How Solid State Flippers Work

Solid State Flippers

A. Rebuilding Flippers
B. How Flippers Work
C. Serial Coils

D. Parallel Coils
E. Computer Controlled Flippers
     Solid State Flippers

  1. Fliptronics
    a) How Fliptronics Works
    b) The Circuit
    c) Optical Flipper Switch
    d) Troubleshooting 
    e) Optical Flippers Controversy
  2. DE/Sega
    a) The Circuit
    b) Troubleshooting
  3. Sega/Stern Whitestar
    a) The Circuit
    b) Troubleshooting
  4. Contact Information / Feedback
  5. External Links

A More Detailed Explanation of How Computer Controlled Flipper work

Since the beginning of flipper pinball games (1947) the flippers have been a weak point that required chronic maintenance.  This made pinball games expensive to maintain and one of the reasons that video games were overtaking pinball in popularity.  In the early 1990’s, pinball companies finally found a way to address this weakness through the introduction of computer controlled flippers.

Unlike earlier flippers, only low power traveled through the flipper switches.  The EOS switches also had only low power and their function changed so that even if the EOS switches were broken the flippers would continue to work (almost) normally.   Finally, optical switches were introduced which may (or may not) be even more reliable than the mechanical switches.

With the introduction of Solid State Flippers, finally the only weakness was the wear and tear of the linkage and the coil stop, plus the dirt issue that never went away.

Fliptronics Parts

Additionally, the computers could control the power of the flippers.  Stern later allowed operators to adjust flipper power to compensate for wear (and, perhaps make the game harder to beat) and Jersey Jack Pinball introduced weaker flipper power during certain gameplay situations.

Fliptronics – Williams / Bally WPC

Fliptronics flipper switch
Low Power Flipper Buttons.

Perhaps the most famous of the solid state flippers is Fliptronics.  It was introduced by Williams (WPC – Bally Williams) in 1992 in The Addams Family which had Fliptronics 1 – the only game produced with version 1.   All later games have Fliptronics 2.  The major difference between these is that Fliptronics 2 has the flipper power supply build in.  All WPC games (WPC89, WPC-S, WPC95) from 1992 on have Fliptronics flippers.  There are two different Fliptronics 2 boards (A-15472 and A-15472-1) and the difference between them is minor – the 50 V power supply 100uF capacitor C2 is removed in the later versions.  These boards can be used interchangeably.

Simple low voltage EOS switch used in Fliptronics.

In addition to using low power switches that (almost) never wear out, Fliptronics EOS switches are different in that they start out open (when the flipper is off), then close when the flipper is activated.  This is completely opposite of what most people expect and can lead to confusion when replacing and adjusting flippers.

In earlier mechanical flippers, the EOS switch turns on the low power (hold) winding, keeping the flipper open with lower power.  With Fliptronics, the EOS switch plays a completely different role.  When the switch is closed, the computer knows that the flipper is open all the way and applies only low power.  But if a force strong enough to push the flipper backwards – such as capturing two pinballs during multiball – the EOS starts to open.  The computer reacts by applying greater force and the flipper then opens back up.

Fliptronics EOS Switch
Fliptronics EOS Switch
Fliptronics EOS Switch
Fliptronics EOS Switch

Just like earlier mechanical flippers, all Fliptronics have two coil windings: 1) power and 2) hold.

For WPC89 and WPC-S, the Fliptronics boards are located in the upper left of the backbox.  For WPC95 the Fliptronics circuitry is incorporated into the driver board.  If confused which version your game is, check the listing in the Internet Pinball Database.

Fliptronics Board – Upper Left

On WPC89 and WPC-S, the flipper switches are wired to the Fliptronics board.  On WPC95 games, the flippers are wired to the CPU board.

Each flipper has two circuits, one for power and the second to hold the flipper.  The power circuit is more robust than pre-fliptronics in that the TIP102 transistor is a pre-driver for a higher current TIP36.  The hold portion of the board has just the TIP102 transistor which should be adequate to hold the flipper open.  

Non-Fliptronics EOS Switch
Non-Fliptronics EOS Switch
Non-Fliptronics EOS Switch

Also, the fliptronics board has a built in (snubber) diode to protect the transistors from the voltage induced as the coil is turned off (back EMF).  In theory, that means that Fliptronics flipper coils do not need to have a diode on the coil.  This coil frequently breaks and fails from vibration of the coil opening.  However, in practice, all coils used on Fliptronics flippers seem to have these diodes installed.

