|
Rpm
SWITCH DESIGN NOTES
There
are literally thousands of ways to home build
your own rpm switch. We prefer electronics
because there are no moving parts and therefore
can be very accurate, inexpensive, reliable and
efficient.
Note:
we sincerely recommend that you build the any
circuit on an electronic test board and test it
with your generator before actually hard
soldering it, so you will know that all the
component values are correct for your
application. This is what we do ourselves,
because it saves a lot of hassle. Every circuit
needs 'tweaking' or 'debugging' or
'optimizing'.
Note:
Seal the circuit (in a box or with epoxy) very
well, to prevent dirt and moisture from getting
on the board. It is designed to be very voltage
sensitive and moisture will skew the
settings.
This
circuit is designed for practical simplicity. It
can work well on voltages between 9 VDC and 15
VDC. It is important that whatever voltage you
choose, that it be as stable as you can make it.
It is a good idea to have the circuit operating
(turned on) for at least a minute before using
it because it needs time to thermally stabilize.
It needs to be very stable as it needs to be
accurate in the millivolt (mV) range.
We
signal a 555 IC (one or more times per
revolution), store the resulting electrical
pulses in a capacitor (C4) and monitor the
capacitor voltage with a 358 (op-amp IC). The
358 signals an electronic relay to connect or
disconnect the generator from the home power
system.
To
initiate each signal event, the 555 needs a
negative going pulse on pin 2, (pin 2 needs to
be normally high). In this example we provide
pulses from the 'coil' negative.
Once
an event is initiated, the 555 will generate a
specific pulse duration from pin 3 and then pin
3 will go low to wait for the next event at pin
2. This is fixed pulse duration is specifically
why we use the 555. We want a specific pulse
width for each event regardless of the pulse
duration of the trigger signal (at pin 2). This
keeps the monitored signal from pin 3 directly
related to the engine rpm.
As
stated, the output of the 555 pin 3 gets stored
in the capacitor C4. The resulting voltage will
be closely relational with the engine
rpm.
The
358 is set up as a very sensitive voltage
comparator. The 358 monitors C4 voltage and
compares the voltage to a set point voltage
(R6). The output signal (358 pin 1) is normally
high (positive) which keeps N1 turned 'on' which
grounds the gate of Fig. 01, P1; keeping the
fuel turned on.
When
the C4 voltage (engine rpm) exceeds the set
point voltage (set just above idle rpm) then the
358 signals the gate of mosfet N1 a negative
(ground) signal. So N1 stops grounding the gate
of Fig. 01, P1. P1 gate goes positive and shuts
off the fuel.
When
the C4 voltage drops below the set point
voltage, the 358 again sends a positive signal
that turns the fuel back on (via N1 and
P1).
Note:
The circuit can have a built in 'averaging' or
delay by adjusting the size of C4. A small C4
will shut off the fuel fast.
(#2
under speed circuit drawing will go
here)
Rpm
switch components:
Resistors
can all be rated for 1/4 watt. Ordinary
abbreviation: k = x 1000 and M = x 1,000,000. We
have used actual values without abbreviation for
clarity. 1,000 ohm = 1k ohm; 10,000 ohm = 10k
ohm.
R1
is 1,000 ohm. It limits the current through the
LED.
R2
and R3 are 10,000 ohm. R3 is a 'pull-up'
resistor, meant to keep the signal to 555 pin 2
positive until the coil ground
signal.
R4
is 100,000 ohm. It limits the current flow to
C4, slowing down the rate at which the voltage
of C4 rises and falls. The 555 acts as both a
voltage source and sink, so it charges and
discharges C4 through pin 3.
R5
is 1,000,000 ohm. It limits the current flowing
to the 358. To keep the 358 sensitive as a
voltage comparator (switches within 0.005 V (5
mV)), it is important to prevent excess current
to or from it.
R6
is a 1,000,000-ohm precision 20-turn
potentiometer. It is meant to control the set
point voltage. In this case, the rpm at which
the fuel is shut off.
We
suggest using a potentiometer case style that
needs a tiny adjusting screwdriver. This will
help prevent unauthorized people from adjusting
your rpm. Besides, once the optimum rpm is
found, you will not be adjusting it unless
something about your system changes.
When
wired as we suggest, R6 turned counter clockwise
will raise the voltage output signal, which will
raise your rpm set point.
R7
is 500,000 ohm. It allows the R6 to be a more
sensitive set point. If R6 set point cannot rise
high enough to equal the voltage at C4, lower
the value of R7. We use high values for R6 and
R7 because they prevent the 358 from 'loading
up' with excess current and allow the circuit to
be extremely sensitive.
R8
is the resistor that controls the duration of
the pulse from the 555 pin 3 (after the signal
to pin 2 initiates it). We do not have the exact
setting you will need at this time. It will be
different for 2, 3, 4, 6, 8, 10, 12 cylinder
engines. We recommend 0 to 100,000 ohm pot. More
resistance will make a longer duration that
makes the voltage of C4 rise higher. When C4 is
at a higher voltage, it takes longer for the
switch to respond (prevents oscillation) and the
switching is more accurate.
We
recommend using a potentiometer for R8 till you
get an idea of the resistance your application
works best at and then put in a fixed resistor
(for ultimate circuit stability.
We
used an oscilloscope to see the pulse duration.
- VERY IMPORTANT - You want the last pulse to be
shut off before the next pulse starts.
Overlapping pulses will prevent this circuit
from working. The more signals per revolution
(as in 8 cylinder will signal 4 times per
revolution and a four cylinder will signal
twice), you'll require less resistance for R8 or
you will get overlapping signals at higher
rpms.
Capacitors
need to be rated for at least 30 volts and be
ceramic whenever possible.
C1
is 0.1 uF. It is the capacitor that sets the 555
operational frequency.
C2
is 0.047 uF. It stabilizes internal values at
pin 5.
C3
is at least 0.1 uF (if running off a battery)
and can be as much as 1,000 uF (if running off a
transformer). It stabilizes the voltage coming
from your power supply.
C4
is about 20 uF; it can be as little as 10 uF
(will switch quickly) or as much as 40 uF (will
switch very slowly).OK to be
electrolytic.
C5
is 0.01 uF. It is used to stabilize the signal
coming from C4, taking out any AC
'bounce'.
S1
is any SPST switch. It turns off the
circuit.
Light
Emitting Diode 'LED1' is green and tells you
when the circuit is turned on. We like to place
it just above the on switch.
The
555 is a standard 'timing' IC available nearly
anywhere, including Radio Shack.
The
LM358N dual op-amp we acquired at Future Active
Electronics.
Note:
It is always a good idea to get the
specification sheets on electronic components
that you use, so you know the minimum and
maximum values.
7809
is a common 9-volt regulator in a TO-220 case
design. The use of a 9-volt regulator in all of
our schematics is to provide stable electrical
voltage to the circuit. The 9-volt regulator
will handle voltages as high as 30 volts. It
requires at least 10.5 volts input to give a
9-volt output.
'N1'
is a IRFD014, N-channel mosfet in a HEXDIP case
design. Can handle 1 amp at 100°C. Is rated
for a maximum drain to source voltage of 60
volts.
We
like using HEXDIPS because they are circuit
board mounted mosfets, taking up little room.
They work great when low amperage switching is
required. You can use any equivalent n-channel
mosfet, including the ones in a TO-220 case
style from Radio Shack.
|
