54. Coil Timers
If you're using any real pinball coils in your virtual pin cab, you
might want to consider using some kind of "time limiter" circuit with
them. The reason is that most types of pinball coils are only
designed to be energized for very brief intervals, typically just a
fraction of a second at a time, and can overheat if energized for too
long at a time. Overheating will melt the wire inside the coil
and destroy the device.
Examples of pinball coils where this is an issue:
- Replay knocker
- Chime units
This is actually a fairly common property of solenoids in general, not
just pinball coils, so it might apply to your pin cab if you're using
other types of devices to simulate any of the above:
- "Open frame" solenoids bought on eBay or elsewhere
- Automotive starter solenoids
How a time limiter circuit works
A time limiter circuit can be used as a fail-safe to protect your coil
devices against getting stuck on for long periods. A time limiter
adds some special additional circuitry to an output controller port
that automatically limits the amount of time the port can be "on"
continuously. If the port stays on for longer than the limit, the
timer circuit forces the port to turn off, overriding any software
signal to the contrary.
The reason to implement this with a hardware timer, rather than with
some kind of software fix, is that the main thing we're worried about
is software failure. A separate hardware circuit won't be affected by
any errors or crashes in the software.
The Pinscape expansion boards include built-in time limiters for the
Replay Knocker port and for all ports on the Chime boards. You can
add your own custom timer to any other type of output controller as
well, which we'll describe below.
Why this is a concern
There are two places in a virtual cab where coil overheating can
be a problem: flippers, and everywhere else.
For flippers, it should be fairly obvious why this is an issue. The
player can keep a flipper button pressed indefinitely to trap a ball,
consider a shot, have a beer... The flipper coil will stay energized
this whole time. It might seem like a typical player wouldn't
typically hold the button on all that long, but it's not the
typical case we have to be concerned about. We have to consider the
longest time that anyone will ever leave hold the button on, since
all it takes is that one time to fry the coil.
For bumpers, slingshots, replay knockers, chimes, and everything else,
there's no reason in normal operation that any of these will
ever stay on for more than a brief instant at a time. So it might
seem like you don't have to worry about them. But you do, because of
the chance of abnormal operation. Specifically, the devices in a real
cab are controlled by software, and software can fail unexpectedly.
There's a particular failure mode in Visual Pinball where VP can crash
just after turning on a device, leaving the device stuck on until the
user can intervene manually. This isn't just a random fluke, either;
if it were, it wouldn't be likely enough to worry about. The reason
it's a concern is that it's relatively likely to happen in this
particular way (device turns on, then VP crashes) because of the way
VP scripts are constructed.
Devices that you don't have to worry about
Not all coils or solenoids have this problem:
- Contactors. These are large electrical relays designed for
high-power switching applications. They're specifically designed to
be activated continuously for indefinite periods (even for months or
years) without overheating. They're popular for simulating flippers
in virtual cabs for exactly this reason.
- Older pinball flipper coils, for machines made before about
1990. On the real machines, they had to deal with exactly the same
issue that we do when it comes to the flippers: the player can press
the button and hold the flipper on for long periods. So the pinball
manufacturers came up with various ways to prevent the coils from
overheating. The technique they used up until about 1990 also works
well on virtual pin cabs, so flipper assemblies for those older
machines are usually safe to use on a pin cab without any special
timer protection. See "Flipper assemblies" below for more on the
- Some other solenoids. Some non-pinball solenoids are built for long
duty cycles and others aren't. If you have a data sheet for your
device, it might list the maximum continuous "on" time or maximum duty
cycle. If not, you can test it. Apply power and carefully monitor
the device's temperature. You don't need a thermometer; it's enough
to check if it's getting hot to the touch. If it starts getting hot,
cut power immediately. If it got hot in a few seconds, you definitely
want to do something to limit the device's "on" time. If you can
leave the device continuously on for a few minutes and it's not
getting noticeably hot, it should be safe to use without any time
limiter. If it's somewhere in between, you should err on the side of
caution and limit its "on" time.
