Laser Maze - Start/Stop Circuit

Discussion in 'The Projects Forum' started by CoachKalk, Jun 17, 2012.

  1. CoachKalk

    Thread Starter Member

    Sep 20, 2011
    Happy Father's Day to all the Dad's on the forum today.

    First, let me just say how grateful I am for this site and the incredible help it is - I am a complete beginner and everyone has been very patient while offering much needed help.

    In my continued mission for a play at home laser maze, I am now working on the laser control portion. Thank to elec mech, my display issues have been resolved.

    After much thought about how to contol the lasers, I have been considering a shift in design. I really wanted a way to control the lasers individually and I think a controller like an arduino may in fact be a great option.

    So, I have a few questions about this simplified laser circuit. If it passes your scrutiny, the end result would have up to 12 of the same set-up.

    Number 1
    Even though an arduino has 40mA per I/O pin, I had read that the I/O pins really shouldn't be used to drive external components. I had already ordered a 5V - 4A power supply back when the arduino was unknown to me, so I plan on just using that.

    But, you will see I am using 2N3907 transistors which will be controlled by the arduino. I am trying desperately to learn from all of the help provided so I would like explain where I came up with my numbers and see if you agree.

    The collector will have 5V from the Laser Power Supply. The laser runs/draws? approx. 25mA. The voltage drop across the 3907 is 0.7V. The base will have 5V from the arduino.

    Based on previous help, I need 1/10th of the 25mA to the base.
    So ... (5.0 - 0.7)/2.5mA = 2K base resisitor. Is that even close?

    When Pin 22 goes HIGH, will the lasers light? Pin 22 LOW = Off? Will the transistor "protect" the arduino from any excess current issues, if that is a potential problem?

    Number 2
    I plan to use SCR's for the difficulty/start buttons. I have never used a SCR (yeah, I know you are surprised) and I am not sure if I can just use a resistor at the gate to trigger the SCR? If yes, is there something on the datasheet that would tell be what size?

    If this new design direction works, I will be able to feed each LDR into an analog pin and "dial in" the sensitivity from 0 - 1023 units. My fingers are crossed that I haven't missed something.

    Also, we sold our short bus that we used on vacations and I made sure I took out the PA/Siren/Animal Noise toy we had bought from Farm and Fleet. Thanks to elec mech, I now know how to measure the current that it draws: 31mA @ 12V. Needless to say I am still trying to figure out a way to get that to the party now too.

    Anyway, I would appreciate feedback reagrding the "new" direction and specifically to my base resistor calculations and SCR question.

    Thanks everyone,
  2. wayneh


    Sep 9, 2010
    That's an NPN transistor that you're trying to use in a high-side switch. It would work better if you moved it down below everything else and let it control the path to ground (low-side switch). Then, pin 22 high = light on.

    If you replace the NPN with a PNP, then the circuit works fine with pin 22 low = lights on (reversed logic).

    Either way, the base resistor does indeed protect the Arduino from an excessive load.

    Why SCRs? Or, put another way, why not a (cheaper) MOSFET?
  3. CoachKalk

    Thread Starter Member

    Sep 20, 2011
    Thanks for the input Wayne. Actually, I had been searching the forum in the meantime and found the exact same advice you just posted. I have updated to a 2n2222 on the bottom of the load. Other than my first calculation for the base resistor being messed up, I got it working.

    The game style PB's I have are either NO or NC, but they return to the original position. When I first started looking over the forums, I found info showing how I could use a SCR for a switch. Have the NO Start button turn the SCR on and then have a NC stop button shut it down.

    Hopefully, my plan is to have the Start PB connected (via arduino) to several laser/ldr units. After the initial start-up, the control of each laser/ldr unit would then be "handed off" to and controlled by the condition of the analog pin that reads the LDR.

    Somewhat off topic, I just realized you are in Roscoe. I am in Lena - just west of Freeport! Small world!