using ESP-12F to power on/off a USB mini projector issue

I have a mini projector (powered by usb 5v) which can be powered on using a push button. Since the push button is actually closing an open to turn the projector on/off, My plan was to create a circuit using ESP 12-f to mimic this operation by sending an http command remotely

This is the circuit I'm using:




Using a multimeter, I am measuring the resistance between the yellow and green wires, to see if my http command works, and indeed the resistance is moving between infinite to none as I send the http command to turn high/low the GPIO4 pin.

so i did the following:
1. connected the green/yellow wires to the projector's push button connectors
2. connected both grounds together (my circuit's and the projector one's)

the behavior of the resistance values are now different - it is continously close to 0, and the http command changes from this small resistance to infinite resistance which isn't good.

What am i missing?

The problem is that you don't know what the button is connected to. It's most likely an input with a pullup resistor that's grounded when the push button is pressed. What you need to do is measure the voltage on both push button wires when the switch is pressed and not pressed with the projector both on and off. One of the connections probably goes from 5V (or, hopefully 3.3V) to 0V when the button is pressed. Once you know this you can them move forward.

Mike.

thanks - it's 2.7v when not pushed, 0v when pushed - same behavior when projector is on or off

Is the other wire always zero?

Mike.

sorry - what do you mean by other wire? oh - green/yellow

Yes, what do the green/yellow wires measure?

Mike.

omg - it was 0.4v - seems like one connection was loose. after fixing it, it shows 2.7v, and it is working now! thx

For buttons, I've used Opto FETS or Optimos relay chips. Probably a better choice if the buttons are scanned.

It gives you isolated control.

Sometimes I use a current source (LM334) and a diode to drive the OPTO LED so you end up with like a 6-30V somewhat protected control input.

The OPTOMOS parts, some need only 1 mA for the LED.

KeepItSimpleStupid said:

For buttons, I've used Opto FETS or Optimos relay chips. Probably a better choice if the buttons are scanned.

It gives you isolated control.

Click to expand...

sounds interesting. these are electronic relays? can you post a link pls?

EXAMPLE P/N: https://www.ixysic.com/home/pdfs.nsf/www/CPC1017N.pdf/$file/CPC1017N.pdf Available at DigiKey

On resistance usually does not have to be low in this application.

will check it out thx

I used these https://logosfoundation.org/instrum_gwr/playerpiano/Optor_VTL5C1_87223.pdf in the mod-1980's to interface a single push button of an audio system. I believe they are obsolete now.

I also used it as part of an exponential ramp of the audio at turn on as well as rapid turn-off of the input to a power amplifier. The leads were useful.