How Fliptronics Works

Fliptronics layoutJust like the mechanical design, power is applied to the coils all of the time.  The flipper is activated when the coil is connected to ground.  In a mechanical flipper, the coil is grounded through the flipper button and the EOS cuts the power back.  In Fliptronics, the computer turns on the flipper, then cuts the power back.

When the player hits the flipper button, a signal is sent to the computer to activate the flipper.  The CPU communicates through the ribbon cable to the Fliptronics board.  Initially, the higher power circuit is turned on.  When the computer receives the signal that the EOS switch closes, the computer turns off the power circuit and energizes the hold circuit.  If the computer never receives a signal from the EOS switch, the computer times out the power circuit and energizes the hold circuit.  This allows the flipper to work almost normally even if the EOS switch is broken.

 The Fliptronics Circuit
Fliptronics circuit
Fliptronics WPC89 and WPC-S

The Fliptronics board has circuits to control up to four flippers.  If a game has less than four flippers, some of the circuity is not used and could be used to control other features such as magnets.

Each of the four circuits have two parts: 1) power and 2) hold.  The power is used initially and only briefly.  Since the coils draw a large amount of current during the power surge, a second more powerful transistor, a TIP36C is used.  The TIP36C is controlled by a TIP102 – the same transistor that controls other game coils.  The TIP102 is controlled by a 2N4403 pre-driver, which is in turn controlled by an IC U2 74374. 

IC U2 is then turned on via U5 which is activated by the flipper switch.  Then the CPU turns off the power circuit and turns on the hold circuit, as long as the player holds the flipper button.

While the EOS switch is not as important, it does play a roll.  If enough pressure is placed on the flipper, such as catching several pinballs during multiplay, the hold circuit may not have enough power.  The flipper then starts to close, the EOS switch opens up and the computer turns back on the power circuit long enough to hold up the flipper.

The Optical Flipper Switch

Williams decided to replace many of the mechanical switches with optos.  Initially, these optical switches were more reliable and did not require adjustment.  But these pins were manufactured decades ago and the LED transmitters get weak and burn out.  Additionally, the optos get dirty and occasionally the receiver goes bad.  Additionally, Williams used a wavelength of LED that is less reliable than the optos used today.  This leads to potential problems with flipper opto switches. 

Mechanical flipper switches are easy to diagnose and replace.  Many owners have removed the optical flippers and replace them with mechanical switches.  

Weak or Non-Working Flippers

When Fliptronics flippers are weak, the problem is almost alway mechanical – rebuild the flippers.  Other issues can cause flippers to be weak.  They are:

1) The power supply.  Check each tab on the coil to insure that +50 VDC is present. Note that this may test as high as +70 VDC because the power is not under load.  If voltage is low, it is possible that the bridge rectifier on the power supply is bad.

2) Connectors and solder points.  Each connector and solder point has the potential to act as a resistor and sap the power.  For WPC89 and WPC-S, the AC power comes into J901 and out J907.  Look for burned or damaged connections and poor solder on the header pins on the board.  The power to each flipper is daisy chained.  Look for strands of wire broken.  The power then comes back into the Fliptronics board at J902, another potential problem.

3) The coil can be damaged.  Check and compare resistance of each coil (power off!) to the others.  Unplug J902 and J907 before testing resistance.  Check both the power and hold sections separately. Unlike non-Fliptronics flippers, the EOS switch does not short the hold portion of the coil.

4) If the flipper does not work at all, try momentarily shorting the tab of the TIP102 (not TIP36C) transistor, for the power circuit, to ground.  If it does fire, then the TIP102 might be bad.  If it does not work, the TIP36C might be bad.

5) If the flipper does not fire when shorting the TIP102 to ground, try momentarily shorting the wire at the connector to the Fliptronics board.  If it fires, then it is likely that the TIP36C is bad.  If it does not fire, then it is a wiring or power supply problem.

If replacing the TIP36C, it is a good idea to replace the TIP102 also.

Optical Flipper Controversy

Opto Flipper BoardThe optical flipper board caused my weak flippers.  Cleaning the opto flipper board made the flippers stronger.

The opto flipper board is digital.  It is either on or off.  The opto flipper board cannot (?) cause weak flippers. At least not than we can figure out – yet experienced pinball repair people continue to insist that opto flippers swithes can cause weak flippers.

Sine in – Square out

The flipper switch optos feed to an IC LM339.  LM339 either turns on or off.  It cannot turn partially on.  As a test, we installed a variable resistor (a ‘pot’) on the output of the opto flipper board.  We adjusted the signal down until the flipper stopped working.  We then slowly increased the signal.  At one point, we could get the flipper to ‘twitch’.  But above that, the flipper worked perfectly.  Below it did not work at all. At no point were we able to get a ‘weak’ flipper.

