An improved optically isolated high
current switch
C. Brent Dane
AMA 345702
Update, December 27, 1998
The original article for the OPTOGLO optically
isolated high current switch appeared in the
December, ‘98 issue of Model Aviation
magazine. That article, as it was published,
documents a good design that will work very
well in almost every R/C switching application.
Since that time, however, I have revisited the
design. Like a couple of commercially available
glow switches, the first OPTOGLO used a bulky
MOSFET transistor with a large heat sink tab
and a separate optical isolator. I have replaced
these components on the OPTOGLO II with a
single small package, called a Power MOSFET
Photovoltaic Relay, which has both the isolator
as well as two MOSFETs built in. The small
photovoltaic relays are much easier to handle
and install than the previously used large power
MOSFET transistors. The result is the most
advanced, compact, and lightweight R/C switch
design currently available, incorporating recently
available, state-of-the-art electronic switching
Onboard glow: a safer alternative
Many of the hazards that we face in R/C model
aviation are centered on the process of starting
and tuning our glow engines before each flight.
It requires us to work in close proximity to the
running engine and often to reach around the
very dangerous spinning propeller. In particular,
removing the glow plug power source once the
engine is running often leads the modeler to
reach over the top of the propeller disk. This
single action is the cause of a large number of
injuries sustained from propeller strikes.
solution, of course, is to securely tie down the
airplane or have a companion hold the craft
during starting. Once the engine is running, the
modeler can then move around to a position
behind the propeller arc and more safely remove
the glow plug connector and make carburetor
mixture adjustments.
However, another
significant increase in safety can be provided by
an onboard glow ignition system. This system
consists of a rechargeable 1.2V NiCd battery
and a switch that can be remotely activated by
the radio control system. The modeler is then
never required to disconnect the glow power
source from the running engine.
A small and simple circuit, shown in Figure 1,
has been designed as a general purpose
electronic switch for radio control (plane, boat,
car) applications. It is primarily intended to
operate on-board glow systems for model
engines but, with an operating voltage of up to
20V and a high current switching capacity, it can
be used for on/off control of a wide variety of
devices such as smoke pumps, lighting systems,
or even electronic ignition systems for spark
Complete optical isolation between the receiver
and the switch circuit eliminates the possibility of
interference from electrical noise generated by
electric motors or metal to metal vibration in
glow engines. Its straightforward design uses
readily available electronic components making
the OPTOGLO II a great project for any R/C
Figure 1
Photograph of the finished
OPTOGLO II electronic switch.
The circuit
pictured is the single photovoltaic relay version
for one glow plug or two plugs wired in series. It
can be used for any switching application with
up to 4.5A of current.
One motivation to use onboard glow systems
has been four-stroke engines that sometimes
benefit from applying current to the glow plug to
© C. Brent Dane 1998
The OPTOGLO II - Brent’s R/C Electronics Page -
provide a smoother and more reliable low speed
These glow systems are either
mechanically or electronically coupled to the
throttle control in such a way that the glow plug
is ignited below a preset throttle setting.
Onboard glow systems are also extensively
used for multi-cylinder two and four-stroke
engines for which it is impractical to make
separate manual connections for each glow plug
every time the engine is started. However, it
should be recognized that the use of onboard
glow systems can add safety and convenience
to any model engine. Even in cases for which
glow current is not required at idle, an electronic
glow system can be connected to an unused
radio channel, allowing the glow current to be
switched on and off with just the flip of a switch
on the transmitter. No more fiddling around with
wires or glow batteries just inches away from a
hungry spinning propeller!
An electronic glow switch has a number of
switches. The first, of course, is the ease with
which the on/off set point can be adjusted.
Another big advantage is the fact that the plug is
automatically extinguished when the receiver is
turned off, eliminating the possibility of
accidentally discharging the glow battery
between flights by leaving the throttle in the
closed position.
Theory of Operation
The common glow plug is designed to be
energized by a single NiCd or dry cell battery.
