The completed Light Phaser to Menacer adaptor

Friday, 10th April 2020

In the last post I was having difficulty with the overall logic for handling the light sensor signal from the Sega Light Phaser and passing that along to the Mega Drive as if it was coming from the Menacer receiver. This involves latching the signal (instead of passing it along directly) and allowing the console to reset it once handled via the TR line. The signal needs to be delayed as well to simulate the delay from the overhead of the Menacer's wireless communications; without this delay the aiming is offset to the left. The signal is also gated using the TL line; I'm not entirely sure why this is done (some form of external interrupt inhibition?) but it's part of the original receiver so I thought it best to implement this for the sake of compatibility.

My main problem was that the CMOS NOR chip I had to hand was not triggering reliably with the signal from the light gun and the RC circuit used to delay that signal; I'd swapped it for a TTL NAND chip which then did work however the logic inputs to the latch were inverted (a simple NOR latch has active-high inputs and a simple NAND latch has active-low inputs) and to fix this would have required a lot of additional logic gates. I ordered an SN74ALS02AN TTL NOR chip and found that this worked very well, so have settled on the following circuit:

Circuit for the Light Phaser to Menacer adaptor

Update (20/04/2020): Please see this post for an updated diagram with a protective 470Ω resistor on the TH output

The four NOR gates towards the top of the circuit diagram are used to gate and invert the inputs from the four input buttons (one main trigger on the Light Phaser and three secondary function buttons on the adaptor itself). This is unchanged from the previous circuit and still works well.

The most important aspect of the circuit at the bottom of the diagram between the two TH pins is the latch made of two crossed-over NOR gates with the output from one going to the input of the other and vice-versa. The two inputs to this latch are normally low (TR from the console is driven directly and normally low, TH from the Light Phaser is normally high but inverted by a NOR gate with its legs tied together). When the reset pin TR goes high the output of the top gate must go low, as a NOR gate will always output low if either (or both) of its inputs are high:

InputsOutput
ABQ = A+B
001
010
100
110

If we assume that the other input to the lower NOR gate is also in its default low state then the two low inputs will result in a high output. This high input travels back to the top NOR gate's other input. This means that even when the TR pin goes back low again there is still a high input on that top NOR gate and so it maintains its low output.

When the Light Phaser sees light its TH output goes low, the NOR gate with its legs tied together inverts this to go high and the bottom input to the lower NOR gate in the latch goes high. This makes its output go low, which travels to the top NOR gate, making both inputs low (assuming TR is still low) and making the top NOR gate output a high which comes back down to the lower NOR gate so that even when the Light Phaser stops seeing light and the other input to the bottom NOR goes back to low there is still a high on the other pin, keeping the overall output low.

Effectively, what this means is that when the Light Phaser sees light the output of the latch goes low, and to return this back to a high the console needs to pulse the TR pin high.

The output of the latch goes to a delay circuit consisting of a resistor, capacitor and diode. This delay is required because the real Menacer introduces a delay between when it sees light and when it triggers the console's TH pin, and games are programmed to compensate for this. Without replicating this delay the aim is offset considerably to the left.

The logic gates are digital devices however still adhere to analogue rules and will treat certain voltages as high or low digital signals when they pass a certain threshold. The delay is implemented by slowing the rise or fall of the logic signal by charging a capacitor via a resistor, so whilst the logic signal at the output of the latch changes very quickly it will take the capacitor a while to charge or discharge via that resistor to a voltage level that is interpreted as a change of state by the following logic gate. The diode is there because we want the signal to be delayed when the gun sees light (when the logic level goes from high to low) but want it to change quickly when the latch is reset (going from low back to high again). When going from low to high there is a higher voltage to the left of the diode than there is across the capacitor to its right and so current can flow quickly through the diode, bypassing the slow resistor and charging up the capacitor quickly. When going from high to low the current can't flow backwards through the diode and so the capacitor has to discharge slowly through the resistor, providing the desired delay.

