Hello and Happy New Year! Can someone please recommend and describe a simple low cost circuit that has the following behavior?


I have a device that when tripped turns a 12 Vdc output high for 1 second. I would like to turn on a 12V LED light and have it stay on until a momentary button is pressed (as to reset it). I don’t care how many times the 12 Vdc output goes high, as I would like the 12 V led to stay lit regardless, until the reset button is pressed.



Once reset, I want it to be able to turn back on the next time the output goes high. I hope this makes sense.


If possible, can someone please list the parts and provide a simple schematic that I can follow? I have basic soldering and electric skills but little theory or design ability.
Thank you,
Matt

 

A simple FlipFlop like the CD4013 will do the job.

Unfortunately I will not have the time to draw something up this year. The following picture gives the general idea.

It is a really simple circuit. The 12V trigger signal is connected to the SET input, the Reset switch to the RESET/CLEAR input (with an pull-down resistor).

Data and Clock will be tied to Gnd.

If you need to be able to reset the FlipFlop's output before the SET input goes LOW you'll need to couple the 12V signal via a RC (the capacitor in series, the resistor as pull-down.)

At the output Q you connect a transistor to switch your load.

 

You might want to think about adding a circuit to define the state of the circuit on power-up, if you care about this. (You may not want the light to come on before the first trigger signal appears.)

Typically, a level comparator is used to generate a brief reset pulse just as the power supply voltage comes on.

 

This is pretty simple, but NOT recommended if your circuit runs on batteries:

S1 is replaced by your momentary 12v input; it will take mere milliseconds to latch it. The circuit will still need 12v applied to the 12v common terminal though.
One normally closed switch for S2.
One relay, either SPDT, SPST N.O., or DPDT.
One 1k Ohm resistor
One LED, your choice of color.

 

Thank you both. As I already have all the parts as described in the Latching Relay method, and being that my device is powered by ac to dc power supply, I believe I will try this method. I appreciate the FlipFlop description. I will look into this as well as I believe this might suit me better if I should need a "memory" I take it?

 

Thank you both. As I already have all the parts as described in the Latching Relay method, and being that my device is powered by ac to dc power supply, I believe I will try this method. I appreciate the FlipFlop description. I will look into this as well as I believe this might suit me better if I should need a "memory" I take it?

There are two differences between the relay and FF solution you need to be aware of and that may have an effect on adjacent circuitry:
1. a relay will probably consume more current. There are also latching relays that can be toggled with a pulse so there is no continuous current through the coil. The 12V signal needs to be able to provide that current or the signal needs to be buffered. The relay solution is however the simpliest to build.
2. the contacts of a relay bounce, meaning when switching they open/close repeatedly for a brief moment.
3. The FF itself can of course not provide current comparable to a relay contact. It needs therefore a transistor and possibly a relay to drive your load.

I don't know what you mean by "memory". The FF will also loose it's current state when DC power is removed. However a supercapacitor or small coin battery can easily provide power to such a circuit.

It is always a good idea to post input/output voltage ranges and current consumption/sourcing/sinking capabilities of a requested circuit. These specifications determine how the circuit should be built.

 

What trips the circuit? I have 555 design that toggles on /toggles off that might work for your needs.

555 Bistable Multivibrator

 

There are two differences between the relay and FF solution you need to be aware of and that may have an effect on adjacent circuitry:
1. a relay will probably consume more current.

How about: "A relay will ABSOLUTELY without a doubt draw hundreds, perhaps many more orders of magnitude greater current than a 4000-series CMOS IC!" - which is why I made the disclaimer about not running with batteries.

There are also latching relays that can be toggled with a pulse so there is no continuous current through the coil.

This is a good idea.
Omron makes an inexpensive ~$3 relay with dual coils (set and reset) that is DPDT and surface mountable; the G6SK-2. Mouser stocks them:
http://www.mouser.com/ProductDetail...GAEpiMZZMs3UE%2bXNiFaVAVkm0GsZcJ1urj1zePrAtM=
Datasheet:
http://www.components.omron.com/components/web/pdflib.nsf/0/3AB780E03885359085257201007DD68A/$file/G6S_0911.pdf
A dual coil latching relay would be much easier to deal with than a single coil latching relay.

The 12V signal needs to be able to provide that current or the signal needs to be buffered.

That's possible. The G6SK-2 12V coil is nominally 1,028 Ohms, for 11.7mA current flow. That's pretty reasonable, but we don't know what's supplying the 12v pulse. A simple small-signal NPN transistor used as a voltage follower could be all that's required.

The relay solution is however the simpliest to build.