As mentioned above, some flipper assemblies are safe for use in
virtual pin cabs, without any special timer protection. Real pinball
machines have to deal with the same problem we do when it comes to
flippers - that the player might press and hold the button for long
periods. The manufacturers came up with a variety of solutions over
The standard approach for many years was to use two coils for
each flipper: a high-power "lift" coil and a low-power "hold" coil.
The lift coil fires briefly when you first press the button, to carry
out the the rapid "flip" action. As soon as the flipper is all the
way up, the lift coil cuts out, and the low-power hold coil takes
over, to keep the flipper flipped while you hold the button. The hold
coil had much less lifting power - just enough to hold the flipper up.
Less power means less heat, so the hold coil stays cool enough that
you can leave it activated indefinitely. Problem solved.
Note that the two coils were always wound together into one physical
unit, so it looks like there's only one coil involved. The
giveaway is that the coil has three terminals instead of the usual
Up until the early 1990s, the two-coil flipper assemblies switched
between the "lift" and "hold" coils using an end-of-stroke switch
embedded in the flipper assembly. The coil was wired directly to
this switch. When the flipper flipped all the way up, it hit the
switch, cutting off the lift coil power and activating the hold
coil power. This system works well in virtual pin cabs, since the
flipper assembly itself takes care of switching between the two
Starting in the 1990s, Williams changed to their "Fliptronic" system.
This also uses the two-coil design, but the end-of-stroke switch is no
longer wired directly to the coils. Instead, it's wired to the CPU,
and the CPU decides when to switch between the two coils. The
Fliptronic assemblies aren't safe with a virtual machine,
because the coil switching is no longer part of the flipper assembly -
it has to be directed by the software. The pinball emulators on the
PC don't have the necessary programming. If you connected one of
these newer Fliptronic assemblies to DOF, DOF would just leave the
"lift" coil on the whole time, and we'd be back to the overheating
Newer Stern flipper assemblies also aren't safe for virtual
pin cabs, because they use an even newer design that uses just
one physical coil and uses software to reduce the coil power
during the "hold" period.
To see if you have a "safe" flipper assembly using the two-coil
design, first check to make sure there's an "end-of-stroke" switch.
If there's not, you probably have a newer Stern assembly that's not
safe for virtual use. If you do have an end-of-stroke switch,
inspect its wiring. Check that it's wired directly to the flipper
coil. If it's only wired to an external connector, you have a newer
Fliptronic assembly that's not safe for virtual use. If it's wired
directly to the coil, you have the older type that is safe
for a pin cab.
If you have one of the newer types, don't panic! There's still a way
you might be able to make it work. If you're using the Pinscape
boards, you can enable the Pinscape "Flipper Logic" feature, which
uses the same PWM power reduction approach that the newer Stern
machines use. Flipper Logic is handled directly in the Pinscape
firmware, so it doesn't require anything on the PC to be aware of it,
meaning it's compatible with all PC software that can access the
flippers at all. See "Flipper Logic" below
for more details.
Pinscape expansion boards
The replay knocker output on the main Pinscape board has a time
limiter circuit built-in. All outputs on the Chime boards feature the
same time limiter. (The time limiter is the whole point of the Chime
Connect devices to the timer-protected output ports exactly the same
way you would connect devices to any of the other Pinscape output
ports. The timer function is built into the port's hardware and is
always active. There's nothing to configure and no software setup
The timers on these output ports cut off power after about 2 seconds
of continuous "on" time. They automatically reset as soon as the port
The short time limit is designed for devices like replay knockers,
chime units, bumper coils, and slingshot coils. In normal use, these
devices always fire momentarily. The instant reset allows the devices
to be fired in quick succession, such as when the ball is bouncing
rapidly back and forth between two bumpers.
DIY time limiter circuit
Here's the schematic for the coil timer circuit used in the Pinscape
expansion boards. This is a standalone circuit that doesn't depend
on anything else on the expansion boards; it can be used with any
type of microcontroller to control just about any kind of coil.