As I began to design the first OPTOGLO circuit,
I was initially surprised to find out that the
average current draw of a typical glow plug is
about 3 amps at 1 volt. This means that the
filament resistance is only about 0.3 ohms. For
this reason, special care must be taken to
design an electronic switch that adds very little
resistance to the glow plug circuit. If the switch
resistance is comparable to that of the glow
plug, then a significant amount of battery power
will be dissipated in the switch itself. This not
only robs power from the plug, it results in
heating in the electronic switch. For this design,
I have chosen a power MOSFET photovoltaic
relay with an on resistance of no more than 0.04
ohms. This results in around 10% of the power
being dissipated in the transistor which is quite
Power MOSFETs with an on
resistance of less than 0.01 ohm are available
but the slight improvement in performance over
the one used here does not justify the significant
increase in price and lowered availability.
My first glow switch design did not incorporate
optical isolation. The voltage to drive the gate
on the MOSFET derived from the 4.8V receiver
pack therefore it was necessary to tie together
the negative terminal of the flight battery and the
glow battery. The result was that the body of the
glow engine was electrically connected to the
negative terminal of the flight battery. The first
indication that this was not a great idea came
when I sometimes noticed small servo twitches
when the receiver was turned on and I would tap
the engine with a metal object such as a
This illustrated the potential
interference that could result due to metal to
metal vibration when the engine was running.
Although a number of commercially available
glow switches are configured this way, I
abandoned this first design at the rough
prototype stage. An ideal solution is one in
which there is absolutely no electrical
connection between the receiver electronics and
the glow battery circuit.
This can be
accomplished with optical isolation.
challenge with optical isolation in the case of
switching glow power is that the battery voltage
on the switch side of the circuit is only 1.2V,
much less than the required 5-15V gate voltage
of a MOSFET transistor. For this reason, a
special optical isolator is used which is called a
photovoltaic isolator. This isolator consists of a
light emitting diode (LED) and a photovoltaic
array, more commonly referred to as a solar cell
array. Current is supplied to the LED which
generates light that is then collected on the solar
cells and is converted into a voltage on the other
side of the isolator.
The PVN012 power
MOSFET photovoltaic relay used in the
OPTOGLO II contains one of these optical
isolators and two MOSFET transistors, all in one
small 6 pin package. The only communication
between the half of the OPTOGLO II circuit
connected to the radio receiver and the half
connected to the glow plug circuit is through
photons of light.
That means that the
OPTOGLO II can be used as an on/off switch for
not just glow plugs but a wide variety of devices
including electric motors, pumps, buzzers, etc.
with no fear of direct interference into the radio
© C. Brent Dane 1998
The OPTOGLO II - Brent’s R/C Electronics Page -
It is worth pointing out that although the PVN012
is called a photovoltaic “relay,” it is not a relay in
the conventional sense. It has no mechanical
accomplished using two high current MOSFET
transistors that are built into the package. Each
relay is capable of handling up to 4.5 amps of
continuous current at voltages up to 20V. The
current capacity of the OPTOGLO II can be
doubled to 9 amps by adding a second
photovoltaic relay as shown in Figures 2 and
Figure 2
Dual photovoltaic relay version of
the OPTOGLO II required for 2 or 3 plugs wired
in parallel or other switching applications with
>4.5A of current.
Remember that the signal from the receiver that
is sent to control each servo is a train of pulses
ranging from about 1 to 2 milliseconds in
duration and occurring at about 60 per second.
A pulse width of 1ms sends the servo output to
the limit of rotation in one direction and 2ms to
the limit in the other. For example, the pulse
width supplied to a servo near the center
position is approximately 1.5 ms. There are
three wires connecting each servo to the
receiver. The first two supply the plus and
minus voltage from the battery pack and are
often red and black, respectively (see Table 3).
The third wire carries pulses which direct the
motion of the servo. The OPTOGLO II has its
own onboard adjustable reference pulse
generator. When the input pulses from the
receiver are longer than the onboard reference
pulses, the electronic switch closes (turns on).
