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For my 10V model train's signals and switches I need a ~500msec power pulse (AC; low Amp). I already have an electronic switch that provides power when triggered. However, the power should be brief, as the signal's solenoid will otherwise overheat and burn with continues power. I am hoping for a simple circuit design that I create for each signal or switch.
My electronics skills are beginner, but I am good at following instructions.

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  • \$\begingroup\$ Please provide an image of the pulse you wish to generate showing numerically all the important features. \$\endgroup\$
    – Andy aka
    Commented Nov 23, 2023 at 21:00

2 Answers 2

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You can use a non-retriggerable monoflop (also called a non-retriggerable monostable multivibrator) to generate pulses with a defined duration.

The 74HC221 is a chip that does this. You need to add external timing components (a resistor and capacitor) to set the pulse duration. The circuit diagram and truth table on page 2 of the linked datasheet show how the chip operates, including the aforementioned timing components. Add a 5V power supply (i.e. from a 7805 linear regulator) and a bypass capacitor, and that's it.

There's apparently also a YouTube video explaining the 74HC221, although I've only skipped over it and not watched it in full.

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This circuit gives you about 500ms pulse to selenoid (or relay) after SW is permanently closed:

enter image description here

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  • \$\begingroup\$ Thx Michal. This seems doable based on a materials search. Are the bottom 4 inverse T connections to ground and upper left 12V as other supply? Sorry, but I am a beginner. \$\endgroup\$ Commented Nov 24, 2023 at 16:08
  • \$\begingroup\$ Ya, all bottom is minus pole of power supply (or Gnd if you want). Most upper line is plus pole. \$\endgroup\$ Commented Nov 24, 2023 at 16:55
  • \$\begingroup\$ Also remember with 4k7 and 10k in base of Bjt you are able to drive about 50mA load ( in collector). If selenoid drains more lower these resistor like 1k and 2k2. Also cap has to be higher value like 470uF (to ensure 500ms timing). \$\endgroup\$ Commented Nov 24, 2023 at 17:12
  • \$\begingroup\$ Great. I will build it and learn, using larger power option. \$\endgroup\$ Commented Nov 25, 2023 at 14:55
  • \$\begingroup\$ I created the circuit in CircuitLab, but cannot upload image to this comment. What are the connections for solenoid? Two wires to solenoid switch? The solenoid goes back and forth with pulse. \$\endgroup\$ Commented Jan 25 at 20:44

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