- VCC is the positive voltage supply for the microcontroller. This
circuit will work equally well with 3.3V and 5V microcontrollers.
- GND is the "ground" for the microcontroller power supply.
Connect this to any GND pin on the microcontroller.
- Input is the GPIO pin on the microcontroller that will be
controlling the device. This is a high trigger circuit, meaning
that the GPIO line must be driven high (to VCC voltage) to turn the
- COIL- is the negative terminal of the coil to be controller.
Connect the other terminal of the coil directly to the coil power
supply's positive voltage.
- COIL PSU (-) is the coil power supply's negative or GND
This circuit is designed to be used with a microcontroller, so the
input takes a "high" voltage value (the same voltage as VCC) to turn
the coil on.
The circuit as shown includes a high-current MOSFET that controls the
coil, so you can connect this circuit directly to the coil without any
external relays or amplifiers needed. The MOFSET shown in the
schematic will handle 14A at 55V, which is enough for any pinball
Adapting for LedWiz use: The circuit could be adapted for use
with an LedWiz or similar device, but I'm going to leave this as an
exercise to the reader, because I haven't actually built or tested an
LedWiz interface version. There are two changes you'd have to make:
- First, you must change the input to trigger on a "low" voltage
value, since that's how the LedWiz signal works. You can do this by
adding an optocoupler to the input stage. Connect the optocoupler LED
anode (+) to +5V, connect the cathode to a 100Ω resistor, and
connect the other end of the resistor to the LedWiz output. Connect
the Input line in the schematic above to the optocoupler emitter, and
also connect it to ground through a 1K resistor. Connect the
optocoupler collector to +5V. Use +5V for VCC throughout.
- Second, you'll have to "smooth out" the PWM signal produced by the
LedWiz. The circuit is designed for a simple digital logic high/low
GPIO output from a microcontroller. The LedWiz produces a pulsed
signal that's never 100% on. Every pulse will reset the timer
circuit, which will prevent the timer from ever expiring, which will
defeat the entire purpose of the timer. To overcome this, I think you
could use a simple low-pass filter on the input from the LedWiz.
I think a simple first-order filter, with one resistor and capacitor,
would do the trick. You'd have to choose the filter parameters
according to the actual controller device you're using; the LedWiz
PWM runs at about 50 Hz.
If you successfully build and test an LedWiz-compatible version of the
circuit, I'd really like to hear about it! Send me the actual circuit
that you got working and I'll publish it here instead of this
"exercise for the reader" dodge.
Pre-built timer device
You can buy a pre-built timer device on eBay that provides similar
functionality, and put it in your output circuit between your output
controller and the coil you want to protect.
The product offerings and sellers on eBay are constantly changing, so
I can't provide you with a link to a specific product; if I did, it
would just be a dead link by the time you read this, and that wouldn't
be very helpful. So I'll have to tell you how to search for it
yourself. I'll warn you, though: this will take some persistence on
your part, and maybe some trial-and-error buying, because the relevant
eBay listings can be difficult to decipher. The products you're
looking for will all be from Chinese sellers who don't always write
perfect English ad copy on their pages.
The basic search term that usually works best is:
multi-function relay timer board
In very vague and general terms, you're looking for:
- a relay to control an output
- with adjustable timers
- and a big list of different timing modes
These will usually have 15 to 20 different "modes" or "functions"
listed. They'll say things like "Function 1: timing on: after power,
time delay pull relay T1, between adjustable 0.1 seconds 270 hours,
CH1 interface to high level signal, repeat function", and then a giant
list of similar tortured sentence fragments.
That much is pretty easy to find on eBay. What's a little harder is
making sure that the list of 15-20 modes includes the exact mode we're
looking for. In my experience, anything that has a big list of modes
like that will have the right mode in there somewhere, so if
you're feeling lucky and don't want to make your brain hurt parsing
Google Translate output, just buy one and give it a try. But if you
don't want to risk the $20, you'll need to scan through the modes and
verify that there's one that does what we need.
