Power Off Time Delay Relay Circuit

Very handy and simple circuit if your looking to have lights, accessories, or other devices power off after a preset time has elapsed after power is no longer being applied to the circuit. Relay contacts can be used on DC or AC loads. The circuit shown here is operated on 12VDC.

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Closed Caption:

okay this is another circuit
very handy to have
and what it is is a power-off time delay
circuit for instance say you wanted to
have you don't light in your car go out
a certain time frame after you take the
key out of the ignition and you get out
of your car at night
alright so this would be an ideal
circuit for that how it works
you would connect this to the power
coming in 12 volt battery so say the
switch went to the light switch going to
the light or whatever else you want to
control goes to this point right here
right now as long as the switches on
this circle always be on
but once you turn the switch off or take
the key out of the ignition or what will
happen this diode keeps the current
right
current flows through the diode which is
just a 401 right and it goes into a
capacitor which goes to ground
basically what you're doing is you're
charging this capacitor and you're
holding the charge here not allowing it
to go backwards towards the car battery
or towards another circuit
all right so it stays there and it
slowly bleeds through a resistor into
the base of a transistor which could be
a to end 39 or 40 or a to end
- - you could probably use others to
start trying out
and what you do is you take the emitter
and you go to ground you take the
collector and you connected to a relay
with a back EMF diode which is probably
just use a 407 and that side goes to the
plus
so you going to take power from the
battery which is always on
all right taking power that's always on
coming into the relay coil through the
back EMF tire into the collector and
emitter to the ground
all right this is a real i'm sure that
when i used a few years this one right
this is the type of relay now you can
put this whole circuit together
just from an old TV set or an old VCR
this is all you need right here
I use a 12 volt because i plan on using
this for 12 volts but you can use a 5
volt relay use a nine whatever you want
to use you use so i use that and this
capacitor electrolytic 25 volt 3300 UF
there's the diode
that's the transistor which is just a
two and three or four and it's a 47 k
with these components i use right here
this is the time i get all right
3300 uf capacitor along with a 47 k
yield six minutes
now if you use it a little lower value
resistor you're going to get four and a
half minutes out of it
27 k you go a little lower down to 10
you might get like three and a half but
you have to realize there's several
factors here is determine how long the
circuit will run a higher value on the
capacitor will give you a longer run
time before the relay disconnects and a
higher value on this will give you a
little longer time also
this is the primary factor for how long
it's going to stay on off and this one
is a secondary you know this one changes
it but not as much as the capacitor this
so this is pretty much what you got to
do you can play around with the values
and that's based on this load this
doesn't draw more than maybe thirty
thirty five milliamps to trigger this
and this will control five amps of
current so if you use a bigger relay
which draws more current this time is
going to change going to be lower and
that's the circuit
it works really good and it has a lot of
uses
you can have a train like a touch
circuit with the transistor right here
that you can touch like a high-gain
darlington that will trigger this
circuit and turn some money maybe a
light or motor appliance and in this
time frame it will shut off