain level – for example turning
on an intercooler fan when the car is
siliconchip.com.au
How It Works
The frequency input signal is applied
to a 10kΩ resistor and then to zener
diode ZD1 which limits the signal to
between +16V and -0.6V. The 10nF
capacitor filters the signal, removing
high-frequency noise. The signal is then
applied to pin 1 of IC1 via another 10kΩ
limiting resistor.
IC1 is a frequency-to-voltage converter. The pin 1 input signal is compared with the voltage at pin 11 which
is set at about +1.8V using 10kΩ and
3.3kΩ voltage divider resistors across
the 7.4V supply. A comparator within
IC1 will provide an output signal if the
signal level at pin 1 swings above the
1.8V threshold for pin 11.
This internal comparator drives a
frequency-to-voltage converter which
charges the capacitor at pin 2 and then
transfers this charge to the capacitor at
pin 4. Trimpot VR1 adjusts the voltage de-
travelling slowly – then you’d configure the Frequency Switch to detect a
falling frequency.
So what are the changes made for
the differing configurations? They’re
simple: for a rising frequency (low to
high) detection, link LK1 is placed in
the (L/H) position (that is, closest to
the top of the PC board when the board
is orientated as shown in Fig.1). For
siliconchip.com.au
veloped at pin 3 with respect to the input
frequency. This voltage is monitored by
another internal comparator which has
its inputs at pins 4 and 10.
Pin 10 monitors the voltage set by
trimpot VR2 (Threshold). The output at
pin 8 will be high (+7.4V) when pin 4 is
below pin 10. If pin 4 goes above pin
10, pin 8 will go low (0V). Hysteresis is
included by virtue of the series 100Ω
resistor, trimpot VR3 and diode D3.
Hysteresis prevents the output from
oscillating when the signal is just at the
threshold point.
The pin 8 output from IC1 drives
transistors Q1 and Q2. Either one
of these transistors can be selected
to drive the relay, depending on the
setting of link LK1. When LK1 is in
the H/L position, Q1 drives the relay
and when LK1 is in the L/H position,
Q2 drives the relay. This enables the
relay to switch when the input changes
from a high-frequency signal to a low-
detection of a falling frequency (high
to low), link LK1 is moved to the H/L
position.
Note that the Frequency Switch in
the photos is configured to switch on
a falling frequency (H/L), while the
parts overlay diagram (Fig.1) shows
the PC board configured to switch on
a rising frequency (L/H).
When assembling the PC board,
frequency signal (LK1 in position H/L),
or when the input changes from a low
frequency to a high frequency (LK1 in
position L/H). LED1 lights whenever
the relay is energised.
Power Supply
Power is obtained from the switched
+12V ignition supply. Diode D1 gives
reverse connection protection, while
the 10Ω resistor, 100µF capacitor
and zener diode ZD1 provide transient
protection for regulator REG1. All the
circuitry is powered from REG1 via D2,
except for the relay and LED1 which are
driven from the +12V supply.
D2 is included to reduce the 8V from
the regulator to about 7.4V which is
necessary for correct operation of IC1
(it prevents an internal power supply
zener diode in IC1 from conducting).
Suggested Uses
• Operate a shift-light at set revs
• Operate changeover inlet
manifolds
• Operate an intercooler fan at
low road speeds
• Over-speed warning
• Control active spoilers
• Auto trans over-drive lock-out
June 2007 83
Where To Buy A Kit
This design was originally published
in SILICON CHIP’s “Performance
Electronics For Cars”. A kit of parts
is available from Jaycar Electronics.
“Performance Electronics For Cars”
is available from Silicon Chip Publications and from Jaycar.
Fig.1: use this diagram and the photos of the completed project when
assembling the PC board. Take particular care with the components that
are polarised – for example, the diodes, IC and electrolytic capacitors.
The Frequency Switch is a multi-purpose building block that can be
used to operate a shift light, alter intake manifold runner length, turn on
intercooler fans at low road speeds – and a host of other uses. Both LED
and relay outputs are provided.
Resistor Colour Codes
Value
4-Band Code (1%)
5-Band Code (1%)
100kΩ
10kΩ
3.3kΩ
1.8kΩ
1kΩ
100Ω
10Ω
brown black yellow brown
brown black orange brown
orange orange red brown
brown grey red brown
brown black red brown
brown black brown brown
brown black black brown
brown black black orange brown
brown black black red brown
orange orange black brown brown
brown grey black brown brown
brown black black brown brown
brown black black black brown
brown black black gold brown
make sure that you insert the polarised
components the correct way around.
The diodes, IC, LED, transistors, voltage regulator and electrolytic capacitors are the easiest to make mistakes
with. During construction look at the
84 Silicon Chip
photos and overlay diagram closely to
avoid making mistakes.
