Using a relay to switch on Lilliput

Vodkapete · 11-13-2004, 04:52 PM

I've got a standard 7" Lilliput screen (not touch screen) that I'm trying to get to auto-on with the power. The PCB layout is different to the touch screen version, so I haven't been able to use Coyote's excellent capacity trick.

However I have built a quick circuit that uses a 555 timer to send a 2 second pulse to a relay which is wired across the contacts of the switch - in principle when the relay is energised, the switch should be shorted.

Unfortunately it does work - at best it's very erratic - sometime it works, but generally nothing happens. Pressing the button still makes the screen come on, and using a bit of wire to short the contacts also makes it come on. Using a multimeter I can check that the relay does close when it should, and the resistance across the pins goes from infinite to zero.

Is there something else going on that I don't know about? I'm only using a small relay - is it too small to provide a decent contact - would a bigger relay be better?

Cheers guys

Ian

Vodkapete · 11-14-2004, 12:29 PM

I think I can answer my own question - I went out today and bought a massive automotive relay, and it works properly. It's too big for permenant mounting, but at least it shows the relay is the problem - I'll try a few alternatives and see what works.

Ian

jcdillin · 11-14-2004, 01:01 PM

maybe the smaller relay is so sensitive that it's actually turning the unit on, then back off again. Thats why the big relay just turns it on. Are you using a reed relay or just a standard relay?

Vodkapete · 11-14-2004, 02:48 PM

That's a good idea - it's not a reed switch, but it is a small fast acting relay, so maybe there is an initial bounce that the IC reads as a 'on' singal, and the main 2 sec hold period is seen as an 'off' signal. Would a small capacitor accross the pins stop this?

Thanks

Ian

jcdillin · 11-14-2004, 04:23 PM

I would try a small cap first and then if that doesnt do it you might have to add small resistor in series with the cap.

The other thing you could do is shorten the pulse and use a reed relay, you might be able to get it down to where to you dont even need to debounce it.

jcdillin · 11-16-2004, 01:29 PM

Did you ever get anywhere with this?

Vodkapete · 11-17-2004, 07:20 AM

Not got it working yet - I thought I'd cracked it last night - I used the existing relay, but connected the normally closed contacts accross the power switch. Then after a delay, the power should open the switch.

My hope was that the N/C power-up state would start the lilliput immediately, and after a couple of seconds the relay would open, allowing the buttons & remote to be used.

It worked last night, and again first thing this morning, but then I couldn't get it to repeat the trick - I'm guessing some capacitance in the circuit somewhere is messing it up, and it needs to be left for a couple of mins between startups for everything to decay back to 0v.

I'm not too sure of my next move... I'm going to have to leave it for a few days anyway - getting married this weekend, and other half seems to think I shouldn't be cluttering up the kitchen table with solder and wires. Girls hey!

Ian

Tony S · 11-17-2004, 09:56 AM

Congrats to you, Ian! I wish you and your fiancee a long, happy life together.

You could power the screen from your PC's power supply. Or, you can insert a switch between the car's ignition-switched power and the carputer or relay. This way you can start the car, then hit the switch and not worry about large voltage flucts.

OR...... if you really want to use a relay and your car's power then.....

Is that relay you have a delay-on-break, etc., or a regular SPST or DPDT relay? If it's a regular instant-on relay than wire it to your car's ignition-switched power. The only problem with this is the screen will turn on, then off, then on again as you turn the key from off/acc/engine crank. This shouldn't damage the screen, but you'd be better off using a timed delay-on-make relay. This way it won't turn on the screen until after x-seconds (you set it). As you turn the key in your car's ignition it'll go thru off/accessories/engine crank, and the relay will reset each time the power cycles but after you crank the engine the power will stay on to the relay. After x-seconds then it will power on the screen. This is probably your best method and safest to the screen.

jcdillin · 11-17-2004, 03:06 PM

Well if I can be of anymore help let me know, congrats on the big day!

Vodkapete · 11-18-2004, 11:57 AM

Got a new problem - after a bit of messing about with different relays and not getting much luck, I now can't get the monitor to auto-on when the button is held in during power up. However once the power is on, a quick press starts the screen straight away.

I'm a bit woried that I've damaged something with the emf from the relay - I stupidly wasn't isolating it with a diode.

Anyway, assuming it still works otherwise, my new plan is to use a roller microswitch to close the contacts as the screen rotates out of the Lexus factory housing.

Ian