Star Trek Red Alert Audio Trigger

For those following my Sega Star Trek Captain’s Chair restoration; I replaced all the bronze plexiglass with some newer Transparent Grey Acrylic which did not have scratches. One of these peices was featured in the Restoration Worklogs at or where I custom laser etched schematics of the Klingon D7 crusier and the Nomad Probe:

At the time; I predicted that the etch would not show up well once it was installed in the chair:

Sadly; that prediction came true. As a result; I decided I wanted to find a way to lite the etch with LEDs. SeaWolf on KLOV mentioned to me that he planned on lighting his Chair with some red lights to give it a “battle stations” feel while playing… I told him at the time; I’d proably steal his idea … which is the update your reading about now.

To light the Etch; I decided to create a Red Light bar under the custom Black Corner piece I created when I destroyed mine. But before we go there; I had to decide HOW to create the light. I decided I wanted mine to “activate” when a new game was started and then automatically turn off when the game was finished. I don’t have access to the Star Trek source code; so a software hack seemed problematic. Hummm… What’s the next best thing? Audio! With the exception of attract speech from Scotty and Spock; the instructional videos are silent. So I decided I wanted my machine to go into “Red Alert” Mode when real game play audio starts.

The Red Alert Audio circuit was born. ;) I spent several nights running spice simulations and came up with this circuit which I’m donating to the public under the TAPR Non-Commercial Open Hardware License which indicates:

You may make products based upon this design, provided you do not make more than ten units in any twelve month period for your personal use.

Schematics :

Click to view Red Alert Audio Schematics in PDF

The only IC in the circuit is U1, a dual JFET opamp – TL062P which operates from a single supply at ~12VDC. U1A makes up a 13.5x amplifier to amplify the ~100mV speaker signal from J1-1. The amplifier operates in non-inverting mode; so the gain is derived from 1+R9/R7. R6 is a series resistor intending to limit the loading of the audio amplifier (prevent distorition) and impedance match closer to the that of the 8 ohm speakers present in the upright version of the Sega Star Trek. D1 and D2 provide over/under voltage protection for U1A; limiting pin3 to 12+0.7V and ground (-0.7V) – a real possiblity given the audio amp is powered from a split supply @+/-24V. C2 is a DC blocking cap to prevent our single supply opamps’ bias voltage from loading the sega’s audio amps. R10 provides a high resistance path to our bias voltage of ~6VDC.

The second opamp (U1B) operates in comparator mode; comparing to the voltage divider made up of R4 and R5. V+U1B is (R5*Vcc/2)/(R5 + R4) or 100k*6V/(110k) or 5.46V. If we have no input audio; Pin1 floats above our bias of 6V; forcing the comparator to drive low providing no voltage to the remaining circuitry… effectively an off. Once the amplified input exceeds (6V-5.46V) 546mV; charging current is sent to D3. For those paying attention; 546mV /13.5V/V… means the input voltage at J1-1 must exceed 40mV regularly to charge the capacitor at C1. Empircal evidence on my Chair indicates that “Thrust” alone generates ~100mV at Pin3 of the U1A. Ofcourse your mileage may vary depending on a number of factors including your current volume level.

Once Pin7 of U1B begins to provide a voltage it is channeled to the RC circuit made up of R2&C1. Since the comparator swings between ground and ~+12VDC; it take nearly zero time for the capacitor C1 to charge to full capacity ~R3*C2 or 3.5mS. D3 provides blocking to prevent U1B from loading the RC circuit and R3 provides load resistance to prevent “shorting” the opamp to ground while C1 charges. Once U1B turns off; the RC circuit becomes a voltage source for M3. R2 slowly drains C1 meaning Toff = R2*C1 or ~8.6seconds. This time delay is necessary to ensure the red alert bar doesn’t go off in the middle of a game… IE between Sectors.

