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I have DPDT relay which I would like to energize using pulse signal (duration of that pulse should be adjustable) and switch. When switch is on, coil of relay is shortly energized by pulse signal and then deenergized when pulse signal ends, although switch is maybe still closed. When I open switch, nothing happens but when I close it, process repeats. Any idea how to implement this circuit? Relay model is Omron DPDT 10A 12V DC. Thanks in advance

Edit: I looked for solution using 555 timer but it will not work in my case. Here is example of monostable 555 timer: http://www.electronics-tutorials.ws/waveforms/555_timer.html

If I put switch between pin 2 and ground and close it, output will be high but it will not go low itself If I keep switch closed. I need output to go low even if I keep switch closed for long time. They say also:

"The Monostable 555 Timer circuit triggers on a negative-going pulse applied to pin 2 and this trigger pulse must be much shorter than the output pulse width allowing time for the timing capacitor to charge and then discharge fully."

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  • \$\begingroup\$ You want a monostable or one-shot between your switch and relay. There are a million 555 timer examples on the web. (On this site we thank in retrospect by voting up and accepting good answers.) \$\endgroup\$ – Transistor Jun 4 '16 at 19:22
  • \$\begingroup\$ Thanks for reply. I will look for 555 examples. I would vote in gratitude but I don't have sufficient reputation to do that. \$\endgroup\$ – zeti Jun 4 '16 at 19:26
  • \$\begingroup\$ That's OK. I only gave a comment and not a full answer. A good one may show up later. \$\endgroup\$ – Transistor Jun 4 '16 at 19:46
  • \$\begingroup\$ Please define what "pulse signal" is? Without knowing what kind of "signal" this is, making any specific useful suggestion is impossible. Is this a brief shot of mains power? Is it TTL or some other logic level? Is it a doorbell? Is it a signal from an RC receiver? Is it some small signal that needs amplification and conditioning before it can be used? Perhaps you don't realize what a remarkably broad range of possibilities there are here. \$\endgroup\$ – Richard Crowley Jun 4 '16 at 19:55
  • \$\begingroup\$ @Richard Crowley, it's 12V DC (to energize coil of relay 12V DC is needed), but it's duration is short (few hundreds of miliseconds to one second, it should be adjustable). Hope it's clear now :) \$\endgroup\$ – zeti Jun 4 '16 at 20:00
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You'll need a lesser-used (albeit still extant) monostable for this job instead of the 555. Given the voltage levels you are dealing with, 74xx parts are out of the question, which leaves us with the CD4000 family's monostables, in particular the CD4047 multifunction CMOS timer:

schematic

simulate this circuit – Schematic created using CircuitLab

Notes and caveats: C1 must be a film type capacitor -- ceramics of that value aren't stable in value enough with voltage for timing applications, and the CD4047 requires a non-polarized cap to work. D1 can be swapped for a 12V zener if the relay turnoff is too slow for you. Q1 can be whatever small-ish power NMOS you have in your junk box (a jellybean BJT can be used provided you put an oh 1kOhm resistor on the base, too). The timing of the circuit is currently set for roughly 250ms, and can be changed by adjusting R1 and C1 in accordance with $$T_m = 2.48R_1C_1$$

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  • \$\begingroup\$ A 555 can drive the relay directly and comes in an 8pin package, so why would you want to use the larger 4047 and spend extra for the relay driver and the PCB real estate? \$\endgroup\$ – EM Fields Jun 5 '16 at 6:46
  • \$\begingroup\$ @EMFields -- didn't realize there was a circuit for the 555 that'd do the job -- I picked the 4047 because the stock 555 monostable was unsuitable. \$\endgroup\$ – ThreePhaseEel Jun 5 '16 at 7:08
  • \$\begingroup\$ ThreePhaseEel, thanks for answer! Regarding to EM Fields comment, I think that coils current is 75mA at 12V so I need driver also in case of 555? \$\endgroup\$ – zeti Jun 5 '16 at 11:25
  • \$\begingroup\$ @zeti: A bipolar 555 will drive the relay directly. Read the data sheet \$\endgroup\$ – EM Fields Jun 5 '16 at 12:26
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In the schematic below, when S1 is made, a low-going spike will be generated when C1 differentiates the level change caused by S1 pulling the junction of R1 and C1 to ground. That spike will pull U1-2 low, trigger the timer, force U1-3 high, and energize the relay.

When U1 is triggered, U1-7 goes open collector and Ct will start charging through Rt until U1-6 senses that it has reached 2/3 Vcc. When that happens, U1-7 will go low, quickly discharging Ct, forcing U1-3 low, and de-energizing the relay.

