Timed switch

[b1mmuo27]

I am trying to make a timed switch with a 555 timer circuit & a latching 2 pole double throw relay. Here's what I need it to do. I have a monitoring circuit with very little current on it. Let's say it will be normaly closed. When the timer circuit is started it needs to count to 5 seconds & open this circuit. A second pulse directly to the second coil in the relay will flip the relay closed again without delay. Voltages for all this are 12vdc.

 

[Hero999]

What's wrong with a normal monostable 555 timer?

You can reset it by grounding pin 4.

 

[b1mmuo27]

Nothing, if I new what you were taking about. I am an electrician by trade that took electronics in collelge 20 some years ago. The circuit I described is what I can remember. I had a thought last night that if I just put a capacitor accross the 12volt relay coil I could get a 5 second delay of the coil opening. This is what I need but the only problem is how much time do I need to charge the capacitor. Assuming the Rcoil=500ohms. C=10,000uF. The coil would only have a pulsed 12volt of a couple of milliseconds. Thanks

 

[b1mmuo27]

From the beginning

I guess I should explain what I am trying to do. I have two inputs from the alarm in my van. Arm & disarm. I have a door locked monitoring line in the drivers door that needs to be removed or opened when the van "arms" . For that I would simply use a 2 pole, double throw miniture latching relay. One problem though, I need this monitor line to disconnect 5 seconds after the arm input hits the relay. When it gets the disarm pulse for the unlock relay in the van it can pulse the other coil in the latching relay & instantly close the circuit again. My problem is I only have 12vdc pulses to open & close this relay. The same if usr an Rc circuit across the coil of the relay on lock pulse. I don't think it will charge in that short of pulse.

 

[Hero999]

I think I understand now.

Build a normal monostable 555 timer. and connect pin 2 to + with a 10k resistor and to negitive with a 1nf capacitor and omit the 10nF capacitor on pin 5.

Now when you connect the power the relay will be energised for 1.1RC then turn off, the 10k and 1nf provide a trigger pulse when the power is connected.

 

[b1mmuo27]

Could you provide a diagram for this? What you just said went right over my head. I can make a 555 timer circuit from a diagram but what you suggest is rocket science to me. Sorry, just being honest. Remember, I'm an electrician not electronics technician. Thanks

 

[b1mmuo27]

OK im looking at a monostable 555 timer circuit. If I use +12vdc for a trigger circuit which would be for at best a second what values C & R would I need to get it to last for 5 seconds on the output?

 

[Hero999]

R1 = 10k
C2 = 1nf

T = 1.1 Rt * C1

C = 10uF
R = 470k
or
C = 4.7uF
R = 1M

The first option is better.

 

[b1mmuo27]

Thanks Hero999 for the diagram. Would I get the 5 second off delay at pin 3 if I only give Vcc +12vdc for less tham 1 second? Thanks

 

[Hero999]

No.

I must've misunderstood you.

What do you want to do?

Do you want it to trigger a 5 second long output pulse if the input pulse lasts for longer than a second?

For this you need two 555s, I can post the circuit if you want.

Do you want to trigger a 5 second long output pulse when the input goes positive?

To do this just connect a small NPN transistor (BC547, 2N2222, BC109 etc.) in series with C2 and a 100K in series with the base which will be the input, you might also want to add 10nF from pin 5 to ground. I'll show you if you don't know how.

 

[b1mmuo27]

I have to pulses from the truck, lock & unlock. These correspond to the alarm, arm & disarm. I have a circuit which normally would not be switched, to be switched, 5 seconds after the lock pulse. I am using a +12vdc mini 2 pole latching relay. This monostable 555 timer circuit would get the 1 second pulse from the lock & then count 5 seconds & pulse the coil of the latching relay. This in turn would open a set of normally closed contacts in the latching relay that the circuit I want to switch would be running through. The lock pulse would simply pulse on the other coil in the latching relay & close the contacts again. So in simple terms I have a 12 volt pulse for 1 second. I need that to last for 5 seconds & then it pulses a 12 volt relay coil for 1 second. Thanks & I appreciate your help. If this doesn't make sense let me know. If you suggestion above would work, yes could you show the diagram. b1mmuo27

 

[b1mmuo27]

No.

I must've misunderstood you.

What do you want to do?

Do you want it to trigger a 5 second long output pulse if the input pulse lasts for longer than a second? Yes, but the input pulse will NOT be longer than a second.

For this you need two 555s, I can post the circuit if you want.

Do you want to trigger a 5 second long output pulse when the input goes positive? NO I need a delay of 5 seconds after the initial input pulse of 1 second. Then I need a 1 second output pulse.

To do this just connect a small NPN transistor (BC547, 2N2222, BC109 etc.) in series with C2 and a 100K in series with the base which will be the input, you might also want to add 10nF from pin 5 to ground. I'll show you if you don't know how.

Show me. I appreciate you help.

 

[b1mmuo27]

Hero999 I also have a constant 12vdc present where this switch will be. If that is needed for this circuit I can use it to but the operation is still the same. A one second pulse starts a 5 second timer cicuit that at the end of 5 seconds pulse 12 vdc for 1 second. Thanks mate.

 

[Hero999]

One more question, do you want it to trigger at the beginning of end of the one second pulse, i.e positive or negitive edge trigger?

 

[b1mmuo27]

What would you mlike to know?

 

[b1mmuo27]

A quick sysnopsis. I have two triggers I want to use. Both are 12vdc. One is for lock. This 12vdc pulse lasts about 1 second. This is the pulse I want to trigger a 5 second delay timer. After the 5 seconds I want a 1 second pulse of 12vdc to pulse the coil of a 12vdc latching relay. That's all I want the timing circuit to do. Thanks

 

[Hero999]

I understand you you obviously don't understand me.

Do you want the five second timer to be triggered when the one second pulse changes from 0 to 12V or 12V to 0V, (when the pulse starts or finishes), i.e. do you want rising of falling edge triggering?

You mean you only want one initial trigger but two outputs, the one second pulse triggers triggers the first timer which outputs a 12V pulse for 5 seconds, when this finishes it triggers the second second timer which outputs a one second pulse.

 

[b1mmuo27]

Sorry, the easiest way to explain is this. I have a 1 second input pulse which is +12vdc. I would like this pulse to start some kind of timer circuit that can count to 5 seconds. At the end of the 5 seconds I need a +12vdc pulse for 1 second output. I also have a constant +12vdc where this relay will be sitting to use if it helps this circuit in any way.
Now having said all that I'll put it in context. When I press the keyless remote to lock/arm my truck I get a lock pulse from the master lock control on the truck. It is +12vdc for 1 second. At this point I have a latching relay that only on "lock" needs to see this pulse 5 seconds after this pulse. After the 5 seconds I need this +12vdc to reappear & pulse the coil on a latching relay. That's it. That's all I need it to do.

Input +12vdc for 1 second. Start a 5 second timer. Then provide an output of +12vdc for 1 second. End of cycle. Hope this helps. PS I'm an electrician not electronics technician. We speak a different language.

 

[Hero999]

I take it you mean positive edge triggering then.

This should do the job, you might want to consider using a 556 to save space.

 

[b1mmuo27]

Thank you Hero999. Your 0v would be the same as ground? A & B are the outputs? + & -