Technical mumbo jumbo:  In looking at the datasheet,  the LM339 can take a sine wave on the input, the equivalent of a weak to stronger signal, and the output is a square wave – or on/off.  It is never weak.

I replaced the flipper board, or cleaned the optos and my flippers are stronger!

We don’t understand that and have been unable to reproduce it.  It is always a good idea to clean the optos, no matter what one believes on this topic.

I replaced the optos with traditional flipper switches and they are stronger.

It maybe a good idea to rip those opto switches out and put in mechanical ones.  Cheap and easy to do.  But….  we give up.  ?

Data East / Sega Solid State Flippers

DE - Sega flipper parts
DE-Sega Parts

Data East beat Williams to the market with the first pinball solid state flipper system.  The first solid state computer controlled flipper was introduced 1989 by DE in Robocop.

Flipper coils used in all but the earliest pins have two windings on a single coil.  One is for power and the second is for hold.  Even the ‘modern’ Williams Fliptronics kept the dual coil design.  But Data East was able to develop a system that required only one coil for both power and hold, by changing the applied voltage.

DE Flipper theory of operation
DE/Sega Flipper Theory

Data East used the pinball machine’s circuitry to control the timing of the high power to the coil. After a short period of time (generally 40 milliseconds), the computer cut the 50 VDC back to 9 VDC to hold the flipper up.  Like Fliptronics, the ultimate advantage of this system is that lower power only goes through the flipper and EOS switches, greatly extending their lifetime.  Additionally, the EOS switch could stop working and the flippers would continue to operate normally – at least in theory.  Data East made a mistake in their first few games (Jurassic Park, Last Action Hero and Tales From The Crypt) and those flippers will not operate properly if the EOS switches are broken or out of adjustment.   Later Data East machines did not have this problem and the flippers could continue to operate with defective EOS switches.   How to modify early Data East solid state flipper boards.

DE/Sega Flipper
DE/Sega Flipper – EOS Switch is closed at rest

Unlike Fliptronics, the EOS switch in DE/Sega/Stern flippers are normally closed, just like regular flipper systems.  This EOS switch serves a similar function in that it tells the flipper board if something is forcing the flipper back closed.  If the flipper is pushed down (by trapping two or more pinballs, for example), then the EOS switch closes and the flipper board sends another 40 millisecond power pulse to open it back up.  For this function to work properly, the EOS switch must be in working order and properly adjusted so that it closes when the flipper is moved back ~1/16″ of an inch.  

No EOS Switch on upper flipper
No EOS Switch on upper flipper

Data East continued to improve on the circuit and this process was continued after DE was acquired by Sega in the mid-1990’s.  Click on the image above for detailed theory of operation in the Data East / Sega solid state flipper.

The DE/Sega Flipper Circuit
Flipper power supply
Flipper Power Supply

Unlike Williams, Data East / Sega had at least 5 different flipper boards from MPU Version 3 and 3b.   On all (? be sure to check the schematic for your game!), the 50 VDC for the flippers is generated on a separate power board (PPB-1) which is a different board from the power supply board.  If there is not any power for the flippers, check the fuses on this board as well as the bridge rectifier BR-1.  

Block diagram for DE/Sega flippers
Block diagram

The 50 VDC for the power stroke is then connected to the flipper board at CN2, pins 8 and 9.  The hold power is supplied via 8 VAC (!) applied to CN2 at pins 6 and 7.  The flipper board converts this to DC through a two diodes for each flipper. 

Unlike all other games, the 50 VDC is applied to the coil only when the flipper is energized.  The coil is grounded via a flipper enable relay located on the CPU board.  To test for presence of 50 VDC, it will be necessary to check it on the flipper board at CN2 pins 8 or 9, not the coil. 

Circuit Diagram
DE/Sega Flipper Circuit

The circuitry for the DE/Sega flipper circuit is far more complex than the Williams Fliptronics board.  The circuit, as described in the DE/Sega Flipper Theory photo consists of IC U1 and U2 which are the timing circuit for the power stroke.  Q3/Q10 TIP36C ultimately turns on and off the 50 VDC to the coil. 

50V & 8V Flow
50V & 8V Flow

The hold power comes from the 8 VDC which is rectified from the 8 VAC supplied to this board via D4, D5, D9, D10.  If the flipper is not held up, check the AC and DC voltages.  Q6/Q12 and SR1/SR2 apply the 8 VDC to the coil.

