This should be a simple timer circuit

[myp71]

This should be so simple that I should be able to find this digram in my books but I can't so now I'm asking for help.<p>I would like to have a timer circuit that has + trigger + output and + cancel wires for this timer.I would like this to go in my truck for my domelight to keep it on for 15 sec or so then dim down or just go out if it dims down it has to have a high M.A output because a relay will not work on It.The cancel wire will go to a wire when I turn the key on to cancel out the remaing time on the domelight circuit so I do not have a light on while I'm driving<p>
Thanks everyone Really big help
Myp71
Ryan<p>[ August 06, 2003: Message edited by: my p71 ]</p>

[haklesup]

At the core, a flip flop would work. The circuit would be continuously powered. The door frame button would serve as the set signal and the Accessory power would serve as the reset signal. Naturally you would need some other components to scale the voltage and interface to the relay but that should work.<p>Another simpler way is to wire a DPDT relay to latch. one switch is for your load and the other switch use the n.o. contacts to keep the relay powered the relay when it is energized.The relay is reset when the power is inturrupted and it opens up again. You can use a NC switch or another spdt relay in series with the battery to do this. If this is not clear, I posted a sketch here [url=http://www.ultratest.com\pub\latchingrelay.pdf]www.ultratest.com\pub\latchingrelay.pdf[/url] <p>Neither of these will dim the bulb just on and off. You will also have to adapt it to your cars wiring. I wasn't sure how your Dome light is wired but you will want this to be in parallel with any other switches so it will still turn on from the dash dimmer knob etc.<p>Chris

[myp71]

There is one little problem this looks like it would be negitive switching on the door trigs but my app calls of everything + .<p>
Thanks
Myp71

[haklesup]

Turn the schematic upside down and replace the ground with the battery except for RY2. It would work also if RY2 were on the other side of RY1 as well.<p>I drew it that way because most door post switches have one wire that contacts to chassis ground. To switch the hot side, you need a switch with 2 wires. <p>Another alternative is to replace the doorpost switch with another relay (RY3) and use the n.o. contacts to activate the circuit. Now you can use the voltage to the dome lamp itself to energize RY3 and activate the latch. <p>I just realized, this is basically an RS flip flop made of relays. The doorpost switch or RY3-coil is the set signal and the point labeled Accessory is the reset and the output is one of RY1's poles. See my redraw at www.ultratest.com/pub/rslatchingrelay.pdf<p>To invert the input voltage on RY2 or RY3 simply connect the other end of the relay coil to batt instead of chassis ground. Use relays or switches at set and reset as required by your application. The two control switches and the coil of RY1 can be in any order as long as they are all in series. This is a very flexible circuit.<p>We use a variation of this circuit for the emergency stop button (EMO) on one of our products. It works at any voltage the coils and contacts are rated for.<p>[EDIT] Oops, I think I got Set and Reset swapped in my drawing.<p>[ August 06, 2003: Message edited by: haklesup ]</p>

[myp71]

O.k will anyone check this circuit out and see if this will work for my app just see if it is a plus trigger and if it has a cancel button and a + output to the dome light if not can it be changed some how <p>
Thanks
Myp71

[haklesup]

As drawn the set and reset are positive and the output goes to 12V when set (remember the set and reset are backwards on my drawing). <p>The real reason I responded was that it was pointed out to me that you would also need some sort of timer on the reset as well to account for the case where you exit the car and the light stays on because you are not there to press the button or put the key in.<p>This can be done with a 555 Timer which is wired to stay low for a predetermined time than toggle back to high. (one shot, monostable mode, long duration pulse)

[myp71]

How stupid of me I forgot to post this link I found on a website for another circuit.I was not questioning your circuit building skills.<p>
Thanks for your time
Myp71<p>I will give this a try.<p>[ August 08, 2003: Message edited by: my p71 ]</p>

[Mike]

A simple way to do this would be to use a 555 timer, power it from 12V, and use a switch as the trigger, another disconneting the power when the key is turned, and a relay as the output. Connect a large capacitor through the 12V from the car battery across the relay contacts, such as a 10,000 - 40,000uf, then the light parallel to the cap. When the timer is done, the power to the capacitor and light will be cut off. Then the cap will drain into the lights, causing them to fade out. <p>-Mike