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Old 24th November 2006, 04:39 AM   #1
Nixie is offline Nixie  Canada
Join Date: Jan 2006
Location: Vancouver, BC
Default Driving a latching relay

What's the simplest way to convert a continuous low/high signal into pulses to drive a dual-coil latching relay? Capacitors are not a robust option here as the signal can have a long rise/fall time, say due to current inrush limiters if the signal is from whether power is on or off in the device.

[Edit] I need a discrete circuit I can build with spare parts, not having to order a special IC. I also need a circuit to generate a pulse to switch the relay back at power off.
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Old 25th August 2008, 04:27 AM   #2
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Join Date: Oct 2004
Bumping this because I have a the same question.
Couple of differences.....

I am working with very small DC signals that need to be noise-free, so I'd like to avoid adding IC's if possible inside the same case.
Battery operation.

Pulse length to coil, for alternating relay latch position....4.5 mS (minimum)
Using these relays:

Teledyne 422D (12v)


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Old 25th August 2008, 04:41 AM   #3
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I'm not sure if I understand you 100 %.
If you need something in discrete form with minimum elements see my "Patent input selector", I used momentary switch buttons with led ind.and with simple non latching relays.
No rf noise!
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Old 25th August 2008, 07:01 AM   #4
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I think this might work.

Unlike the original poster, I need these to "stay" in the same direction when powered off.
There can (and will) be more relays and switches added.
Each momentary switch turns one specific relay "on" in one direction and all the other(s) "off" in the other direction.

Maybe adding some polarity reversal protection diodes for safety (??) (added noise??)

Metal case will be grounded for these particular TO-5 style Teledyne relays (hard-to-find ) .
One concern I have is that each time a switch is pressed, one relay will activate, but the rest will be (re)energized in a direction that is already latched...I hope this will not create problems down-the-road, or shorten the effectiveness of the relay..... Although these won't be subject to massive usage....maybe 200 times a year.

I don't know if these are "make-before-brake" or "break-before-make"....I will test them.


Click the image to open in full size.

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Old 25th August 2008, 08:51 PM   #5
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...adding more relays means having to use a more complicated switch.
For example, 4 relays would require 2 4PDT momentary/NO (Normally Open) ....such as:
C&K #7405 momentary (mom-off-mom) Digikey# CKN1138-ND
(up or down toggle would trigger/switch between two relays)

This drawing shows four 4PST switches (same as using half a 4PDT per relay).
4PST mom switches are VERY hard to find.

Click the image to open in full size.

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Old 28th August 2008, 07:41 AM   #6
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What about the classic "555" multivibrator or whatever they called it.
One R and one C sets the time constant (pulse width).
They've been around for literally decades.
There are probably CMOS versions.

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