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# IR Lap Timer for PC

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#### mdearth

##### New Member
Im working on a lap timer for RC Cars that will use my Laptop along with Lap Timer 2000 Software. It calls for IR Photo transistor connected to the joystick port of the PC. The IR Recieving side with the Joystick port was layed out clearly...

My Question is about the IR LED. How do I power a single IR LED?

IR LED Specs
• Radiant Intensity (mw) 7.368
• Peak Wavelength (nm) 940
• Forward Voltage (V) 1.28
• Half View Angle (Degrees) 36.36

Power Supply Specs
• Selectable Output Voltage: 1.5, 3, 4.5, 6, 7.5, 9, or 12 VDC
• 300mA

What voltage should I select? What size resister should I put on the Cathode side? Is a single resister all i need?

If I used 2 IR LEDs. Paralell or series? What voltage and and resistors should be used?

You basically just need to put a resistor in series with your IR diode and your power supply. The resistor value is easily computed by the following:

R = (Vs - Vf) / If

So in your case, if you use the 3V setting, you would need an 86 ohm (or thereabouts) resistor. If you use 2 diodes, just add their Vf together, plug them into your formula and set your supply voltage a little higher. BTW, I assumed the diode drew 20mA which is probably low, but it wasn't in the spec you gave.

Its the IR LED from Radio Shack 276-143. It did not list a ma rating, however on superbrightleds.com they listed their IR LED at 20ma.

I might use a 12v power supply, as i have a free one available.

12V - 1.28v / .02 = 536 ohm resister neded. Rite?

Would a 560 ohm 1/2w resister do the job? Does it matter how many ma my power supply puts out? 800+ ma

If 2 or more IR LEDs are used, in paralell, would each get its own 560 ohm resistor?

If a single IR LED only requires 1.28v & 20ma, could i power it from the 15pin joystick port on the PC?

Here is the "traditional" pinout of a 15-way joystick port:
1 +5v
2 Button 1
3 Joystick 1
4 Gnd
5 Gnd
6 Joystick 2
7 Button 2
8 +5v
9 +5v
10 Button 3
11 Joystick 3
12 Midi Out
13 Joystick 4
14 Button 4
15 Midi In

The Lap Timer Joystick Interface uses...
Pin 1-3 100k R
Pin 1-6 100k R
Pin 4-2 Photo Cell 1
Pin 4-7 Photo Cell 2
Pin 4-10 Photo Cell 3
Pin 4-14 Photo Cell 4
https://www.hoslotcarracing.com/LapCounter.html

Then pins 5, 8, 9, 11, 12, 13, 15 are unused.

Anybody know if I can use pins 5&8 to power a single IR LED? Along with a 186 ohm resistor. is there enough amperage?

Found at https://www.epanorama.net/circuits/power_from_pc.html

Joystick port
Joystick port in PC has +5V power output which is directly connected to PC +5V power line. The advantage of this is that there is quite much power available (up to few amperes). The disadvantage is that in short circuit condition there can be over 20 A current flowing and this can easily burn PCB traces and small in your circuit or inside your PC. Here is the pinout of the joystick port:

.-----------------------.
\ 8 7 6 5 4 3 2 1 /
\ 9 10 11 12 13 14 15 /
~~~~~~~~~~~~~~~~~~~

You can get +5V power from pins 1 and 9. The ground can be found from pins 4 and 5.
Note: Some new PCI soundcards can supply 3.3V instead of standar +5V in the joystick connector. According some articles in the news DIAMOND MM SONIC IMPACT S70 has a 3.3volt game port and it is listed in their readme file for the product. I can just wonder what kind of incompability problems this can cause to existing joystick products and joystick port MIDI interfaces which are designed to use that standard +5V power from normal joystick port.

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