Troubleshooting DE/Sega Flippers
Flipper board
Flipper board is located in the cabinet on the left.

If the flippers do not work at all, check the operation of the relay on the CPU board.  This relay grounds the coils (the opposite of what happens on Bally / Williams pins where the relay applies the voltage to the coils).  

Check that there is 50 VDC coming into the flipper board at CN2-8 & 9.  Check there is 8 VAC at CN2-6 to 7 (measure AC across these two wires).  Also check the fuses and their fuse holder clips.  Check for ~8 VDC at the band side of diodes D9/D10 and D4/D5.  On early games, defective EOS switches can cause issues, but always clean and adjust the EOS switches anyway.  Note that there is not an EOS switch on DE/Sega upper flippers.

The LEDs on the boards confirm that the flipper buttons are being pressed. 

Test the TIP36 transistors using the normal transistor / diode test.  Values of 0.5 to 0.7 are good.

Always check and compare the flipper coil resistances.  We like to unplug CN2 when performing this test to insure the circuit board does not affect the reading.  

If the power works, but the hold does not, it is the 8 VDC portion of the circuit.  If the power does not work, it is (hopefully) the TIP36C transistors.  In the low power portion of the board, it is possible to use a digital probe to compare the working side vs. the non-working side of the circuit.

Sega / Stern Solid State Flippers

Sega/Stern Flipper
Whitestar Flipper

In the mid-1990’s, Sega introduced a new CPU board system called Whitestar.  When Stern purchased Sega, they continued production with the Whitestar system for several more games.  The first two, Apollo 13 and Goldeneye used a separate flipper board located in the cabinet like the DE/Sega games did.  From Twister on, the flippers were controlled by the power driver board FETs and the CPU.

Twister Flipper Parts
How It works

While the DE / Sega system had a 50 VDC and a 8 VDC power supply, the Whitestar system makes use of only one power supply,  50 VDC.  The power stroke is a 40 msec pulse like it was on the DE/Sega board; but the hold circuit is now also 50 VDC, but just for 1 msec every 12 msec.  This short burst of full power mimics the hold of an 8 VDC power supply because it is a very short duration.  The 50 VDC is generated in the power driver board and is also used for the other coils, but the flippers have their own fuses.  One fuse is on the power driver board and goes to both flipper coils.  The other two fuses are located near the flippers.  Note that there may be some variation of this setup from game to game.

Lord of the Rings flipper wiring
Flipper wiring (LOTR)

The EOS switch plays the same role as before:  it detects if the flipper is being pushed too much and tells the computer to increase the power.  The flipper can operate with a defective EOS switch.

According to Sega / Stern: The EOS switch should be adjusted so that it opens 1/16″ when the flipper is open.  During this period of time, the computer applies the short pulse of 1 msec ever 12 msec.  If the flipper is pushed down, so that the EOS switch closes, the computer then applies 50VDC for 40 msec.  If the EOS switch is missing or misadjusted, then the flipper could be forced closed while trapping pinball(s) with the flipper.

The flipper switches are dedicated switches to the CPU coming in on CN6.  Closing the flipper switch grounds the connection to the CPU.

Stern Flipper Software Upgrade
Sega / Stern Whitestar

Note that later Whitestar games, such as Lord of the Rings, had a software update allowing the operator to change the strength of the flippers by modifying the timing of the power stroke.  These are Standard Adjustments 53, 54, 55.  Stern partially explained the purpose of these adjustments, so we did our best to interpret them.

Flippers will now fire when the button is pushed until the End-of-Stroke (EOS) Switch closure is seen. When EOS is seen, continue firing for the amount of time in Std. Adj. 55. When this time is expired, if the minimum time has not yet been met (Std. Adj. 53), then keep firing until it has. Otherwise switch to hold power. • If the flipper has been firing and it reaches the maximum (Std. Adj. 54), then switch to hold power. • If the flipper caves-in (EOS re-closes) while the button is still held, then refire the flipper at full power. Switch to hold power as soon as EOS is seen, no minimum times are enforced in this situation. • If the flipper caves in several times on the same single flipper button press, then stop refiring at full power and just stay at hold. • If the flipper fails to open the EOS switch several times, then assume it isn’t working and fall back to a “safe” fire time so as not to burn up the coil. Three (3) new STANDARD ADJUSTMENTS were added to control the flippers.

To us, it seems as if the EOS switch now plays a much greater role than it used to, which it just detected the flipper being forced closed.

53 FLIPPER ADJ 1: Set between 05 to 40. Default is 10.
This adjustment sets the minimum fire time for a flipper coil in milliseconds.

