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Stretch 0.5sec pulse to 6sec

Discussion in 'Electronic Basics' started by amdx, Feb 2, 2013.

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  1. amdx

    amdx Guest

    I have a 4 volt pulse that is 0.5 sec long.
    I need a 6 sec pulse that will switch a 2n3904.
    The 4 volts has about a 400 ohm source impedance.

    I could use a 555, but first I need to invert the signal then
    drive my 2n3904 switch, so I have 2 transistors and a 555. Oh yes I
    could probably differentiate the 0.5 sec pulse to trigger the 555, so
    maybe only 1 transistor and a 555.

    Could someone help with a pulse stretcher circuit.
    Maybe an RC, transistor, or, I have some FETs, J301 or 2N3819, probably
    a diode. That will drive my final 2N3904 switch.
    The 2N3904 switch is across the alert button on a Walkie Talkie and
    shorts about 3.5v to ground.

    The longer story;
    This started with "Have 5v regulator need 9 volts"
    That particular problem is solved. everything has good clean power.

    I have a Harbor Freight *Driveway sensor.
    The sensor triggers a receiver with RF.
    The receiver in turn triggers the alert button on a **Walkie Talkie.
    After a temporary install, I noted the Talkie alert signaled 10 times.
    That is to much, three times is fine.
    After trying several things to eliminate the 10 alerts, I finally
    wrapped the walkie talkie in aluminum foil. Problem solved. I'll deal
    with this later by putting the Harbor Freight receiver inside my boat
    and the walkie talkie outside where it's RF can get out. (it's an all
    Aluminum boat)
    The 0.5 sec pulse causes one alert signal. A six second pulse causes
    three alert signals. Perfect!



  2. amdx

    amdx Guest

    You presume correctly.
    But you already new that. ;-)
  3. amdx

    amdx Guest

    I have breadboarded the two transistor, 555 circuit.
    Three alert signals as wanted. I can wire it on a pcb
    in the morning, if no one comes up with a simple RC one transistor
    pulse stretcher. I couldn't make it work, maybe it can't be done.
    Seems the timing would be difficult, but for my circuit 20% fluctuation
    would be ok.
  4. Jasen Betts

    Jasen Betts Guest

    you can't just use the discharge pin of the 555 instead of the collector of the
    2n2904? is 200mA not enough?
  5. Jamie

    Jamie Guest

    | |1MEG PTT load (Radio)
    | | |
    0.5s Pulse 1k '-' ||-+
    ___ | ||<-
    + |
    1N914 + |
    --- 10uf ===
    --- GND
    | Power NMOS or what ever
    (created by AACircuit v1.28.6 beta 04/19/05

    Depending on your Vgs(th) spec you use for the NMOS will determine
    the time off delay, which is what I think you're after.

    This is one should be ~ 5 to 6 secs.

    I didn't do a sim on this, just used a calculator but it should be
    THe pulse should pull the output down fast and the off delay
    will give you a slight ramp due to the way fets are biased and behave
    when the drain starts to get higher in voltage than the gate. This
    should work out.

    The diode is there to prevent back drain from the pulse source.

  6. amdx

    amdx Guest

    Do you mean the discharge (pin 2) or output (pin 3).
    The pin from the walkie talkie already has a voltage on
    it, pin 3 will source current to the walkie talkie before
    it sinks current. I need an open collector to pull the Walkie talkie
    low. That's my understanding for now.
  7. Jamie

    Jamie Guest

    if you drive the trigger input via the output of the timer through a
    R of lets say 100k for example. Connect a 50uf cap from the trigger
    input pin to common and then use a diode in parallel with the 100k R
    to quickly recharge the cap when the output is high. The anode of the
    diode will be on the output while the cathode is on the trigger input.

    WIth this configuration, you can drive the threshold pin from your 4
    volt source for the TX PTT signal. This will force both the output pin
    to go low along with the discharge pin also going low. Take your pick
    which one you want to use. I would use the discharge pin since it is a
    open collector and not yet in use. The output pin will be busy
    discharging the cap so the trigger input will reset the output when it
    reaches 1/3 of the Vcc voltage.

    And that is another issue. Since you only have 4 volts to drive the
    threshold pin for the TX signal, your Vcc can not exceed 6 volts abs which
    is on the edge, I would use a common 5 volt post reg to be sure for the

    The diode that is placed there for quick reset can be tailored with
    adding a R in series with it if you don't want it respond to another
    input pulse too early.. THat is up to you.

    THis is just my opinion of course... others most likely have other ideas.

  8. amdx

    amdx Guest

    Thanks Jamie,
    That seems to be working well. I have it breadboarded using 500k and
    47uf with an IRFBE30* MOSFET, because that's what I have.
    I'll work with it a bit more to get me centered between 2 alerts and 4
    alerts. Right now it is alerting 3 times but I don't know if it is
    centered between 2 and 4 alerts.
    This will fit better in the case.
    Thanks again, Mikek

    Went back to the bench with a 1 meg pot, Found two alerts at 344k and 4
    at 513k, centered at 428k and it consistently alerts 3 times. (47uf)

    * IRFBE30 800V 30amp 3 ohm on resistance. Overkill but I have it.
  9. amdx

    amdx Guest

    Jamie's RC, power FET circuit worked good and fit easily, no power
    supply needed, only 5 parts.

    Bonus, I had all the parts.
    I'm in Panama City Fl. The highway doesn't make it here. We are at
    the end of the line. Nothing goes past us to get somewhere. I have a
    radio Shack and a one man audio repair shop, that sells some parts.
    Thanks, Mikek
  10. Guest

    Not so great with that kind of delay.
    I'll bet even DigiKey (via UPS) ships to that end of the line. ;-)
  11. amdx

    amdx Guest

    But I want it now! :)
  12. Jasen Betts

    Jasen Betts Guest

    pin 2 is trigger one of the main inputs.

    Discharge is an open collector output it's on pin 7
  13. Guest

    It's way off (three orders of magnitude off) the chart for the HC
    types, at least according to TI. This sort of thing is right up the
    alley for a 555, though.
  14. Guest

    I don't believe what's printed. Drawing lines outside what's printed
    is suicidal.
    555s are designed to work with much higher resistors. The '123 really
    isn't very good.
    Use a PIC. ;-)
  15. Fred Bartoli

    Fred Bartoli Guest

    Tim Wescott a écrit :
    I just saw a nice CSS555 (available at Jameco)
  16. I stopped buying stuff from jameco after a bad batch of 79L12 voltage
    regulators.... cost me too much time.

    George H.
  17. amdx

    amdx Guest

    I had the 555 circuit breadboarded, it worked.
    I'm sure a 74123 could work.
    A CD4060 and a dual J-K flip-flop could work.
    A pic would work.

    But Jamie's circuit worked with 5 parts, I had on hand.

    Check with my 6 months from now, if it still works, ya'll can call it
    a success.
    I'm calling it now.

    For Jim,
    Alert, Alert, Alert.
    That's what it does now.
    Alert, Alert, Alert.

    Thanks, Mikek
  18. Fred Bartoli

    Fred Bartoli Guest

    George Herold a écrit :
    I just mentioned jameco because the OP seems to be an hobbyist, but have
    a look at the datasheet. It's a pretty interesting son of a ...555
  19. No problem Fred, I use to buy lots of things from Jameco.
    That is an interesting part. low voltage and an internal eeprom
    controlled divider.

    George H.
  20. Jamie

    Jamie Guest

    I remember having a few of those too, and the voltages were all over
    the place. I don't remember where I got them from but it would appear to
    me they may have been seconds that I got from some surplus dealer.

    Sometimes you never know.

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