When the input pulses are shorter, it switch
opens (turns off). A jumper on the OPTOGLO II
allows the direction of operation to be reversed
so that it is open for long pulses and closed for
short pulses. The jumper therefore functions in
a way similar to a servo reversing switch.
Figure 3 shows the electronic schematic for the
The set point for on/off is
changed by adjusting potentiometer R4. R4,
resistor R5, and capacitor C2 determine the
reference pulse duration. The outputs from pin
5 and from pin 12 of the integrated circuit IC1
have reversed polarity.
Moving jumper J2
applies the output from either pin 5 or pin 12 to
the photovoltaic relay IC2 (and optional IC3).
Notice that the only link between the power
switch side of the circuit and the side connected
to the receiver is through the optical isolator.
The red LED D1 lights to indicate that voltage is
being applied to the photovoltaic relay, closing
the MOSFET switch inside the relay. D2 is a
catch diode placed across the device that is
being powered by the OPTOGLO II switch.
When inductive loads such as a mechanical
relay or motor windings are de-energized, they
produce a voltage spike which can cause erratic
shut-off characteristics.
The catch diode
suppresses the possible spike. Finally, note that
the schematic illustrates the connections for a
glow plug and glow battery. The glow battery
can be replaced with any DC voltage source of
up to 20V and the glow plug can be any of a
wide range of electrically powered devices.
Table 1 is a complete parts list for the
OPTOGLO II with catalog part numbers from
Digi-Key Electronics.1 Figure 4 is a 2X scaled
pattern for the printed circuit board. It should be
reduced to 1/2 size when it is reproduced. The
best way, I believe, of making this board is with
a product called "Press-n-Peel."2 It is a blue
plastic film which can be run through a
conventional photocopy machine or printed onto
directly with a laser printer.
Following the
instructions supplied with the film, the image in
Figure 4 should be copied onto the dull side. It
is then ironed onto a copper clad board (such as
Radio Shack 276-1499) using a household iron
or your airplane film sealing iron.
experimenting with the best temperature, I find it
useful to securely tape the film to the board
along one edge. I can then carefully peel back a
© C. Brent Dane 1998
The OPTOGLO II - Brent’s R/C Electronics Page -
small portion to test the adhesion of the pattern
to the board. If the transfer was incomplete, the
film is let back down and additional pressure at a
higher temperature used.
After successful image transfer, the board is
etched in a solution of ferric chloride (Radio
Shack 276-1535) for 30-60 minutes or until all of
the unwanted copper is gone. Note that this
particular blank board is two-sided so the copper
will be completely removed from the back side.
After etching, the mounting holes should be
drilled with a #65 drill bit.
This is easily
accomplished since the copper surrounding
each hole location accurately guides the drilling.
Slightly larger holes will be required for the glow
plug and battery wires. The smallest possible
holes should be drilled in each case. Those
modelers interested in building a circuit but who
do not want to manufacture a PC board may
contact the author. Alternatively, a complete
unassembled kit, minus the wiring harnesses,
can be obtained.3
5 - Photograph of the required
electronic components to build the OPTOGLO II.
Figure 5 is a photograph of the complete set of
electronic components and Figure 6 shows a
schematic of the installation of these
components onto the board. They should be
inserted from the side opposite from the copper
circuit pattern.
Table 2 is a list of the
components with their required values. All of the
resistors should be mounted vertically as shown
in the photographs in Figures 1 and 2. Jumper
J1 is simply made from a left over piece of
resistor lead.
Notice that if the single
photovoltaic relay version is built, a second
jumper J3, made the same way as J1, should be
installed. Pay special attention to IC1, IC2,
optional IC3, D1, and D2 since the direction they
are installed is very important. Notice that the
LED D1 has a small flat spot which should be
oriented as shown in Figure 6. This indicator
LED can also be placed on extension wires to
locate it in an easily visible location on the
airplane such as in the cockpit area. Diode D2
should be installed with the striped end facing
down. For high current applications (4.5-9A), an
optional photovoltaic relay IC3 can be installed
in the second board position.
