2013-04-05 Pulse Generator, Two Transistor, Variable Frequency

I assembled the circuit to see if I could come up with an alarm.

Wanna make an Open Garage Door Indicator Alarm


The circuit uses two transistors in a circuit that needs only a single timing capacitor and timing resistor to determine the frequency.  It’s somewhat similar to the flasher circuits I’ve been working on, but it uses 9V supply voltage.  It could be as low as 5V, or maybe even lower, but I figure that the alarm will only be on for very short periods of time, a 9V or 6V battery should last for a very long time.

The circuit will be two of the pulse generator circuits, one running at mid audio frequencies, and the other will be running at a pulse every 1/3 second or so.  This will vary the audio pulse generator up and down.  Then that output will be used to frequency modulate the RF generator.  I know how to do that, but I’m not yet sure where the 1 second pulses will be fed into the audio generator to vary it up and down.  I may connect it through a high value resistor to the base of Q1.  The change in frequency doesn’t have to be a lot, just enough to make it distinctive and easy to recognize.  Maybe a few hundred Hertz will be enough.

I built up the circuit in the schematic above, and it worked good.  I could vary the frequency from 1300 Hz to 30 kHz.  I touched the base of Q1 with my finger while I was connecting the scope probe, and I noticed that the scope showed the frequency was being modulated by the 60 Hz hum from my body.  Apparently the circuit is sensitive at that point and I should be able to feed the slow pulse generator into that point through a resistor and get it to vary the audio tone.

I have also been thinking about how to use a lower battery voltage.  I could feed the 1.5V battery into one of the flasher circuits, and use it to boost the voltage up to 3 or 4 volts.  I could also use a Joule Thief to boost the voltage up to 6 or so volts, and run the whole circuit off that.  This method seems very attractive, since I have most of the stuff already laying around.  The only question is will the output have enough current to run the RF section and the pulse generators?  I’ll have to check out the current drains.

Of course the single circuit without the warble can be used to do whatever a 555 timer chip can do in the astable mode.  The 555 made it so easy to just put the chip into a circuit, that the Real Engineers all forgot how to do it with a simple  two transistor circuit like this one.  It’s probably cheaper, and probably isn’t much different in the number of parts and ease of building.  Once you get to know how the different values of R and C interact in the circuit, it should be easy to guesstimate the approximate value of the timing capacitor and resistor.

Warble Circuit

I clipped a 120 ohm earphone element from a telephone handset to the output.  I could adjust the frequency from  I clipped a 10 uF electrolytic capacitor across the .01 uF, and measured the frequencies.  I could vary the pulse rate from 1.25 Hz at the pot maximum, to about 30 to 40 Hz near the pot minimum.  I’m guesstimating that a frequency somewhere around 3 Hz should be a good warble sound.

Another use(s)

Another thought that I had was to make a subcarrier.  I could set the frequency of the pulses at some high frequency above the audio band, say 38 kHz.  Then I could put a microphone with a single transistor preamp on the pulse generator and generate a frequency modulated carrier.  I could feed this into a red LED and send the audio modulated carrier on a light beam.  I could also use it to  modulate the RF carrier, and that would conceal the audio, so it could not be heard until someone used a subcarrier demodulator to recover it.

Another useful tool is the wire tracer.  I have one of these, the little box that clips on to the pair of telephone wires and sends the “deedle-deedle-deedle” tone down the wire, so it can be traced by the probe.  I could put this circuit into a small box with a pair of clip leads and m,ake one just like the commercial models that sell for $30 or more.  I really don’t need to. because I picked up a genuine “Bell System 139B Test Set” at a swap meet, and I took it apart and fixed the intermittent contact problem with the battery holder.  That’s one thing that will really make a person angry.  I use the tone tracer box at work to find a telephone line among the hundreds or thousands of others.  One end may be located a thousand feet away on the other end of campus.  If I put the tone tracer box on the phone line and walk all the way down to the other end of campus and can’t find the line because the box stopped working, and I have to walk all the way back to fix it, then walk all the way down again, it can get really frustrating.

I continue this in my following blog.

6 Responses

  1. biju says:

    this is good circuit. i try this. but i wan to adjust 0hz to 30khz. how can it possible.

    my requirement is to control a “controlled thyristor” using pulse transformer.
    input volt ac 380 output dc 0 to 380v.

    can u help me with ur valuable experience.

    • admin says:

      In the schematic I wrote that when C equals 0.01 uF, the frequency is 1.3 to 30kHz. If you need a lower frequency, then add a switch and a larger capacitor. You can add more capacitors for lower frequencies but it can never go to zero Hz.

      I will offer one very important rule that every technician who works on high voltages should obey. When you have your hand near or in any high voltage equipment, always put the other hand in your pocket. If you don’t, high voltage may go through both of your arms and through your heart, causing electrocution – you die.

      Because of this danger, I am sorry, but I am not willing to give advice about dangerous projects such as you may want to build. And I am not responsible for anything that happens to you or anything you build.

  2. andrei ruiz says:

    can i use 13003 transistor or s8050??? and can i put 5k pot and .1uf 50v cap???

    • admin says:

      The 13003 transistor is made for high voltage and high power, and has low current gain. It might work okay for the second transistor. The s8050 should be okay for the first transistor. I say ‘might’ and ‘should’ because i’m not sure until I have built the circuit with those parts. The circuit is not critical and should work with a wide range of part values. I would not use less than 1000 ohms for the freq. adj. pot. The 0.1 if capacitor should be okay. It can be much larger, but the pulses may be slow and sound like a ticking clock. This is a good circuit to experiment with because it allows you to change C and the freq. adj. pot and make pulses from over 100 kHz to 1 pulse a second or even slower. You can also put a resistor in series with C to change the width of the pulse. Have fun!

  3. Dhanraj Chauhan says:

    this circuit not create any type of frequency

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