So I'll try to describe the specific mode you need, in terms that
hopefully are comprehensible. Don't look for my exact
words. I'm trying to use sensible English, which the ads generally
don't. I'm trying to give you an understanding of what the mode is
actually supposed to do, on the theory that if you can internalize
this with a solid mental model of the desired function, you'll have a
better chance of fitting the poor descriptions on eBay to your good
internal understanding. So here's what you're trying to accomplish
with the device:
- You fire a trigger signal (by "you" I really mean "the LedWiz" or whatever:
the point is that the signal is being fed into the relay board from
an external source)
- In response to the trigger signal, the relay board immediately turns
the relay output on
- After a timer expires, the relay board turns the output off, even
if the trigger signal is still on at that point
- When the trigger signal turns off, the cycle resets, so that the
board is now ready to receive a new trigger signal and start the
whole process over
As an example, here's some actual verbiage I just pulled from a random
eBay ad for a random device that does what we're looking for:
Function 14: Disconnect and then pull the trigger timing: After power
relay does not act, a high-level interface to CH1 pulse signal,
immediately pull the relay, the relay off delay time T1 after arrival;
T2 arrive after disconnecting time relay, delay time between T1 and T2
in 0.1 seconds -270 hours adjustable, repeat CH1 interface to a high
level pulse signal, repeat the above functions;
Again, don't try to look for these exact words. This is just
one example. Every eBay seller will use their own words for this, and
they'll all be different, and they'll all be bizarre and hard to read
in their own way. But let's pull this particular example apart, as a
practice exercise and to help illustrate how this relates to the
functions we're looking for:
- "After power relay does not act": this is telling you that the timer
doesn't do anything special when first powered on. Weird that they
say what it doesn't do, right? Well, they're just spelling
this out because a lot of the other functions of these boards
are all about doing something special when the power first
comes on. So they're trying to be helpful by saying explicitly
that this function isn't related to the power initially coming on.
- "...a high-level interface to CH1 pulse signal": this is the
user-generated trigger signal. "CH1" probably stands for "channel 1",
which is the input that you'd connect to the external signal source,
such as the LedWiz output we want to use as the control. Some of
these boards have more than one trigger input, so they might have
"CH1" and "CH2" for the two inputs. We only need one input for our
purposes, but if the board has two or more, that shouldn't be a
problem, as long as there's a mode that operates on the basis of a
single input. This particular mode's description doesn't say anything
about any other inputs being involved. "High-level" means that the
board's input terminal senses the trigger when the connected voltage
goes "high", meaning it goes from 0V to 5V or whatever the supply
- "...immediately pull the relay": the relay turns on. Just what we
want. I'm not sure why the verb is "pull"; maybe they're thinking of
the relay's magnetic coil turning on and tugging on the switch
contacts to toggle the switch. I bet it makes more sense in Chinese.
The Google Translate round trip probably looks hilarious when you
translate it back.
- "...the relay off delay time T1 after arrival": the relay turns
back off after a delay. "T1" is one of the programmable delay
- "T2 arrive after disconnecting time relay...repeat": everything
resets after the further delay time T2, also programmable. In other
words, you get a forced "off" time of T2 before you can fire the relay
again. That's not part of what I said to look for, but we can
effectively ignore this by setting T2 to a very short delay time
(minimum 0.1 seconds according to this ad).
High-level inputs: As with my "DIY timer circuit" above, most
of these relay boards require a "high-level" input, meaning that they
trigger when the input signal changes from 0V to 5V (or whatever the
logic voltage is for the board). And as I mentioned with my DIY timer
circuit, that's the opposite of how LedWiz's work. See "Adapting for
LedWiz use" in the DIY timer circuit section above for some pointers
on interfacing to an LedWiz.