Carefully solder the installed components to the
copper circuit pattern, taking care to avoid solder
bridges across gaps between the traces. It is
extremely important to use a good quality resin
core electronics-grade solder and to brighten the
copper traces with fine steel wool or a scotchbrite pad before starting.
Advice and/or
instruction from a fellow modeler with circuit
assembly experience could be useful here. The
excess resin left from soldering should be
removed with a solvent such as lacquer thinner
(I use K&B Superpoxy thinner since that's what I
have in my shop) and an epoxy brush with the
bristles cut back to 1/4".
A receiver lead
compatible with your radio system can be
purchased at your local hobby shop or you could
use the wiring harness from a worn out or
damaged servo. Table 3 summarizes the wire
color codes typically used for some common
radio systems. Using Figure 6, carefully attach
the leads according to this chart. Airtronics
users should be cautioned that the V+ and Vwires are reversed in the servo harness from
those for Futaba, JR, and RCD radio systems.
Since the Airtronics harness has two wires of the
same color (black), carefully note which one is
denoted as the center wire in Table 3.
Installation of the OPTOGLO II
Figure 8 shows a completed OPTOGLO
onboard glow system, including the 1.2V NiCd
battery and glow plug connector. The finished
circuit weighs only about 1/2oz. The complete
onboard system with wiring and a sub-C battery
weighs 4oz. In the set up shown, the battery
and glow plug in-line connectors are gold plated
Ultra Plugs made by W.S. Dean and available
through most hobby retailers (Tower Hobbies,
© C. Brent Dane 1998
The OPTOGLO II - Brent’s R/C Electronics Page -
part #WSDM3001). The stainless steel glow
plug connector is manufactured by McDaniel
R/C (McDaniel R/C #448). The small circular
lug should be attached to the engine case. This
can be conveniently accomplished by placing it
under one of the engine’s mounting ear screws.
Notice that an extra electrical lead is installed on
the sub-C battery for charging and is terminated
with a Deans 2-pin connector (Tower Hobbies,
part #WSDM3002).
number means bigger wire) should be used to
avoid undesirable voltage drop in the harnesses.
When using the OPTOGLO II to power the two
glow plugs on a twin cylinder engine, there are
two possible connection schemes, as shown in
Figures 9(b) and (c). For the parallel scheme in
Figure 9(b), the negative wire is attached to the
engine case and the positive lead is attached to
both glow plugs. The two plugs in parallel draw
twice the current of a single plug therefore it is
recommended that two batteries in parallel be
used as a power source. In this case, the
optional photovoltaic relay IC3 should be
installed. Figure 9(c) shows the alternate series
connection scheme. In this case, the positive
wire is attached to one glow plug and the
negative wire to the other glow plug. The
electrical current through the OPTOGLO II for
series wiring is the same as for a single glow
plug but twice the voltage is required, therefore
two batteries in series are used. Since the
current is not increased, only one photovoltaic
relay is required for the series connection
Figure 7 - Completed OPTOGLO II as viewed
from the trace side of the board.
When installing the in-line connectors, be sure to
place the female half of the connector on the
battery side of the battery wiring harness and on
the circuit side of the glow plug wiring harness.
This reduces the risk of a short circuit when the
plugs are not connected. Great care should be
taken when hooking up the battery since
installing it backwards will quickly damage the
MOSFET switch in the photovoltaic relay or the
catch diode D2. A simplified wiring schematic
for a single glow plug system is shown in Figure
Care should be taken in choosing the wire size
used to connect the glow battery and the glow
plug connector to the OPTOGLO II circuit. 20
gauge copper wire has a resistance of .00085
ohms/inch therefore up to 24 total inches of wire
can be included in both harnesses before the
resistance of the wire will have an resistance
comparable to that of the MOSFET switch. The
total length of both wiring harnesses should be
kept as short as possible. If more than 24
inches of wire are required, 18 gauge (smaller
Figure 8 - A completed onboard glow system
including the 1.2V NiCd battery and glow plug
connector. This setup is illustrated with the
original OPTOGLO design using a discrete
MOSFET transistor.