Note in the plot below that even if the switch is held closed for a long time, when the trigger spike dies away U1-2 will be pulled up to 12V, inhibiting U1 from being triggered until it times out, S1 is released and then pressed again, starting a new cycle.

enter image description here

And, just in case you want to play with the circuit, here's the LTspice circuit list:

Version 4
SHEET 1 948 680
WIRE -1120 -80 -1168 -80
WIRE -1040 -80 -1120 -80
WIRE -992 -80 -1040 -80
WIRE -896 -80 -928 -80
WIRE -800 -80 -896 -80
WIRE -688 -80 -800 -80
WIRE -320 -80 -688 -80
WIRE -288 -80 -320 -80
WIRE -1040 -32 -1040 -80
WIRE -896 -32 -896 -80
WIRE -688 -32 -688 -80
WIRE -800 -16 -800 -80
WIRE -464 16 -576 16
WIRE -256 16 -400 16
WIRE -1040 112 -1040 48
WIRE -896 112 -896 48
WIRE -896 112 -1040 112
WIRE -800 112 -800 48
WIRE -800 112 -896 112
WIRE -576 112 -576 16
WIRE -576 112 -800 112
WIRE -544 112 -576 112
WIRE -256 112 -256 16
WIRE -256 112 -320 112
WIRE -688 176 -688 48
WIRE -544 176 -688 176
WIRE -288 176 -288 -80
WIRE -288 176 -320 176
WIRE -688 240 -688 176
WIRE -544 240 -688 240
WIRE -224 240 -320 240
WIRE -288 304 -320 304
WIRE -1040 368 -1040 112
WIRE -976 368 -1040 368
WIRE -832 368 -896 368
WIRE -288 368 -288 304
WIRE -288 368 -832 368
WIRE -1168 400 -1168 -80
WIRE -1040 400 -1040 368
WIRE -1216 416 -1296 416
WIRE -832 416 -832 368
WIRE -688 416 -688 240
WIRE -1296 432 -1296 416
WIRE -1296 544 -1296 512
WIRE -1216 544 -1216 464
WIRE -1216 544 -1296 544
WIRE -1168 544 -1168 480
WIRE -1168 544 -1216 544
WIRE -1040 544 -1040 480
WIRE -1040 544 -1168 544
WIRE -832 544 -832 480
WIRE -832 544 -1040 544
WIRE -688 544 -688 480
WIRE -688 544 -832 544
WIRE -256 544 -256 112
WIRE -256 544 -688 544
WIRE -1296 640 -1296 544
FLAG -1296 640 0
FLAG -224 240 OUT
FLAG -320 -80 TRIG
FLAG -1120 -80 SWITCH
SYMBOL Misc\\NE555 -432 208 M0
SYMATTR InstName U1
SYMBOL res -912 -48 R0
SYMATTR InstName R2
SYMATTR Value 10K
SYMBOL res -1056 -48 R0
SYMATTR InstName R1
SYMATTR Value 10K
SYMBOL cap -928 -96 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C1
SYMATTR Value 100n
SYMBOL voltage -1040 384 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 12
SYMBOL res -704 -48 R0
WINDOW 0 40 37 Left 2
WINDOW 3 41 63 Left 2
SYMATTR InstName R3
SYMATTR Value 1meg
SYMBOL cap -704 416 R0
WINDOW 0 27 3 Left 2
WINDOW 3 45 34 Left 2
SYMATTR InstName C3
SYMATTR Value 100n
SYMBOL voltage -1296 416 R0
WINDOW 0 -53 5 Left 2
WINDOW 3 -242 110 Invisible 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V4
SYMATTR Value PULSE(0 12 250m 1m 1m 1 0 1)
SYMBOL sw -1168 496 M180
WINDOW 0 32 15 Left 2
WINDOW 3 32 44 Left 2
SYMATTR InstName S1
SYMBOL res -880 352 R90
WINDOW 0 0 56 VBottom 2
WINDOW 3 32 56 VTop 2
SYMATTR InstName R4
SYMATTR Value 100k
SYMBOL cap -848 416 R0
SYMATTR InstName C2
SYMATTR Value 10n
SYMBOL diode -816 -16 R0
WINDOW 0 40 31 Left 2
SYMATTR InstName D1
SYMATTR Value 1N4148
SYMBOL cap -400 0 R90
WINDOW 0 0 32 VBottom 2
WINDOW 3 32 32 VTop 2
SYMATTR InstName C4
SYMATTR Value 100n
TEXT -1280 576 Left 2 !.model SW SW(Ron=10m Roff=1G Vt=6 Vh=0)
TEXT -1280 608 Left 2 !.tran 2 uic
TEXT -672 48 Left 2 ;Rt
TEXT -672 480 Left 2 ;Ct
TEXT -520 440 Left 2 ;Tout = 1.1 Rt Ct
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  • \$\begingroup\$ Hello, thanks for answer! This is what I need. +1 as soon as I get sufficient reputation to give positive vote :) Best regards! \$\endgroup\$ – zeti Jun 5 '16 at 11:01
  • \$\begingroup\$ @zeti: You're allowed to switch your accept. \$\endgroup\$ – EM Fields Jun 5 '16 at 12:22

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