Momentary switch for on-off operation

[chimpboy]

I have a momentary switch and want to use it to turn a rear screen demister on and off, preferably in a way that doesn't "remember" that it's on once the ignition is turned off.

I think I can do this with a jk flip-flop and a relay, but it's getting outside my comfort zone so I wanted to check with you electronics guys.

It seems like I could take this from jaycar:

http://jaycar.com.au/products_uploaded/ZC4027.pdf

... leave S and R at 0, J at 1, connect Q to K, connect CL to the button, and use Q1 as the output for a relay to the demister.

Does this make any sense at all or am I truly electronically n00b?

(The momentary switch is built into a unit so I don't want to use a different switch if I can help it.)

Jason

[jet-6]

Its not hard to do at all, if you want i can draw it up for you and email you the link?

[chimpboy]

Its not hard to do at all, if you want i can draw it up for you and email you the link?

For sure, that would be great.

[DamTriton]

555 timer or similar set for about 10 mins to automatically turn off, that way it never gets left on accidentally.

[jet-6]

Yer now i think about it, a multi funtion timer with a pulse on would be a much better option, i have a few timers here that would do the job, one button press and it would run for what ever time you set, and would not start until you press the button again

[chimpboy]

Actually that is not a bad idea although different from what I had in mind.

However, if I turned it on accidentally would I then be stuck with it on until the timer ran out?

There is an indicator light built into the switch.

[DamTriton]

Actually that is not a bad idea although different from what I had in mind.

However, if I turned it on accidentally would I then be stuck with it on until the timer ran out?

There is an indicator light built into the switch.

Maybe the South Koreans do know how to do something right, this is how the rear demister works on the Kia (and I'm guessing on most other vehicles too...).

So what if it is left on for an extra couple of minutes, would it make any real difference?

[chimpboy]

Actually that is not a bad idea although different from what I had in mind.

However, if I turned it on accidentally would I then be stuck with it on until the timer ran out?

There is an indicator light built into the switch.

Maybe the South Koreans do know how to do something right, this is how the rear demister works on the Kia (and I'm guessing on most other vehicles too...).

So what if it is left on for an extra couple of minutes, would it make any real difference?

So what is the cheapest robust way to do it? Is there a self-timing relay I could pinch from something at a wrecker?

[stuee]

Actually that is not a bad idea although different from what I had in mind.

However, if I turned it on accidentally would I then be stuck with it on until the timer ran out?

There is an indicator light built into the switch.

Maybe the South Koreans do know how to do something right, this is how the rear demister works on the Kia (and I'm guessing on most other vehicles too...).

So what if it is left on for an extra couple of minutes, would it make any real difference?

So what is the cheapest robust way to do it? Is there a self-timing relay I could pinch from something at a wrecker?

My parents 2004 Toyota Corolla will leave the demister on until it is switched off again or the ignition is switched off. So don't look for an auto timer off one of those

My disco has either a self timing or heat sensing circuit in the demister that magically turns off the demister when the window and mirrors are clear. (Never timed it but it appears to vary in time to me which is highly unlikely but i like to think that LUCAS was practicing dark magic when they made it...).

So I don't know if you'd find a specific demister timer circuit, but I'm sure you could adapt a body control module of some sort. Failing that, I'm sure jay car make a basic timer kit which would be suitable (Its based on a 555 timer I'm fairly certain). Soon as the ignition is turned off the timer would reset. Like DAMKIA said does it really matter if its left on for an extra few minutes?

edit* and there's no cheapest way with anything English

[stuee]

Jaycar basic kit which could run of car 12v power supply $20
http://www.jaycar.com.au/productView.as ... rm=KEYWORD

Complex kit for having a demister that can run when it doesn't need to $43
http://www.jaycar.com.au/productView.as ... rm=KEYWORD

You could probably even find a kit to power you demister in time with your stereo beat.

[DamTriton]

My disco has either a self timing or heat sensing circuit in the demister that magically turns off the demister when the window and mirrors are clear. (Never timed it but it appears to vary in time to me which is highly unlikely but i like to think that LUCAS was practicing dark magic when they made it...).

Nah, it just stops working when it gets hot like all Lucas electricals....

------------------------

Seriously, the $20 jaycar kit would do the job. You could spend as much at a wreckers and still be left with something dodgy.

[chimpboy]

My disco has either a self timing or heat sensing circuit in the demister that magically turns off the demister when the window and mirrors are clear. (Never timed it but it appears to vary in time to me which is highly unlikely but i like to think that LUCAS was practicing dark magic when they made it...).

Nah, it just stops working when it gets hot like all Lucas electricals....

------------------------

Seriously, the $20 jaycar kit would do the job. You could spend as much at a wreckers and still be left with something dodgy.

Heh.

Can I just ask though, the jk flip-flop option is like two bucks, would it work the way I've suggested?

[macca81]

hows this??

http://store.autospeed.com/Items/22901? ... 20Prebuilt

[jet-6]

Actually that is not a bad idea although different from what I had in mind.

However, if I turned it on accidentally would I then be stuck with it on until the timer ran out?

There is an indicator light built into the switch.

You would need a second switch, or wire it to the key, just turn it off then on, fixed, easy

[stuee]

Nah, it just stops working when it gets hot like all Lucas electricals....

Gues chimpboy could just plug the demister into a Lucas box and be set

Can I just ask though, the jk flip-flop option is like two bucks, would it work the way I've suggested?

Whats the current rating on the flip flop? Often IC's aren't suitable for driving relays directly so you will require extra circuitry to drive a relay or pass through the current required for the demister.

[chimpboy]

Nah, it just stops working when it gets hot like all Lucas electricals....

Gues chimpboy could just plug the demister into a Lucas box and be set

Can I just ask though, the jk flip-flop option is like two bucks, would it work the way I've suggested?

Whats the current rating on the flip flop? Often IC's aren't suitable for driving relays directly so you will require extra circuitry to drive a relay or pass through the current required for the demister.

Can you advise me on what the extra circuitry would be?

Or do I just want something like this? http://www.mcmelectronics.com/product/28-5115

[stuee]

Can you advise me on what the extra circuitry would be?

Or do I just want something like this? http://www.mcmelectronics.com/product/28-5115

That board shown in the link basically shows all the extra circuitry that would be required. Basically a transistor switching circuit with protection diodes, you may get away without a voltage regulator seeing as the first flip flop has a wide supply voltage. (You also need the pcb, connectors etc)

With the flip flop you will need to remember to turn it off every time otherwise it will be left on, and unless the flip flop resets to the off position once it looses power then you will need to remember to turn off the demister next time you get into the car if you forgot to turn it off last time. I'm not sure of the behavior of flip flops in powered and un-powered states as I hated that unit at uni and didn't pay much attention so I cant say for sure how the flip flop will act.

My personal preference would be the timer circuit from jay car for $20 bucks. I've used a few jay car kits now and they are hard to stuff up the building (except when you don't read the instructions and fail to add a link on the underside of the pcb that's not marked on the top layer...). They also proven devices that are generally pretty reliable.

[chimpboy]

With the flip flop you will need to remember to turn it off every time otherwise it will be left on, and unless the flip flop resets to the off position once it looses power then you will need to remember to turn off the demister next time you get into the car if you forgot to turn it off last time. I'm not sure of the behavior of flip flops in powered and un-powered states as I hated that unit at uni and didn't pay much attention so I cant say for sure how the flip flop will act.

Ah, I definitely don't want it to do that. I want it to always be in the "off" setting when the car has been switched off. So I will have to check.

I will mull over the timer vs on/off button options!