pulse on, pulse off latching switch

[b1mmuo27]

Hello. I am asking this question here as the relays I have bought for this purpose don't do what I want. First the load I am trying to switch is carrying milliamps. That is why I think I can do it with electronics. Control voltage is 12 volt dc. I need to have two 12vdc +pulse inputs in to control this switch. I need a N.C. contact at rest. Pulsed on one input it would open to a N.O. position & stay their until it was pulsed again throught the other input & it would then return to N.C. If possible I would like the N.C pulse to delay for 5 seconds before it opened. The return pulse to close would be instantanious. The application is on my car alarm. I am using the lock pulse to open the N.C. circuit & the unlock pulse to return the switch to N.C. position. I am trying to take a lock sense circuit out of the circuit while the alarm is on & reuturn it back to normal when the alarm is off. I hope this makes sense to some one. I can buuild the circuit but my electronics was 20 years ago & I am an electrician. Thanks

 

[mstechca]

sounds like an oscillator with an unusual duty cycle.

You can make a two-transistor astable multivibrator (square-wave oscillator) to produce your pulse description.

Normal oscillators run at a 50% duty cycle, where the result is high and the result is low at the same time. Adjusting the resistors connected to the NPN's bases, or the capacitors can change the duty cycle.

There are transistors that can handle milliamps, and even amps.

try TIP31 or TIP32.

 

[b1mmuo27]

Tanks, but.......

I understand they are transisters but what I am asking is what the circuit would look like. I am an electrician, I could do it with relays but what I am looking for is a wiring diagram thanks. :roll:

 

[DigiTan]

You could feed a 555 timer output to a Toggle flip-flop. If so, you'd just take a JK flip-flop like the 74LS109 and wire the J and K pins to +5 Volts. Sending a clean pulse of any length the CLK pin will toggle the output and latch it. If you are allowed to order additional parts, the 74XX109 itself would replace about 16 transistors. Is this possible?

 

[john1]

If you use a small relay,
you could put a capacitor across the coil,
which would hold it for a few seconds,
so that it wouldn't drop out straight away.

But it would still close quickly.

John

 

[mstechca]

...you'd just take a JK flip-flop like the 74LS109...

wait a minute here.
b1mmuo27 didn't specify how many milliamps he is dealing with here. IC's can handle only a limited number of milliamps, and I see the LS series being the worst, when it comes to dealing with high current.

as for circuit diagrams, goto:

**broken link removed**

 

[b1mmuo27]

Current

I don't believe their is any "real" current running through the switched wire as it is can bus . Door lock telling the body controller I'm open or closed. The conductor I am trying to switch is only 20 gauge.

 

[Gorgon]

Hello. I am asking this question here as the relays I have bought for this purpose don't do what I want. First the load I am trying to switch is carrying milliamps. That is why I think I can do it with electronics. Control voltage is 12 volt dc. I need to have two 12vdc +pulse inputs in to control this switch. I need a N.C. contact at rest. Pulsed on one input it would open to a N.O. position & stay their until it was pulsed again throught the other input & it would then return to N.C. If possible I would like the N.C pulse to delay for 5 seconds before it opened. The return pulse to close would be instantanious. The application is on my car alarm. I am using the lock pulse to open the N.C. circuit & the unlock pulse to return the switch to N.C. position. I am trying to take a lock sense circuit out of the circuit while the alarm is on & reuturn it back to normal when the alarm is off. I hope this makes sense to some one. I can buuild the circuit but my electronics was 20 years ago & I am an electrician. Thanks

Hi,
If I understand you correct you want to use 2 pushbuttons? To solve this with logic you only need a flipflop made from 2 nand gates, a delayed activation of a small relay that is NC. When activated you go open. I think you could do this with a quad nand Schmitt trigger chip. If you want to use only 12v you need to use the 4093 chip.

TOK

 

[instruite]

as you said you are electriocian then I guess you will be able to wire this circuit with just one relay see the attachement

the only thing is you wont get a 5 second delay which you want for that you will need to use a Turn off delay timer relay

 

[john1]

Hi Gorgon, and b1mmuo27,

I also got the impession that the poster was describing a arrangement
with two press buttons, one to engage, the other to disengage.