Which method for twin cylinder hook up is right
for you? There are good arguments in favor of
both wiring methods and I will summarize these.
The advantage of the parallel wiring scheme is
© C. Brent Dane 1998
The OPTOGLO II - Brent’s R/C Electronics Page -
increased reliability since, if one plug fails in
flight, the other plug will still light, providing the
best chance of keeping the motor running at
idle. However, some prefer the series wiring
scheme since if one plug does fail, neither plug
will light, preventing the modeler from
inadvertently starting the engine on the ground
on only one cylinder. The two glow plugs in
series draw the same current as a single plug
meaning that the same power is dissipated as
heat in the MOSFET switch (meaning that IC3 is
not required). However, if the plugs are not well
matched in resistance, the power delivered to
each plug may not be equal when they are wired
in series. In fact, since the plug resistance
increases as the filament heats, the power
imbalance between the plugs could increase
with the hotter plug getting hotter and the cooler
plug getting cooler. Therefore, for series wiring,
the two plugs should be as well matched as
possible. The parallel wiring scheme, however,
results in good heat balance between the plugs
even in a case of mismatched resistances. A
final consideration is the charging of the glow
battery. Since it is generally not advisable to
charge two NiCd cells in parallel, they should
theoretically be removed from the circuit for
recharging. In the series scheme, the batteries
can simply be charged in place. Surveying
REC.MODELS.RC.AIR will find modelers who
prefer and successfully employ both series and
parallel wiring schemes. Even so, I believe that
the series scheme may offer a few more
advantages and an easier wiring job overall.
placed in the position that lights the glow plug
when the throttle position is below the set point,
near idle. The potentiometer R4 should be
adjusted so that the glow plug comes on at the
desired throttle setting by observing the indicator
LED. Remember that the LED shows that the
electronic switch is closed, not necessarily that
the plug is glowing. The indicator will light
whether or not the glow battery is properly
charged or the plug is burned out. A second
hook up option is to connect the OPTOGLO II to
a separate receiver channel. Electronic mixing
on the receiver can then be used to slave that
channel to the throttle servo channel. This is my
preferred set up since, with a computer radio,
the mixing can be deactivated if desired and the
glow power can be applied when needed with
the flip of a switch for the glow channel.
Using a single photovoltaic relay, the OPTOGLO
II will easily handle the 3 amp current
requirements for a single plug or the series
connection scheme of dual plugs. For three
cylinders or more, the parallel arrangement is
the only possible wiring scheme since each plug
is connected to the common engine case. Two
photovoltaic relays are required for both the 2
and 3 plug parallel connection scheme. More
than three glow plug connection is not
recommended with the OPTOGLO II.
Figure 10 - Photograph of author’s Pica 1/6
scale Cessna 182 with onboard glow connector
in place.
Using the OPTOGLO II
There are two ways to connect the OPTOGLO II
circuit to the radio control receiver. The first is to
connect the circuit to the throttle servo channel
using a Y-harness. The jumper J2 should be
Sliding the finished circuit into a short length of
1.25” diameter heat shrinkable tubing, as
pictured in Figure 8, provides protection for the
OPTOGLO II circuit. Be careful not to overheat
the circuit when shrinking. A set of access holes
can be cut through the covering to allow set
point adjustments and to move the reversing
jumper J2. If the circuit is connected to a
separate receiver channel as described earlier,
access to these adjustments may not be
necessary since this can be accomplished by
receiver programming. I like to wrap the finished
circuit in a layer of 1/4" latex foam held with a
© C. Brent Dane 1998
The OPTOGLO II - Brent’s R/C Electronics Page -
couple of rubber bands to shield it against
engine vibration.
commercially and I highly recommend one of
Controlling other on board devices
The author
As I described in the introduction to this article,
the OPTOGLO II can be used as an on/off
switch for a variety of devices operating with
battery voltages of up to 20V. Among the
possible applications are the control of smoke
pumps, lighting systems, buzzers, and electronic
ignition systems for spark engines.
