I have a project where I have a single board PC (sorry, Arduino is slow for this one, it's an x86 PC) and a pico projector inside a custom device.
I need both the projector and PC to turn on from one click.
Problem with my (and seems most) pico projectors is you need to press and hold the On/Off button few seconds for it to turn on.
Anyway I could hack the PCB of the projector and tell Arduino to "fake" a 3 second button press? How would you do it?
You need to know how the existing switch works.
If it simply makes a contact that must be held for a period of time it should be possible to make an Arduino replicate that. The simplest thing might be to use a relay to close the contact and use an Arduino to control the relay. That will completely isolate the Arduino voltages from the projector voltages.
Of course this assumes that you can solder a wire to each of the contacts in the existing switch.
It is possible that the switch makes some more-complex connections that could not easily be replicated
...R
Yes, it's a simple button which creates a contact, then the microcontroller probably just starts counting how long there has been non stop contact.
What kind of relay do you have in mind?
wonxega:
What kind of relay do you have in mind?
cheap ?
You need to choose a relay based on 2 considerations.
The switch contacts must be rated for the voltage and current that will be switched. It sounds like your application will have a low voltage and very low current. It does not matter if the relay is rated for higher voltage and current.
And the relay coil must work with the voltage you plan to use to activate the relay. Don't be tempted to power a relay coil from an Arduino - the Arduino pins can't provide enough current. You need to arrange for the Arduino to operate a small transistor which in turn will switch the power for the relay. That means the relay is not limited to using a 5v coil. There may be example circuits on this Pigihx website.
You can get solid state relays that can be controlled directly from the Arduino but they tend to be more expensive.
...R
I wouldn't mind using SSD relays. But all I found were for controlling AC current via DC signal.
wonxega:
I wouldn't mind using SSD relays. But all I found were for controlling AC current via DC signal.
Post a link to the datasheet for the most suitable looking device.
...R
Not sure what you need.
wonxega:
Not sure what you need.
I assume you have identified an SSR that you would like to use except that it seems to be for AC only. Post a link to its datasheet.
...R
Hi,
What voltage/current are you switching?
This is very important to give you a proper answer.
If all you need is a three second ON pulse from a momentary pressbutton, why not use a 4000 series, monostable IC.
Use hardware, bit of a waste to use a microcontroller just for that.
What is your electronics, programming, arduino, hardware experience?
Tom.... 
TomGeorge:
If all you need is a three second ON pulse from a momentary pressbutton, why not use a 4000 series, monostable IC.
NE555 (preferably CMOS version).
TomGeorge:
Hi,
What voltage/current are you switching?
This is very important to give you a proper answer.
I don't know, I'm hacking a commercial device, not working on an open hardware. It's a 12V device and for power on/off switch I'd assume it uses very low current.
If all you need is a three second ON pulse from a momentary pressbutton, why not use a 4000 series, monostable IC.
Use hardware, bit of a waste to use a microcontroller just for that.
I don't know, to be honest my electronics experience is pretty limited, I usually just solder wires and mostly do programming for such projects, that's my thing. If the device (a pico projector) had a switch button instead of a press button like some others I've had do I wouldn't need to hack it at all, the switch button could just be set to On and it would just turn on when powered automatically.
I already use an Arduino Nano for other parts of the device (menu buttons and such and communicating with an integrated x86 PC) so I can use it for this too.
you can charge a cap in an instant.
then that cap can power a transistor that sends out that power to your other unit.
you press the button, the cap charges.
the cap saturates the transistor /FET, turns on power. that power charges the cap, you have a sustained ON
all simple parts, a resistor, cap, FET.
is that all you need ?
I have tried to see my had speed of a button press and I cannot press for less than about 8ms.
that is about 800 times slower than you need for this to work.