In this third article, Todd "Pinted" Andersen, President of the TCFPA moves on from looking at lamps and LEDs to explaining how to control them with a simple circuit.

Automated & Isolated -

Controlling your pinball mods safely and automatically

I thought of this idea when I read about all of the pin-toppers and pin-mods on RGP. I realized that some circuits could be made that didn’t use the already over taxed pinball power supply or dog down some reset line on the game’s MPU.

I grabbed a few electronic parts out of my spare parts bin. From inception to creation, I produced this neat little circuit, and tested it in a game in just a few hours.

The circuit consists of just 7 - components. There are 3 - resistors, 1 - diode. 1 - relay, 1 - circuit board, and 1 - transistor. In the pictures below, the big blue resistor and red LED should be familiar from a previous article, but are not part of the circuit. They are a test replacement for the device the circuit is going to power. The operation of this small circuit is much like when you turn on your television. For the circuit, the ambient light of the general illumination lighting inside your game is like the action of you using the infrared light from your remote control to turn on your television. In the context of the circuit, the television is the relay. And the images and sounds you enjoy are akin to the device you are trying to control in or on your pinball machine.

Photo taken with room light on.
Photo taken in dark room with flashlight.

As stated above, this little circuit does not rely on any voltage or signal from the pinball machine’s boards. The circuit may either be powered from the auxiliary service jack in most pinball machines or from a wall outlet. The control for the circuit is the light emitted from the general illumination. Because the circuit uses this light, whatever you are powering with it turns on or off when the general illumination lights turn on or off. So, the add-on turns off when the game is off or during the times of ‘black out’ during game play. The shared black out effect of the pinball machine and add-on makes whatever you added to the machine seem more like part of the game. This circuit uses a relay. The use of a relay isolates both the circuit and its load from the game. Lastly, the board that the relay circuit is built on is very small. It will fit under the playfield of almost any pinball machine - electromechanical or solid state. And, the circuit’s simple design allows for substitution of all its parts.

I take no responsibility for anything that happens to any pinball machine utilizing this little circuit or to anyone who attempts to build the circuit. The circuits that I have installed in several different pinball machines have worked flawlessly. But, due to the unknown condition of the wall power for the circuit’s converter and game the circuit is to be installed in, I cannot guarantee that the circuit will work flawlessly for you. Any add-ons or conversions you choose to do to your pinball machine(s) are your responsibility.

The little light-sensing relay can be used to control many pinball add-ons or that require a small voltage at a low level of direct current.

Here's a picture of the circuit powering an under playfield strobe light in “Attack from Mars”.

Inside cabinet photo of “Attack from Mars”. Courtesy of "LTG".

The under playfield strobe turns off when the game is turned off.

Inside cabinet of “Attack from Mars”. Courtesy of "LTG".

The under playfield strobe also turns off when the game goes into black-out mode. Below you only see the strobe from the playfield’s “mother ship”.

Upper playfield of “Attack from Mars”. Courtesy of "LTG".

The circuit is controlling a skull topper on “Monster Bash”.

Backbox of “Monster Bash”. Courtesy of "LTG".

The same skull turns off when the game is off. But, the converter is still plugged in.

Backbox of “Monster Bash”. Courtesy of "LTG".

The circuit is used to control this small skull-mod in “Sorcerer”.

The same skull turns off when the game is turned off - without unplugging the circuit.

The barely visible skull also turns off when the game goes into black-out mode.

I found room to install the small circuit on the crowded playfield of “Addams Family” shown below. Notice the two green glowing lights on the right side added-on near the “swamp”.

The circuit was installed to add light to Addams’ swamp. The swamp now has a cool but spooky green glow.

This picture shows that the newly added swamp lights turn off automatically when the game is turned off.

The circuit is even small enough to fit in the limited space of a “Dracula” playfield.

I have no intentions of producing this little kit. And I do not stock any parts. This circuit was conceived for the pinball community. Most pin-heads should be able to make their own Light Dependent Relay - LDR for about $20.00. If anyone wishes to reproduce this circuit or produce a kit, please feel fee to do so.

As I’m always telling RGP’ers to build at least one electronic kit before they work on their own pinball boards for the first time, I recommend that you make this simple little circuit.

With that in mind: a parts list, parts suppliers, and a schematic are listed at the end of this article. I did NOT list the power supply for several reasons.