OPTOGLO II can easily handle 4.5 amps of
current with a single photovoltaic relay installed
with no heat sinking required. This current
handling can be doubled to 9 amps by adding
the additional optional photovoltaic relay IC3. It
should be noted that devices such as lighting
systems and smoke pumps typically draw much
less current than a single glow plug! Consult the
documentation supplied with these devices to
determine the actual requirements. Remember,
as long as the battery voltage is less than 20V,
only the current (in amps) determines how much
power is dissipated in the power MOSFETs
inside the photovoltaic relays.
Brent Dane has been an AMA member and an
R/C model pilot since 1990. He is a member of
both the Livermore Flying Electrons and the
East Bay Radio Controllers, two AMA chartered
clubs which have flying fields near Livermore,
CA. Brent is a physicist in the laser program at
Lawrence Livermore National Laboratory. For
answers to questions or for additional
information about the OPTOGLO II, Brent can
be contacted on the internet at email address
[email protected] Additional information on
other R/C electronic projects can be found at
The original OPTOGLO could also be used as a
small and inexpensive switch for the increasingly
popular Speed 400 and similar electric motors.
In theory, the current draw required for these
motors could just be handled by the OPTOGLO
II using two photovoltaic relays. However, it is
very near the design limit and I do not
recommend that the OPTOGLO II be used for
Speed 400 on/off control. There are now a
number of very compact, lightweight proportional
speed controllers for these motors available
End notes
1. Digi-Key Corporation, 701 Brooks Ave.
South, Thief River Falls, MN 56701-0677, (800)
2. Press-n-Peel is manufactured by Techniks
and can be ordered from All Electronics Corp.,
Van Nuys, CA 91411, (818) 997-1806.
3. To receive an etched printed circuit board,
send a check or money order for $10.00 per
board to Brent Dane, 678 Crane Ave, Livermore,
A complete unassembled kit,
without wiring harnesses or battery, can be
obtained for $25.00 (includes postage and
handling). Add an additional $5.00 for a second
photovoltaic relay for applications requiring >4.5
amps (but not more than 9 amps) of current.
© C. Brent Dane 1998
The OPTOGLO II - Brent’s R/C Electronics Page -
Table 1
# parts
1 (2)
Digi-Key part number
1/4W resistor 470Ω
1/4W resistor 47k
1/4W resistor 100k
1/4W resistor 1M
100k rotary potentiometer
polyester film capacitor 0.010µF
polyester film capacitor 0.056µF
red LED with diffuse lens
600V silicon diode
dual retriggerable monostable multivibrator
power MOSFET photovoltaic relay
3 pin square header
shorting jumper
Table 2
yellow, purple, orange *
yellow, purple, orange
brown, black, green
rotary trim potentiometer 104
brown, black, yellow
yellow, purple, brown
yellow, purple, brown
0.056µF **
0.010µF ***
red LED
Integrated circuits
IC1 MM74HC123****
IC2 PVN012
IC3 PVN012
Dual resetable monostable multivibrator
Power MOSFET photovoltaic relay
Power MOSFET photovoltaic relay (optional)
NOTE: The optional relay IC3 is needed for 2 or 3 glow plugs wired in parallel
or for applications with current draw of >4.5A. A single glow plug or dual plugs
wired in series do not require IC3.
The last resistor color band is gold for ±5% resistors.
0.056µF capacitor is labeled 563.
0.010µF capacitor is labeled 103.
The MM74HC123 works differently than other 74HC123 and 74HCT123 ICs. If you don’t use the
MM74HC123 then change C1 to 0.1µF (104) and C2 to 0.022µF (223).
© C. Brent Dane 1998
The OPTOGLO II - Brent’s R/C Electronics Page -
Table 3
Negative voltage (A)
Positive voltage (B)
Signal (C)
Airtronics / Sanwa
Hitec / RCD
KO Propo
Kyosho / Pulsar
Tower Hobbies
black (center wire)
black with red stripe
red (center wire)
red (center wire)
red (center wire)
red (center wire)
red (center wire)
red (center wire)
© C. Brent Dane 1998
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