Delayed on / off switch

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robnorton

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Hello,

I have an invertor in my car, and would like to have to switch off automagically when i turn the key on & off, I know I can get a 12V ignition line (same line as what signals my carphone to turn on & off) to drive a relay for the invertor.

However, I would like the invertor to only switch on after 20seconds (approx?) after I turn on the key, and turn off approximately 20seconds after I turn off the key. I would also like a override toggle switch for Off, Auto, On ... and if possible a LED to indicate that the invertor is on.

I have looked at voltage switches (the type that turn off after the battery reaches a certain voltage etc).. but dont like that concept..

I am able to follow diagrams etc, but cannot design this kind of thing myself

Any help would be appreciated...

Many thanks,
Rob
 
Hi,

Yeah, I have a understanding of how things work, I'm able to convert a diagram into a working circuit,
Just unsure how all the components interact..
 
Sounds like you need to read up on 555 timers
 
Funny you mention that,

I was reading up on the 555 timer last night, but am unsure on which way to go about it, this is the site that I was reading last night:

**broken link removed**
 


Attached is something that should work. If you can put this together then we can fill in the missing details.
 
Thats brilliant, what size resistors & capacitors will I need to get me going?

Is the 12V line going into pin8 a permanent 12V supply? or the same supply from the ignition line?
 
robnorton said:
Thats brilliant, what size resistors & capacitors will I need to get me going?

Is the 12V line going into pin8 a permanent 12V supply? or the same supply from the ignition line?
Click to expand...

See attached schematic. The 12V line into pin 8 must remain connected to the battery for the circuit to operate. It will use about 8 mA which is not too taxing on a car battery. The relay can be replaced with a solid state type if a suitable relay cannot be found.

It is prudent to put a .1 uf capacitor across pins 1 and 8 of the 555. It is also prudent to put a 15 volt zener diode, or transorb, across pins 1 and 8 to make sure voltage spikes do not damage the 555. I did not show these items on the schematic.
 
You don't really even need a 555 for that, a simple RC time-constant circuit would accomplish what you need..
555 is definately sufficient for this though if you got one..
(Its also prudent to tie pin5 to gnd through a small cap)..
 
Hello!

Ok, so I've built the circuit you gave me, and it works very well, I could only make up resistors to get 91K (stupid irish shops..), but still has the desired effect...

about the zener diode, does it matter which way it is orientated?
 
robnorton said:
Hello!

Ok, so I've built the circuit you gave me, and it works very well, I could only make up resistors to get 91K (stupid irish shops..), but still has the desired effect...

about the zener diode, does it matter which way it is orientated?
Click to expand...

I'm glad to hear it worked. Thanks for letting me know.

The resistors along with the capacitor determine the time delay. Larger value resistors and larger value capacitor, give greater time delay. The topmost resistor determines ON delay. The lowermost resistor determines OFF delay.

The cathode of the zener (the end with a band on it) should connect to pin 8 (+12 Volts). Of course, the other end (the anode) connects to pin 1.

If there are voltage spikes in the +12 volt power with high energy content above 15 volts, the zener will overheat and short out. I can't say this is true in your case, but a fuse between the battery and the circuit is a good idea in any case. Should you find that the fuse blows because the zener conducts too long and overheats, then you can put a small valued resistor (500 ohms, max) between the +12volts from the battery and the cathode of the zener to help absorb some of the energy of the voltage spikes, preventing the zener from getting damaged. Automobile power can be quite dirty, with lots of voltage spikes. The larger wattage the zener, the better too.
 
great.. have done that too, I built the circuit in a simulator too and fiddled with it to put in a switch which will give me the options of having the invertor always-off, always-on, timed by cutting the 12v line to the 555 chip, disconnecting the RST pin & having it like it is now (respectively, for each option...) it seems to work in the sim, is there any reason why I shouldnt do it this way?

Also, I'd like to add 2 LED's red & green, the red one should be on when the ignition key is on, and the relay is off, the green one should come on when the relay turns on, but the red one should switch off.. how difficult would it be to add this?

Many thanks,
Rob
 

Disabling the timer's control over the relay, thus forcing the relay to be energized by holding the RESET line low is fine. Another way is to place switch contacts in parallel with the relay contacts.

Your green LED can be placed in parallel with the relay coil. A resistor must be put in series with the LED, say 470 ohms.

Your red LED can be connected between pin 3 of the 555 and the ignition switch, with the anode at pin 3. Again, a resistor must be put in series with the LED, same value as for the green LED
 
ok, i've done that, although, the red LED doesnt operate as expected - pin3 goes high while the relay is off, currently the red LED glows dimly (while the capacitor discharges) after the relay is off, and the ignition line is disconnected, am I going to need a logic circuit (AND) to achieve the red light - so that there is a signal from pin3 and the ignition for the LED to work?
 
Further reading leads me to believe that a transistor could work too?? pin3 to the base of the NPN transistor, ignition signal into collector, emitter to ground??
 
Arrr, yeah I see my error. Do you have an unused pair of relay contacts? If so, they can provide a ground for the red LED instead of pin 3.
 
robnorton said:
Further reading leads me to believe that a transistor could work too?? pin3 to the base of the NPN transistor, ignition signal into collector, emitter to ground??
Click to expand...

Yeah, sort of. You would need a resistor between pin 3 and the Base, say 4700 ohms. The Collector connects to the ignition switch through the red LED (anode to ignition switch) and 470 ohm resistor. Emitter to ground.
 
A quick question

I'm starting with a 13.8VDC power supply not a car battery. I'm thinking about using this circuit in a similar project though im not useing an ignition switch but a key'd on/off DC switch so very similar. I'd like to have one Red LED blinking while the switch is in the off position. When the key is turned to the On position i'd like there to me a 3-4 second delay before the relay throws. Durring that 3-4 second period I'd like for the Red LED to continue blinking untill the time is up when the relay throws
i'd have a Green LED that would go on. But when the key is switched off the Red LED should go back to blinking. I think the basic circuit here would work

Thank you for any thoughts and advice you might have.
Trav
 
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