After this delay comes another NOR gate with its other input coming from the console's TL pin. I'm not entirely sure what this is for, as mentioned above, but it does at least allow us to buffer the signal from the RC delay circuit. The real receiver circuit then uses a final NOR gate with its inputs tied together to invert the latched signal before passing it to the console via a 470Ω resistor, but as I am already using an extra NOR gate to invert the input from the Light Phaser and have therefore used all four gates on one chip I instead use a transistor with a 1K pullup to invert the signal. The Light Phaser uses such a transistor circuit with the same 1K pullup resistor on its TH output so I thought it safe to use it here as it would be electrically compatible. The real receiver also includes 10K series resistors on its inputs from the TR and TL lines – I'm not entirely sure why but in case there was a good reason I thought I should use them here also!

Photo of the enclosure and switches

With the circuit appearing to work well I thought it was time to put it in an enclosure. I found a box that was a comfortable size to hold in one hand – it's about the same overall size of a Master System control pad other than being quite a bit fatter. This allowed me to put the three push buttons down the side of the box so they can all be pressed easily when gripping the box in one hand. The lid of the box has a lip that would interefere with the ability to tighten the nuts for the buttons and so I cut a small piece of acrylic that is slightly thicker than the lip and short enough that the box can be closed with it in place.

Photo of the connectors and cables

Of course, we need something to plug the Light Phaser into and a cable to plug the adaptor into the console with. A good source for these connectors is an inexpensive extension cable, so I bought one and cut it in two so I could use it for this project. After cutting the cable in two I installed strain reliefs and crimp connectors, and definitely remember to do that in that order! Interestingly I've bought a few of these cables recently from different suppliers (and with different moulding designs for the console plug and controller socket) and they've always been about half the length advertised but sold in packs of two.

Photo of the circuit coming together

I normally use pad board to solder my final circuits together. I start by roughly placing the components on the board where I think they should end up going, and when I've got the main parts in place I flip the board over and start soldering them down, using bare pieces of wire to make connections between components. I lay the bare pieces of wire as straight as possible and try to avoid crossing them over; where necessary I'll add a piece of insulation to a wire where it needs to cross another. Once as much of the board is placed in that fashion I'll solder thin wires between the points that can't have direct connections. It may not be the prettiest job (and making corrections afterwards is a bit of a pain) but it's worked well for me and I've found it allows for more compact layouts than stripboard.

The circuit assembled in its enclosure with the lid off

Some crimp connectors were then added to the three push buttons and the whole circuit was installed in the box. Before the lid was screwed down the delay to correct the horizontal aim was calibrated using the 470Ω variable resistor. To adjust it the variable resistor was first set to its smallest resistance value and the tip of the light gun was touched to the centre of the screen. The aiming reticle is quite far to the left by default so the resistance of the variable resistor was then gradually increased to move the aiming reticle in the game right until it sits under the tip of the gun. The aim was then checked at the extreme left and right sides of the screen and if there were any dead zones the variable resistor was adjusted to move the aiming reticle away from the edge of the screen until it had a good range of motion across the entire width of the screen.

Regular grip styleReversed grip style
The two different grip styles for the adaptor buttons

The adaptor can be gripped in two different ways – in the regular position you have direct access to all three extra buttons with your fingers, and in the reversed position you have access to a single button at a time with your thumb. None of the games I've played on the Mega Drive use more than two extra buttons (and usually one of those is the bottom "Pause" button) but it's always good to have options!

I have tested the adaptor with the three Mega Drive games that support the Menacer — the six-in-one pack-in cartridge, T2: The Arcade Game and Body Count and am happy to report that it works with all three. I am not going to be able to test it with any Mega CD games as I do not have a Mega CD or a compatible flash cartridge, I'm afraid to say, but hopefully it will work just as well with those games.

The red and black theme was chosen to match the other accessories and I think it fits in rather well, even if it is a bit boxy-looking. If you are reading this and build the circuit yourself I'd love to hear how you get on!

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