First, the input voltage for the transformer will vary from country-to-county or even region-to-region of the same county. The input voltage for the circuit is the same as the voltage required to run your add-on. The circuit’s designed working voltage range is: positive 9 to 18 - Volts of Direct Current. So a “9 - Volt transistor” battery could be used to power the circuit. But, small AC / DC converters, sometimes called “wall warts”, work wonderfully for this purpose.

Which brings us to the second reason for NOT listing a specific voltage source, current requirements. Pick an AC / DC converter that will handle twice the estimated current consumed by your add-on. For instance, two #47 bulbs in series take about 12 Volts to operate at about 1/2 Amp. So, for this application, choose a 12 VDC, 1 Amp converter.

Most LED add-ons seem to be set up to run off of +12VDC and usually have 2 - 12 LEDs. Each LED needs about 0.30 mA of current. The relay I used can easily switch these small loads. The relay will even switch small inductive loads such as small DC motors. Remember to choose a relay and power supply that match. For example, I used a +12VDC (1 Amp): power supply, relay, and add on.

Start by gathering all of your parts and prepping your circuit board. First, cut part of the trace in the upper right corner. Then, use an eraser to clean the board.

Notice the shiny left side.
Notice the partially cut trace.

Remember that with the exception of the resistors, the parts in this circuit only go into the circuit one way. And, be careful to remember positive and negative for your input and output power connections.

All parts get installed on the bare side of the circuit board. Start out by installing the: relay, diode and first three jumpers. Excess component leads or a paper clip can be used to make the jumpers.

Install the transistor next. Then, bend both leads of the 1K Ohm resistor down at 90° right next to the resistor’s body. This resistor will “lie down” when installed on the board. Now, bend only one lead of the 10K Ohm resistor down at a 180° bend to this resistor’s body. Bend this lead right next to the body of the resistor. Keep he other lead straight. This resistor will “stand up” when installed on the board.

Then, install the two resistors. The body of the 10K Ohm resistor should go towards the transistor and the bare lead of the 10K Ohm resistor should be mounted away from the transistor. And lastly, install your wires.

Notice the three jumpers.
Notice the jumper made from a cut lead.
Notice the two added silver jumpers.

All components are soldered.

The board has been laid out to keep wire combinations together. Wires were added for: power in, power out, and sensor connections. The INput power connections are the RED (positive) and BLACK (negative) wires near the BROWN and RED SENSOR wires. The OUTput power connections are the RED (positive) and BLACK (negative) wires on the opposite side of the board. You may want to use different colored wire to keep track of INput and OUTput. Be especially sure you keep track of both of the positive and negative sets of wires. This will help ensure that you apply the correct polarity to your: pin-toppers, add-ons, or pin-mods.

Use your small, flat screwdriver to “feel” in between the adjacent traces that you soldered. The screwdriver may also be used to break any solder bridges that may have inadvertently formed. Large bridges may have to be cut with the small knife.

Double-check all of your work. Be especially mindful of the power input and output connections. Then, clean up the solder side of the board. I’ve found that 90% isopropyl alcohol - IPA and a toothbrush work best for cleaning solder flux residue off of circuit boards. Allow the board to dry thoroughly. Then, continue to the last step.

And lastly, test the circuit out of the game before you install it. Apply power to the circuit. You will know that the circuit is probably working when you hear the tiny “click” of the relay. If you hear the click, try testing the add-on or pin-mod out of the game first.

If the circuit does not appear to work correctly, remove power, go back, and recheck all of your work. Ensure that you have installed all of the components in the right holes and that all components are facing in the right direction. If not, it may be best to start over.

This little circuit is very sensitive. If you can’t get the circuit to toggle the output power on-and-off, try making the room completely dark. Then, shine a flashlight on the light dependent resistor - your sensor. A small click should be heard from the relay and the device you are controlling should turn on. Move the flashlight’s beam away from the sensor and the device should turn off. A small click should also be heard from the relay. If you got these results, the circuit should be working properly.


Pictured below are most of the tools and supplies I used while making the relay circuit.

TOOLS LIST (clockwise)
1) Soldering iron
1a) Selectable / replaceable tips
1b) ESD safe
1c) Temperature controlled

2) Non-insulated wire (thin gauge)

3) Small Straight Screwdriver

4) Solder

5) Eraser (pen), a standard pencil eraser will work

6) Insulated stranded wire (thin gauge), stranded wire can take more flexing than solid or single strand wire

7) Small utility knife, a box knife or an “X-Acto” knife will also work well for cutting the trace or solder bridges

8) Wire Stripers

9) Side cutter

10) Needle Nose Pliers, a large tweezers will work as well

Remember to turn the power off or even unplug any machine that you are working on, including installing this little circuit in a pinball machine.