But i wasn't sure.
And the more times i read through that post, the less i understood it.
After reading it many times, i still wasn't sure.
I'm still not sure if thats what is meant or not.

Assuming that it is, then i will offer this possible solution.
Its just a suggestion, i haven't made it.
The values are just tentative, you would have to see what works best.

The circuit is very simple, it relies on an SCR as a latch.

Almost any relay with a fairly low current coil would do although it
was intended for the newer type of reed switch with a NC contact,
these are now common, yet for years they were available but rare.
I assume the extra contact is brass or some non ferrous material.

The R1 resistor is to limit the surge through the SCR, maybe 500
ohms or so would do for a start ?
The R2 resistor is to keep the gate shut except when operating,
maybe 1k would do ?
The R3 resistor has to be capable of operating the gate, maybe 20k
or so ... ?

The SCR is just a normal sort of thing, see what the shop has got,
they are fairly cheap, i would expect 1 amp or so at a couple of
hundred volts working to be normal stock. Its best to go over the
top on the spec for use in a motor car, it won't need to pass an
amp, and the kick-back voltage from a reedswitch is typically low
probably cos they aren't particularly inductive like a relay with
iron bits around it.

SCRs are quite responsive to even tiny spikes. You may need to do
something to stop it responding to spikes from the vehicle.
Maybe clamp it with a twenty or thirty volt zener to soak any spikes
that might come along. Vehicle wiring can be very harsh.

This circuit is offered only as a suggestion, it is where i would
start if i wanted to do what you described. Where vehicle wiring is
concerned my policy is to keep things as simple and minimalist as
possible.

Best of luck with you project, John

 

[john1]

Hi instruite,

Your circuit is indeed very simple,
and of course would work ok, as it is.

But if a delay were introduced to pause the relay dropping out,
then the 'hold-in' contact across the 'start' button would stay
for a while, so the user would have to keep his finger on the
stop button,
if a delay were introduced.

Also these flat little membrane type press buttons are always a
normally open type. If a normally closed is wanted such as the stop
button in your diagram, then a suitable switch with NC contacts would
have to be found or bought.

But the circuit is fine as it is, with no delay.

Best Regards, John

 

[b1mmuo27]

ideas

Thanks for all your ideas. I think I should start from square one as the idea has somehow been missed.
First I would like to do this through CMOS. NOT relays. I could use relays as I now how to do it that way except it takes up too much space.
Here is the operating senario.

I have a lock pulse which is at +12volt dc. It lasts for a fraction of a second. What I need it to do is count 5 seconds after this pulse then open a normally closed electronic relay. Through this relay is a can bus signal which is of very small current. (maybe 200 milliamps). It's a communication signal. At this point I want the circuit open.
I have a second input which is at +12volt dc which will be instantanious & close this normally closed contact. This will allow the can bus signal through again.

So two seperate +12 volt inputs controlling a "dry" set of normally closed contacts. (electronic relay). The input that opens the contacts needs to have a 5 second delay before it opens the contacts. If I could draw this I would to clarify this. However if I could do that I wouldn't be asking for help. Thanks & sorry for the confussion.

 

[Gorgon]

Hi,
If I understand you correct you have 2 signals that is active(pulsed) to +12v? What is the input state when not activated? 0V or floating(not connected)

And the output from this, the electronic relay, how do you control this?

Is this controlled from the 2 inputs today? If so, is the electronic relay a two input latching thingy?

I might extrapolate to far, but is what you want in fact a 5 sec delay on the relay 'on' input?

TOK

 

[instruite]

Hi john1

here is the complete solution as per the problems you said
Actually I thought b1mmuo27 wanted to do with realys only
thats why I suggested that it is used mainly in case of PLCs or hardwired electrical circuits

 

[john1]

Hi instruite,

Yes, that looks perfectly good to me.
After i had figured out that 'T' is a timer-relay operating the contacts 'T1 NO'.

Personally, i would be happier with electro-mechanical devices in
a motor car, than i would be with home-made electronic circuitry.

So if it were me, i would go for a relay solution, like that.

Regards, John

 

[instruite]

Hi john1
you post remionded me of the mistake in diagram
that is the symbol of timer realy is wrong
I made the timer relay contact symbol right but forget abt the relay symbol itself
https://www.plcs.net/chapters/time16.htm
check the ablove link for the same
Regards