Project TG-16 - Modding part 2

In addition to the previously discussed audio/video mod, I also added a reset switch and power LED mod to the TurboGrafx-16.  Much to my amazement, the TurboGrafx-16 did not come with either a reset switch or a power light.

Reset Switch
Adding a reset switch is fairly easy, all you need is the switch and two pieces of wire.  Again, here's the pinout of the back connector of the TurboGrafc-16.

In column 22 is a "/RST" switch.  We need to solder one wire onto this point.  The second wire gets soldered onto any ground potential point in the system.  This can be any of the large copper traces around the edge.  You connect these two wires to your switch - but you can't use any old switch.  You need what's called an "off-mom" switch.  This means the switch is off by default, but momentarily the switch is on while you press it.  But as soon as you let go, the switch goes back to off.  I used Digikey part number 509PB-ND.

To mount the switch I drilled a single hole in the side of the case.  The switch works as you'd expect, press the switch and the system resets.  Here's a picture of the final product.

Power LED
I've seen a number of power LED mods for the TG-16.  But I think my favorite was one that lights up the logo on the top.  Since the logo has color in it, the best would be a white LED mounted behind it.

For this mod we need 1) a power source, 2) the LED, 3) a resistor, and 4) some wires.  Most LEDs take between 2 and 3.5 volts to light up, so we need to find a power source that is at least a volt or two above that.  Although ideally not too much higher.  Fortunately the TG-16 has a 5 volt regulator (highlighted below).

Turn the TG-16 on, and using a multimeter identify which pins are ground and 5V.  Then solder a wire onto each.  Here's a closeup of the voltage regulator after attaching the wires.

Next you need to solder the LED and resistor onto these wires to make one big loop (don't forget about LED polarity).  It is true the LED will run off of 5V without the resistor, so why add the resistor?  If you run an LED off of more volts than it was designed for, the LED will 1) generate more heat, 2) burn out much quicker, and 3) may even shine at a different color.  But what value resistor do we need?

To calculate the resistor value we need 3 pieces of info.  First we need to know the total volts being used to power the LED (let's call it Vt).  Second we need to know the voltage of the LED, usually called the Forward Voltage (let's call it Vf).  And finally we need to know the current in amps of the LED (let's call it I).  Using my multimeter I tested the 5 volt regulator and got a value of 5.03V, so this is my Vt value.  For my LED, I chose Digikey part number 1080-1006-ND.  According to the datasheet on this LED, it has a forward voltage of 3.2 (Vf) and a current of 20 milliamps (I).

Our total voltage is 5.03, but we only want 3.2 for the LED,   So we need to "burn off" 1.83V in the resistor (let's call that Vr).  We only need one formula, V=IR (volts equal amps * resistance).  Vr = I * R.  We know Vr and I, so the formula becomes 1.83 = .02 * R.  When you divide that out you end up with 91.5 ohms.  So you need to use a resistor close to that value.  I used a 91 ohm resistor.

Next solder it all up.  Be sure and cover the exposed LED and resistor leads with heat shrink or electrical tape to prevent shorts.  I then taped the LED onto the top of the metal RF shield around the circuit board.

The final step is to enlarge the hole for the light to illuminate the underside of the logo.  If you use a razor blade you can very easily remove the TG-16 logo.  I then used both a Dremel and razor blades to cut a square hole in the case.  Be sure to leave a lip around the opening so you can glue the logo back down.  Here is a picture of the opening.  Notice the LED inside the hole.

The last thing to do is glue the logo back down.  It looks pretty cool when you turn the system on.

This was a fun project to work on.  I have a nicely modded TG-16, and with the exception of the reset switch in the side it looks completely stock.  Now it's time to turn it on and enjoy some great retro games!