Discussion in 'The Projects Forum' started by ponderosa_74, May 20, 2013.

  1. ponderosa_74

    Thread Starter New Member

    May 5, 2013
    (This is an update of a previous post.)

    I am building a scale airport for my almost 3 and almost 5 year old sons for their Siddley the Spyjet airplanes from Pixar’s Cars2. I’m posting an updated version of my original circuit, for which I got lots of input from this forum – thanks again!

    Please see the attached schematics for the new circuit. I want lights on the jetways, alternating lights for the top of the air traffic control tower and 2 rotating radars – all with individual switches to turn them on/off.

    I used an iPad app called iCircuits, which also simulates the circuit and includes a multimeter and oscilloscope. In most of the attached pictures there's a box in the bottom right (see below) which is the multimeter and the corresponding component is highlighted in blue on the schematic.

    The circuit has 4 LED’s in parallel with each other, which are in parallel with two DC hobby motors which are in parallel with a 555timer sub circuit, that alternately flashes a red and a green LED.

    Component parts include:
    All LED’s are 2.2V, 20mA
    2 x Hobby motors – 3V, 260mA, 30rpm
    Various resistors
    1 capacitor
    555 timer
    All powered by a 4.8V NiMH


    Re: MOTORS
    The hobby motors are 3V and supposed to draw 260mA of current at 30RPM. BUT The circuit simulator shows they are running at 60RPM and 65.645mRPM. For the parameters of the motor see the attached picture. I had no idea what the “stall current” was so I input 530mA. I also didn’t know other parameters such as stall torque, inductance or inertia so I left those with the default settings of the simulator.
    1. What’s up with the RPM? Why such drastic changes, is that going to be evident in the final product?
    2. Should I be concerned about the other parameters?
    3. Is there a way to figure out the stall current? I tried attaching my multimeter and stopping the motor but couldn’t get a stable reading, it was just fluctuating all over the place.
    4. Also, the current of the motor shown by the simulator is fluctuating from 182mA to 183mA to 667mA. Since 667mA is way more than the 260mA draw current for the motor is this a problem? I’m thinking its okay but just wondering.

    Re: 555 Circuit
    This is a circuit based off a RR Xing blinker from
    1. Is there a way that I can make these LED’s alternatively blink like they are but make them fade in and out?

    Re: Zener Diodes
    I was told the zener diode voltage should be whatever your battery voltage is minus the voltage requirement of the motor.
    1. The current of the zener diode is all over the place, from 184mA, to 320mA, to 667mA, to 208mA, to 2.3A. Is that a problem or is that normal?
    2. Also, the 2 settings in the simulator for the zener diode were “Fwd Voltage @1A”, which I left with the default, and “Zener Voltage @5mA”, which I set at 1.8V. Is this correct and should I do anything to change the “Fwd Voltage @1A” setting?

    Thanks for any help/advice, its much appreciated.
    Last edited: May 20, 2013
  2. LDC3

    Active Member

    Apr 27, 2013
    First, the zeners are used in the reverse direction. So in your schematic, the zeners are backwards. Instead of zener diodes, use two diodes in series to drop the voltage by about 1.4 volts. For circuit protection, place a diode across the motor in the reverse direction so when you open the switch, no large voltage potential is created.

    Second, you want to increase the 150Ω resistors for the white and red LEDs to at least 240Ω to limit the current below 20 mA. I am guessing you need to do the same for the red and green LED for the same reason.
  3. wayneh


    Sep 9, 2010
    That would work, but his zeners may work fine too if they are turned around. They need to be rated at more than the stall current. Two times normal current is a good guess, but then you need to add in a safety factor. If they're really rated 1A or more, they'll be fine.

    You can absolutely do a fade-in, fade-out on the red and green LEDs.
  4. LDC3

    Active Member

    Apr 27, 2013
    OK, I've had a chance to calculate the timing and with the 20k resistor and 100μF capacitor, you get the green LED on for 1.3 sec, then the red LED for the same time. Changing the resistor to 7.5k will give a little over 0.5 seconds each. A 3.7k resistor will give about 0.25 sec each.

    The easiest way to have the LED fade in and out is to place a capacitor across the resistor. It will need to be at least 500μF, which should give you about 0.25 sec of fade time. Smaller capacitors will be faster fade times.
  5. wayneh


    Sep 9, 2010
    If you want longer fades without big capacitors, you can use a transistor. A small current - your RC tank current - goes through the base-emitter circuit and controls the larger LED current through the collector-emitter circuit.
  6. LDC3

    Active Member

    Apr 27, 2013
    Probably a better way to go.