"Slow-blow" fuses are designed to perform a similar function to the
type of timer we've been talking about. Rather than blowing
immediately on overload, a slow-blow fuse is designed to tolerate an
overload for a certain amount of time, eventually cutting off power if
the overload goes on too long.
I'd consider a slow-blow fuse to a be a good back-up to a timer. Add
one if you want to be extra-cautious, just in case the timer doesn't
do its job. But I wouldn't consider a fuse to be a good replacement
for a timer. The problem with fuses is that they're not precision
devices; they don't blow after precise time periods or when exact
loads are exceeded. You can count on a timer to have precise timing
characteristics; you can't expect the same thing from a fuse. And, of
course, fuses are expendable, and must be replaced if they ever
actually do stop an overload; a timer can be triggered over and over
without any harm done.
for information on selecting slow-blow fuses
and how to predict their timing and loading properties. That chapter
includes a section on pinball coils and how to select appropriate
slow-blow fuses for them.
Pinscape "Flipper Logic" setting
The Pinscape Controller firmware has a Flipper Logic feature that
can be used to provide similar timer protection at the controller
The Flipper Logic feature is implemented in software, so I wouldn't
consider it as bulletproof as a dedicated hardware timer. Software is
just categorically less reliable than hardware. But I'd consider it
far more reliable than software on the PC, since it runs in the KL25Z,
which is a much more isolated and controlled environment. So it's a
good first line of defense, and much better than nothing. If you're
using the Pinscape software, and you don't want to go to the extra
trouble and expense of implementing hardware timers, you can greatly
reduce your coils' exposure to overload by enabling the Flipper Logic
feature for them.
What Flipper Logic does: The feature wasn't actually created as
an alternative to coil timers, but it can serve that purpose anyway.
What it was actually created to do was to emulate the "lift" and
"hold" power settings in real pinball machine flipper assemblies. As
we mentioned above, most real pinball flippers generally use two
physical coils, a high-power "lift" coil, and a low-power "hold" coil,
which are engaged, respectively, as the flipper is first activated and
Some newer Stern machines do the same thing with a single physical
coil, simulating the low-power "hold" coil by reducing power to the
main coil. That's exactly what Flipper Logic does. Flipper Logic
provides full power to a port for a given initial interval immediately
after the port is first activated, and then reduces power to a lower
PWM level if the port is kept on beyond that initial period.
How to use it as a coil timer: The trick to using this as a
software coil protection timer is that the "hold" power can be set to
zero, cutting off power to the coil entirely. When the hold power is
set to zero, it's effectively the same thing as a coil timer. The
only difference is that it's implemented in software rather than as
a dedicated hardware circuit.
How to use for flippers: For flipper emulators, you don't
want the flippers to un-flip when the time limit expires. You want
them to act like real flippers and stay flipped, just with reduced
"hold" power, to avoid overheating. So you want to find lowest hold
power level that keeps the solenoid activated. The required power
level is a function of the specific coils or solenoids you're using,
so you'll have to experiment. Start at the lowest setting, and turn
it up until the flipper stays flipped. Once you find that level, test
it to make sure it doesn't overheat the coil! Carefully monitor the
coil while holding the flipper on - you can just check to see if it's
getting hot to the touch. If it stays cool after being on for a
couple of minutes, it should be safe indefinitely. Note that a little
heating is okay, but it shouldn't ever start feeling hot.
Enabling Flipper Logic: Flipper Logic can be enabled
individually on each output port via the Pinscape Config Tool. Go to
the Settings page and scroll down to the Output Ports section. In
each port, you'll find a little "flipper" icon in the group of control
icons at the right side of the page. Click the flipper icon. This
will let you enter two parameters: the initial full-power time
allotment, and the "hold power" PWM level. To use this as a coil
timer, set the initial full-power time to a suitable value (around
500ms should be good for any momentary device like a bumper,
slingshot, knocker, etc), and set the hold power level to 0.
Remember to click "Program KL25Z" after you've finished making changes,
to save the new settings to the controller board.