triggering a 555 monostable without a push button/switch trigger?

Discussion in 'The Projects Forum' started by 43sqn, Nov 5, 2014.

  1. 43sqn

    Thread Starter New Member

    Nov 5, 2014
    1
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    Well not exactly! Very new to electronics cct design, and I have this problem......Any advice would be very welcome.

    I'm after the following solution, a toggled switch when selected, will operate a green LED, then after a time delay of 10 - 15 minutes extinguish, simultaneously operating a red LED to remain on until the toggled switch is selected off.

    I selected a 555 timer as the reviews appeared quite robust and relatively easy to operate for beginner-ish level. I thought potentially, I could operate the 555 in a monostable configuration. With the output driving a relay, this would switch over the earth paths when energised, ensuring one LED would always remain illuminated. Satisfaction of my relay solution then turned to dismay........

    The problem!!!
    The toggled switch must supply the power for the circuit and also be the trigger for the timer. All the other circuits I have seen have the circuit powered with a supply, then triggered from a push button switch either high or low.

    Is this possible? Or am I chasing my tail with this philosophy?

    I have attached a circuit diagram of my proposed design, which has taken about a week of research! But im starting to lose confidence in my abilities now.

    Further info on the stipulations for the proposed circuit;

    The 24VDC supply is attained through a toggled switch (not push button type), I've incorporated a 7812 Voltage regulator to step the voltage down to a more friendly 12VDC.

    Any comments or feedback would be greatly appreciated. Many thanks for your time.
     
  2. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    It is on the sketch pad, sim. to your drawing , minus relay, NPN changes to PNP. Out of time, be back later tonight.
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,977
    3,221
    Getting a 10-15 minute delay is problematic with a 555 due to the high value of timing resistor and capacitor needed. Better would be a circuit using a CD4060 counter-timer such as this. Note that for driving LEDs you just need to add a PNP transistor to the circuit and can eliminate the relay. Drive the PNP transistor from the same CD4060 output driving the NPN, (emitter to V+ and collector driving the LED connected to ground) giving an inverted output. The NPN transistor drives the Red LED and the PNP transistor drives the Green LED.
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    395
    The toggle SW can be in AC line feeding power supply,PS output , or output of 12 V reg. C1 holds 555 trigger low as power comes up turning on green LED. When 555 times out, red LED comes on. Tried several different C,s, 2200, 1000, 820 uF, only one 1000 uF was too leakey. R3 can be 1 M pot in series with 100k, as it stands, timing is about 20 min.
     
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