That's basically why I threw it up there; almost everyone who has been in the hobby for any length of time would have the parts for the latching relay thing in their Ye Olde Junque Box. It used to be that you could go to the neighborhood Mom & Pop electronic supply shop and get whatever you needed; now it's order stuff online and wait.

2. the contacts of a relay bounce, meaning when switching they open/close repeatedly for a brief moment.

Very true, and they may bounce several times and perhaps as long as 100mS or even more - but with a 1 second input pulse, the relay will be latched long before the pulse is over with.

3. The FF itself can of course not provide current comparable to a relay contact. It needs therefore a transistor and possibly a relay to drive your load.

The load is an LED. I figure 10mA will be plenty bright. At worst case, another NPN emitter follower could be used to amplify the Q output current.
I don't know what you mean by "memory". The FF will also lose it's current state when DC power is removed. However a supercapacitor or small coin battery can easily provide power to such a circuit. [/QUOTE]
If a memory is desired, the F/F output could drive the gate of a logic-level MOSFET for the LED load. That way, there would be no added current requirement.

It is always a good idea to post input/output voltage ranges and current consumption/sourcing/sinking capabilities of a requested circuit. These specifications determine how the circuit should be built.

Unless I'm missing something, I think they pretty much did that?

 

What trips the circuit? I have 555 design that toggles on /toggles off that might work for your needs.

555 Bistable Multivibrator

The 12 Volt signal is coming from a 3rd party alarm system. I read through your post, but I don't exactly understand what this would be used for. Do you mind explaining what applications make use of a a Bistable Multibibrator? I'm curios. Thank you.

 

The load is an LED.

Yeah, apparently. I never know what is meant with "LED". You think of a normal LED and then (after 25 posts or so) it turns out to be 10 LED strips each drawing 2A, LOL.

 

So far my main requirements have been fulfilled (simple and low cost circuit). Thanks to all again for the help. I have another request on this project, and keeping it in the spirit of simple and low cost, can you guys help me figure this one out...

As I described earlier, the 12vdc alarm output signal is high for 1 second. I timed it and actually it stays high for 1.2 seconds. I need to send a 120vac signal to my lighting system and in order to trigger it, and it requires a pulse within one second (high/low). Since my alarm is 1.2 seconds, and I have no way to change this as this is controlled internally within the alarm manufactures circuit, how can I create a simple, quick, on/off circuit to trigger the lighting system? Is there a way to do it without a 555? Maybe just using a relay and discharge capacitor somehow? Thank you again in advance.

 

I need to send a 120vac signal to my lighting system and in order to trigger it, and it requires a pulse within one second (high/low).

Do you mean you want a pulse shorter than 1 second from the input pulse?

What exact length do you want?

 

Do you mean you want a pulse shorter than 1 second from the input pulse?

Yes, exactly. How about .5 seconds.

 

In other words, my signal pulse right now is 1.2 seconds 12vdc. I need to use that signal to create my own .5 sec 12vdc pulse to send 120vac to my lighting system.

 

In other words, my signal pulse right now is 1.2 seconds 12vdc. I need to use that signal to create my own .5 sec 12vdc pulse to send 120vac to my lighting system.

Simpler than a 555 monostable ? That would also include some components like a transistor, a few resistors, capacitor etc... I don't think it gets much simpler than the 555. Maybe someone will prove me wrong.
The only thing with the 555 is you need to invert your input signal since you want to use the positive going edge of the output signal.

 

The 12 Volt signal is coming from a 3rd party alarm system. I read through your post, but I don't exactly understand what this would be used for. Do you mind explaining what applications make use of a a Bistable Multibibrator? I'm curios. Thank you.

It is a simple flip flop. Push the button once it turns the LED on, push the button again and the LED turns off. It would be a piece of cake making this circuit respond to an external change, such as a voltage going high.

A 555 has plenty of current to drive LEDs.

About your additional request, does this alarm have a 120V input? There are devices such as SSR (solid state relays) that could translate a DC pulse into an AC one.

 

Simpler than a 555 monostable ? That would also include some components like a transistor, a few resistors, capacitor etc... I don't think it gets much simpler than the 555. Maybe someone will prove me wrong.
The only thing with the 555 is you need to invert your input signal since you want to use the positive going edge of the output signal.

Okay okay! lol My local Radio Shack actually had a 555 so I went this route. It was pretty simple. I'm just needing this pretty quickly and don't want to order stuff on-line and wait for it. Needing to get this off my plate pretty quick.

Everyone here has been very helpful. I can't thank you all enough! Very nice forum and very informative.