When this time [Adj 55] is expired, if the minimum time has not yet been met (Std. Adj. 53), then keep firing until it has.” This appears to us to set a minimum time to fire the coil at full power, when the switch closure occurs.  Oddly, if Adj 55 is set to zero or any value less than Adj 53, wouldn’t Adj 53 always control?

54 FLIPPER ADJ 2: Set between 40 to 60. Default is 50.
This adjustment sets the maximum fire time for a flipper coil in milliseconds.

This allows the operator to adjust how long the power stroke is applied, no matter when the EOS switch opens.  Interestingly, the default is 50 msec when it used to be 40.  This is especially beneficial if the EOS switch breaks.

55 FLIPPER ADJ 3: Set between 0 to 10. Default is 3.
This adjustment sets the amount of time to fire the coil after the EOS switch is seen in milliseconds.

Flippers will now fire when the button is pushed until the End-of-Stroke (EOS) Switch closure is seen” is confusing.   If this were to mean closing, then this seems to us to apply if the flipper is forced closed due to a force such as trapping a pinball.  The amount used to be 3 msec, which is the default here.

The Sega/ Stern Whitestar Flipper Circuit
Whitestar Flipper Diagram
Circuit Operation

This is the simplest circuitry of them all – it works like any of the other coils on the game.  The 8A TIP102 (or TIP120) are no longer used.  Instead, a more robust 100V 20A  MOSFET STP 20N10L is in its place.  IRL540 is a direct substitute.

To fire the flipper, data is sent over from the CPU which tells IC U2 to turn on.  The signal comes out to the FET transistor controlling the flipper.  The FET turns on, completing the circuit to ground. 

Circuit layout
Circuit Design

Note that there is not a snubber diode on the I/O board.  If the diode on the coil is broken or open, the FET is likely to blow.   The transistor used to activate the flipper coil may vary from machine to machine, so it is important to look at the coil chart table to identify which transistor is used.

Troubleshooting Sega/Stern Whitestar Flippers
transistors that control the coils
Control Transistors

If both flippers do not work, check the fuse located on the power driver board. Check your manual for the correct value of the fuse.

If one flipper does not work, check the fuse to that flipper.  The fuse should be located underneath the playfield in the vicinity of the flipper.

twister flipper parts
Flipper parts (Twister)

Check for voltage present.  Note that this may test higher than 50 VDC when it is not under load.

If the flipper still does not operate, it is possible to check the circuit the same way that coils are tested in older WPC pins: briefly short the tab of the FET to ground.  The flipper should activate.  If it does, it is likely that the FET has failed.  Replace it with an IRL540.  Important: do not use a TIP 120/102 transistor.

If the flipper activates (locks on) when the game is started, then the FET is likely shorted.  Replace it with an IRL540.

If the FET has failed, be sure to inspect the diode on the coil.  If the diode is broken, missing or open, then the FET will blow again.


Comments, including suggestions, improvements, errors, etc. are welcome (see below).

If you have a specific question about your game that does not directly apply to solid state flippers, please see our FAQ section.

External Links

DE/Sega/Stern History

Copyright 2006 – 2021, all rights reserved.

2 thoughts on “How Solid State Flippers Work”

  1. Hi,
    If I am building my own pinball machine from scratch, do you see any drawback to having the EOS switch go back to simple gates which directly switch off the Power Coil and switch on the Hold coil, given my using the Williams driver approach/schematic? I could directly use the Button logic level output to make it all happen, keeping my microcontroller out of it (although letting it disable the path when not in play mode).

    1. Congratulations on building a pinball machine from scratch! What a cool undertaking.

      We believe that the main reason that pinball manufacturers introduced computer controlled flippers was to increase their reliability. With non-computer controlled, the EOS switch wears and the flipper loses power. And a EOS switch that does not open means a burned coil.

      Computer control removes the wear from the switches and potentially allows computer adjustments, something that Stern added with Lord of the Rings. With computer controlled flippers, it is usually the linkage and coil stop that wear first. Plus the EOS switch can break and the flipper will still work.

      With home use pinball, wear and tear is less of an issue. So using non-computer controlled flippers (which is what we think you are asking) should not be a significant disadvantage.

      We would suggest using the pre-Fliptronics WPC design. The parts are readily available, the exterior spring is more reliable, and the parallel coil combined with the capacitor on the EOS switch means a long life. We also suggest that you use a 50 V DC power supply for the flippers.

      If you buy the assembly from Pinball Life, be sure to add the capacitor to the switch.

      Best of luck. Please share photos of your progress.

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