R1, Q2, and Q2 make up a BJT Current Mirror with M3 becomming the “on/off” switch for said mirror. When the gate exceeds approx 3.3VDC; M3 turns on conducting R1 to the top of Q1. Since we tied V+ (J2-1) to the 12VDC PSU in the sega G08 Card Cage; We can calculate the reference current into Q1. Iref is (V+ – Q1_VEB)/(R1 + M3_Rds); assume Q1_VEB=0.7V and M3_Rds from datasheet is 5ohm max. Iref is (12-0.7)/(681+5) or 16.5mA. Given Q1=Q2; ILED @ Q2 (J2-2) is 16.5mA; meaning we now have the means to pull 16mA thru up to 5 LEDs. Given my planned use is ~2V super bright Red LEDs; we can fit at most 5 LEDs in the leg of the current mirror before we run out of voltage. Q4 is a secondary Current Mirror “channel” to allow me to drive another 5LEDs… so I now have a means to drive 10 LEDs in the Red Alert Lightbar.

The powersupply for the circuit is made up of D4, C4, and C5. D4 provides a blocking diode incase I screwed up the wiring. C4/C5 provide filtering given the 12VDC source is about 12feet away. D5, R11, and Q3 make up a simple linear regulator to derive the 6VDC (VCC/2) biasing voltage to U1.

Whew! – clear as Mudd? There was some detail there; hope some of it made sense. :)

The PCB is designed as a two layer PCB with through-hole parts on the top layer to enable those less skilled with a soldering iron to build the circuit.

Red Alert Audio Fab B Top Layer w/ Silkscreen

Red Alert Audio Bottom w/ Silkscreen

For those without PCB layout tools; a bare PCB is available from the batchpcb service for under $15. You can purchase the boards from this link:

The Bill Of Materials (BOM) of the board is available from Digikey for under $6 (as of the time of this post). A CSV file with the digikey part numbers is here: RedAlertAudio_FabB_bom.csv

Here’s a package of the materials above as a single download:

Here is the the prototype assembled:

Now I needed to find a way to mount the 10LEDs under the black corner piece. This was done by laying out a 1inch strip of Red Acrylic in Corel Draw and laser cutting said peice:

I decided the LEDs would be wired as a “circuit” on the Red Acrylic. My LEDs are 5mm in size; so these would edge light the 1/4inch red acrylic. I put in 40mil traces in the design which are etched on the laser cutter to give us groves to put the LED leads in:

5 of the LEDs were placed on the right side with the Cathode of LED1 at the far left side end. LED2’s cathode is wired to LED1’s Anode repeating until the LED 5’s Anode which is then wired back to the left hand side. When the LEDs touch; they are soldered in place. When they don’t touch a piece of Adhesive red rework wire is soldered between the leads:

The Anode is wired back to the left side by the red rework wire. The Anode of the second 5 LEDs is then tied to the Anode of the first 5LEDs and the cycle repeats in reverse until the cathode of LED1 is at the right most side. It is then tied with a peice of Black adhesive rework wire back to the left hand side. This provides a “single” connection side for all the LEDs.

Here’s the finished Circuit on Acrylic:

I twisted about 12feet of yellow and black wire together and wired it into the connector on the G08 PSU at the +12VDC lines; it terminates on pin 1 of J1. The LED connections also terminate on J1. I drilled a couple of 7/16″ holes on the right side of the chair near the speaker using the circuit board as a template; making sure not to go all the way thru the side wood – just deep enough for the hex standoff threads. I then tapped these holes with a #6-32 tap; and placed a set of #6-32 hex standoffs in the holes. A couple of #6-32 screws and the board is now mounted at the top of the chair:

I found that I didn’t need to wire the speaker ground from the PCB to the speaker; instead only wired from the + side of J1-1 to the + side of the speaker. A small 7/16″ hole was drilled thru the wood to allow the 3 rework wires to come thru to the PCB. Given rework wire is fragile, small, and solid core; I put about 3″ of stranded 26 gauge wire before the crimp style connector. All of the excess wire was tye-wrap anchored to side of the wood -leaving plenty of “service loops” to enable future changes.

So… was it a success?

Does it Activate as expected?

While the circuit works as expected; it fails to deliver the intended effect of lighting the Window etch properly. Still a cool effect; one I’ll keep… but I’ll need to give the etch lighting some more thought. I’m also going to try and increase the current thru the LEDs by adjusting the R1 reference resistor. The LEDs should be able to handle close to 20mA (package says 24mA max); so the increase in current should brighten them up a bit.

This entry was posted in Arcade, audio amp, DIY, NCL, sega, TAPR, vector, Video Game and tagged , , , , , . Bookmark the permalink.

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