Set-Up & Fitting
The Frequency Switch can be set to
two broad frequency ranges: 10-100Hz
or 50-500Hz. If measuring engine RPM,
this corresponds to 600-6000 RPM for
the first range or 300-30,000 RPM for
the second range.
To set the required range, connect
+12V and ground and then measure
the voltage between TP1 and ground.
Adjust VR2 for 1.5V if you want the
first frequency range or to 6V if you
want the second range. In most applications, the first (ie, lower) range
will be required.
That done, you can install the unit
in the car. In addition to providing
power (switched +12V ignition supply) and earth connections, you will
need to tap into the frequency signal
that you want to monitor and connect
this signal to the input terminal. This
signal wire can be from the:
• Road speed sensor
• ECU tacho output
• Switching side of an injector
• Crankshaft or camshaft position
output sensor
At this stage, don’t connect anything to the relay – you will be able
to see when the relay clicks over as
the LED will light (and in quiet environments you’ll also hear the relay
change over).
So how do you set the trip point?
You might want to have the Frequency
Switch trigger a shift-light at 6000
RPM. But you don’t have to start off
holding the engine at six grand – instead, adjust VR1 until the LED comes
on at (say) 3000 RPM and then goes
off as revs again drop. By adjusting
the hysteresis pot (VR3), you should
be able to alter how much the engine
speed drops before the LED turns off.
(Hint: if the LED flashes on and off
around the switch-off point, increase
the hysteresis by turning VR3 anticlockwise.)
With the system working as it
should, turn trimpot VR1 a little more
anti-clockwise to increase the trippoint frequency and then blip the
engine until it again switches on the
siliconchip.com.au
Parts List
Fig.3: the on-board relay can be used to switch large loads. For example,
as shown here, a high-power shift light could be wired into place.
1 PC board coded 05car051,
105 x 60mm
1 plastic case, 130 x 68 x 42mm
(optional – not in kit)
5 PC-mount 2-way screw
terminals with 5mm pin
spacing
1 12V PC mount DPDT 5A relay
(Relay1)
1 3-way header with 2.54mm
spacing
1 jumper shunt with 2.54mm
spacing
1 1MΩ horizontal trimpot (VR1)
1 2kΩ multi-turn top adjust
trimpot (VR2)
1 10kΩ horizontal trimpot (VR3)
Semiconductors
1 LM2917 frequency-to-voltage
converter (IC1)
1 7808 3-terminal regulator
(REG1)
2 BC337 NPN transistors
(Q1,Q2)
1 5mm red LED (LED1)
2 16V 1W zener diodes (ZD1,ZD2)
3 1N4004 1A diodes (D1,D2,D4)
1 1N4148 switching diode (D3)
On modified engines with changeover intake manifolds, the frequency switch
can be used to set the revs at which the runners swap from long to short length.
LED. By making changes to VR1 and
then assessing the results with blips
of the throttle, you should be able to
quickly and easily set the trip point at
the correct engine revs.
Note that VR1 is a multi-turn pot.
This has been used so that the trip
point can be adjusted very precisely –
however, if you’re not used to this type
of pot, be aware that you can keep on
turning it endlessly and never reach
a clear “stop”!
In the above example, you’ll probably want only a small hysteresis (ie,
a small difference between the switchon and switch-off frequencies). But in
some cases, a much larger hysteresis
works very well.
For example, if you use the Frequency Switch to turn on an intercooler
siliconchip.com.au
fan at low road speeds, the adjustable
hysteresis can be used to keep the fan
running until you’re again travelling
fast enough to push air through the
core. In this case, you could set the
turn-on at 10km/h and then adjust the
hysteresis so the fan doesn’t turn off
until 35km/h.
This works well in practice where
heat-soak of the intercooler is more
likely to have been occurring after
you’ve been stopped for awhile and
are driving off slowly.
The device that is to be triggered by
the relay will normally be switched via
the Normally Open (NO) and Common
(C) relay contacts. Fig.3 shows these
connections. Note that because a double-pole, double-throw (DPDT) relay
has been used, another completely in-
Capacitors
2 100µF 16V PC electrolytic
2 10µF 16V PC electrolytic
1 1µF 16V PC electrolytic
1 22nF MKT polyester (code 223
or 22n)
1 10nF MKT polyester (code 103
or 10n)
Resistors (0.25W, 1%)
1 100kΩ
2 1kΩ
7 10kΩ
1 100Ω
1 3.3kΩ
1 10Ω
1 1.8kΩ
dependent circuit can also be switched
simultaneously. This other circuit can
even turn off the second device as the
first is switched on.
Note that if you just want to simply
monitor a frequency (eg, engine revs),
you can delete the relay and just mount
the LED on the dashboard instead.
In most applications, once the
Frequency Switch is set, it won’t
need to be altered again. The PC
board fits straight into a 130 x 68 x
42mm jiffy box, so when the system
is working correctly, it can be tucked
SC
out of sight.
June 2007 85
|