HELP with pulsing circuit!

Discussion in 'General Electronics Chat' started by electronice123, Oct 29, 2008.

  1. electronice123

    Thread Starter AAC Fanatic!

    Oct 10, 2008
    I have built the following circuit, shown here:


    The circuit first regulates the voltage to 10V using the 5K pot, the frequency is an adjuatable square wave with the 100K pot and is adjustable from 0-10kHz, the Counter, NAND, and AND gates have been set up so every 20th pulse will be blanked out.

    After hooking up a light bulb as shown to the source voltage and the transistor I have the following problems.

    The light bulb stays on constantly regardless of frequency!

    I am only gettting 1.49 V to the base of the NTE11 and I think that might be my problem.

    ANY help would be greatly appreciated to get this circuit working properly!
  2. SgtWookie


    Jul 17, 2007
    The LM317 requires a minimum 10mA load to guarantee regulation. Otherwise, the output voltage will rise to perhaps within 1.7v of the input voltage.

    Use a 120 Ohm resistor from the OUT to ADJ terminals. Since Vref (measured between OUT and ADJ) is nominally 1.25v, a 120 Ohm resistor will have at least 10mA current through it. From ADJ to ground, use a 330 Ohm resistor in series with a 1K pot to give you a decent range for 4000 series CMOS IC's.

    Use a 0.1uF/100nF and 10uF cap on the output of the LM317.

    You have a lot of inputs that are not connected. ALL 4000-series CMOS inputs must be connected to something, either Vdd or ground. Inputs left floating will cause random oscillations, overheating, and hard-to-diagnose problems.

    Use bypass capacitors (100nF, or 0.1uF) across the Vdd and Vss/ground pins of each IC.

    On the 4046, connect pin 4 to pin 3.
    Last edited: Oct 29, 2008
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    You symbol is incorrect - it is for a FET, not an NPN transistor.

    The transistor needs a current limiting resistor in series with the base. And CMOS logic is not meant to drive transistors directly. The 4093 is probably fried from too much current into the transistor base.

    All logic needs to have unused inputs tied high or low. With CMOS, just wire direct to Vcc or ground. If you don't, the inputs tend to wander up to the high/low operating point and cause high frequency oscillations. That tends to cook the chip.

    As is, you would do well to replace the NTE11 with a FET. If the bulb is less than 250 ma, then a 2N7000 willl work as a replacement. For more current, get something like an IRF520. Do add a small reisistor between the 4093 output and the gate - about 100 ohms. The variable resistance should move to the drain side of the FET.

    For better operation with a FET, run the Vcc up to 12 - 15 volts.

    A FET's equivalent to the emitter is the source, for the base it is the gate, and the onefor the collector is the drain.
  4. electronice123

    Thread Starter AAC Fanatic!

    Oct 10, 2008
    Thanks you two for the help. I appreciate it!

    I will do all the things you bothe mentioned and see how everything works, and I'll probably replace a few chips as well.

    Thanks again.
  5. Audioguru


    Dec 20, 2007
    Of course the light is always lighted. It is on most of the time and the small time it is off does not allow it to cool unless the frequency is extremely low.

    Incandescent light bulbs draw 10 times their normal operating current when cold (turning on) so maybe the tiny transistor is blown up.