I secured the small circuit board with just one small wood screw in two opposite corners. DO NOT OVERTIGHTEN the screws or damage to the circuit or your game may result. Only tighten both screws to the point where the board is flush with the playfield and it is not loose; it does not wiggle when you apply slight pressure with your finger.

For the circuits I built, I chose to add connectors for the: input power, sensor, and output power. But, you can use wire-nuts or solder the wires directly. I recommend the use of wires to extend the: input power, sensor, and output power. For small DC loads, there is no need for heavy gauge wires. Thin wires can be easily routed. But, be sure that the wiring itself does not interfere with the game and that the sensor can be installed at the best location for picking up the light from at least one general illumination bulb.

Because of the added lengths of wire, the circuit works especially well for pin-toppers. The converter may be plugged into a nearby standard wall outlet. The circuit can be mounted behind the game’s backbox. And, the sensor can be carefully inserted through one of the vents in the backbox. Remember to run the wires so they does not interfere with the game or short anything out. Some sensors have a metal body. Ensure that when you place your sensor, it does not short anything out or interfere with the game in any way.

The “Electronic Goldmine” ships parts all over the world.
Small SPDT 12VDC Relay (HB1-DC12V) $1.49
TIP31C Power Transistor $0.58
IN4005 Rectifier Diodes (Package of 10) $1.00
1K 1/4 watt Resistors (Package of 100) $3.50
10K 1/4 watt Resistor (Package of 100) $3.50
1/4W Resistor Kit (470 ohm to 39K) $10.95

"Radio Shack"
Dual General-Purpose IC PC Board $2.29
CdS Photoresistor (Package of 5) $2.79
IN4005 Rectifier Diodes (Package of 10) $1.00
1K 1/4 watt Resistors (Package of 5) $0.99
10K 1/4 watt Resistor (Package of 5) $0.99

D1, any rectifying diodes can be used - 1N4004, 1N4005, 1N4006, or 1N4007.
IC Board, any small one-sided copper clad board should work.
R1, several will work. “Radio Shack” sells a pack of five. Use the one that works best.
R2, a 1K Ohm resistor rated at 1/4 or 1/2 - Watt may be used.
R3, a 10K Ohm resistor rated at 1/4 or 1/2 - Watt may be used.
REL, any small DC relay that matches your converter voltage can be used.
T1, any TIP 31 transistor may work - TIP31, TIP31A, TIP31B, or TIP31C.


Two small, short wood screws; I recommend 1/4” Hex-drive or Phillips head screws.

The light dependant resistor senses light and applies power to the transistor; which, in turn, turn on the relay. The relay switches the input power that is powering the control circuit to also apply power your device.

D1, is a protection diode. It protects the entire system (pinball machine, converter, circuit, and device) from the large voltage spike generated from the coil in the relay when its voltage is removed. This spike is called counter electromotive force - CEMF.

R1, is the Light Dependant Resistor. It’s value decreased in light and increases in darkness.

R2, is used to protect the transistor and help ensure that it doesn’t go too far into saturation.

R3, is the pull-down resistor. It determines the set point for the transistor to turn on.

R1, R2, and R3, together make up a voltage divider bias for the transistor. This is an electrical “ladder”. At a certain position on the ladder, the transistor turns on. The level of ambient light determines the actual dynamic level.

REL, is the relay. As the designed operating voltage of the circuit is from 9 to 18 - Volts of Direct Current, for my circuit, a small +12 Volt Direct Current relay was selected; the contacts of which can handle 1 - Amp of Direct Current. The relay is small enough to fit on the tiny circuit board. The relay uses the input power and switches in the same power, via a parallel path, to the device. Other relays may be substituted but the circuit may have to be slightly modified (laid out) to match the new relay’s connections.

T1, the transistor is a TIP 31C. Any transistors in this family may be used. In fact, due to the simplicity of the circuit’s design, almost any NPN transistor that can handle at least 40 - Volts and 3 - Amps should work. The transistor is just used in switching mode.

Unless otherwise noted all pictures and schematics courtesy of Pinball Renaissance - TCFPA. Also unless otherwise noted, all pictures taken with a Sony 3.3 mega-pixel MVC-CD300 digital still camera set to take fine pitch JPGs in: auto, inside, micro mode.


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