How to wire opto switches & opto switch boards in a FAST Neuron-controlled pinball machine¶

Opto switches are a bit more complex than mechanical ones, but still pretty straightforward. An opto switch is made of two parts:

• An infrared LED (called an "IR emitter") that shines a beam towards an IR detector.
• An IR detector which operates like a switch which is "on" when the IR beam is hitting it, and "off" when no infrared light is hitting it.

The following drawings illustrate this process. The IR emitter is an LED that's always on, nothing fancy. It's a normal LED except instead of its color being red or blue or green, it's infrared. The IR detector is like a normal transistor, except instead of the base leg being the wire where a little current controls the transistor's "switch" effect, it's a light sensor. That transistor is packaged up as an IR detector by being wrapped in a special translucent plastic that only allows infrared light to pass through. (That's why they look dark purple, like the lens covers on your 1990s VCR.)

When the IR light is hitting the detector, the switch is "on" and reported as active.

When the IR light does not hit the detector, like when a pinball or plastic tab is blocking it, the switch is "off" and reported as inactive.

One thing about opto switches it that they're backwards / inverted (or "normally closed / NC" as you recall from the previous guide on mechanical switch wiring). So an opto shows as open/inactive when a ball is there or a target is down or a button is pushed, and it shows as closed/active when it's actually not being blocked. This is no problem, as you can configure the switch profile to be NC in the FAST hardware or the switch settings in your software and then treat optos like any other switch.

Wiring optos is also straightforward. Since each opto switch is a paired IR emitter and IR detector, we'll cover the wiring of each of them separately.

Wiring your opto emitters¶

Opto emitters are just regular LEDs that shine infrared light instead of a color humans can see. Nothing special other than that. They are always on. No pulsing or codes or PWM. Machine on = optos on. Easy peasy.

Of course LEDs come in many shapes and sizes which means infrared LEDs also come in many shapes and sizes. Some are the old-school beefy through-hole designs, and others are teeny-tiny surface mount ones which are attached to little boards. Sometimes you'll wire up a single emitter (like for ramp entry detection), and other times you might have a board with a whole bunch of emitters (like a trough opto emitter board).

As with all LEDs, you need to provide them with the proper voltage so they can consume their correct amount of current. If you just hook up an LED to your 12V power supply it will most like explode instantly. (It's fun, like tiny popcorn.) You need to do some math to calculate the watts and ohms for a resistor to add to the circuit to drop the voltage to a suitable level. And then you need a way to mount that resistor, and ... this is the reason the FAST 4-channel constant current opto emitter board exists:

In a Neuron-controlled machine, you can power this board directly from one of the 0.100" 12V low current output headers on the Playfield Interchange Board. The opto emitter reduces that voltage and current to a level that's appropriate for LEDs. The output pins are labeled "Anode" and "Cathode" (A or K, yes cathode is abbreviated K) which will match the labeling for the IR LED you're using.

Wiring your opto detectors¶

While the opto emitters are just IR LEDs, the IR detectors are essentially just switches remotely controlled by IR light. IR detectors are wired like any other switch, directly to one of the 11-pin switch headers on a FAST I/O board.

The only "catch" with IR detector wiring is that polarity matters. You must connect the "Collector" (C) to the numbered switch input (orange wire), and the "Emitter" (E) to the shared switch ground return (purple wire). (If you get this wrong, it probably won't break anything--it just won't work.) Other than that, treat an IR detector like a normal switch. You can use the same daisy-chained purple wire to connect multiple mechanical switches and IR detectors (to the "E" pin). Just remember that both leads of the IR detector must go back to the same I/O board!

As mentioned already, IR detector switches will appear logically inverted, in that they will be active when they are not blocked, and blocking them makes them inactive.

Opto sender/receiver pairs¶

Many pinball machines use small (maybe 1cm x 4cm?) IR sender and receiver boards with surface mount components. These are nice because they fit into tight spaces, like the sides of a ramp or around a tight loop. FAST Pinball has created these and will soon offer them in the web shop. Reach out if you want some in the meantime!

"Long Range" opto emitters¶

A regular IR LED only had a range of about 3 inches or so. Beyond that, it's too weak to have enough power to trigger the phototransistor. You can, in theory, up the power you're sending to the LED, but that risks them burning out.

The solution for this is to use much higher current, but with a low duty cycle / PWM. (So you pulse the LED at something like 100mA instead of having it on solid at 30mA.) Then you need some circuitry on the receiving side that can detect that specific pattern. (This is exactly how an IR TV remote works, with different pulse patterns for each button.)

FAST Pinball doesn't have a specific board for this for sale now, so if you need it, reach out to me (Brian, brian@fastpinball.com).

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