I thought I knew how this works but I'm having a bit of a brain freeze with a project here.
I have bought a set of 3 remote controlled plug-in power sockets. It came with a remote with 6 buttons; one on and one off per socket. You guessed it, I want to control the sockets from an Arduino by cannibalising the remote control PCB.
The buttons are all connected to +12V on one side (yes, it really is 12V, there is a "23A" 12V battery in the remote, I was surprised too). The other sides of the buttons go to a HS2260A-R4 chip via a diode network.
So, since the buttons switch to +12V (and not GND), I decided I needed PNP transistors to "press" these buttons from an Arduino.
I came up with the attached circuit (shows a single channel). I added the NPN transistor mainly to avoid having the 12V on the Arduino pins. The part in the thick frame is on the original circuit board of the remote controller.
It does work but not for very long.
Looking at the transistor data sheet the maximum Emitter-Base voltage of the PNP Transistor (a 2N3906) is 5V. The max Collector-Emitter voltage and Collector-Base voltage are both 40V.
When my circuit is active (+5V on the base of the NPN transistor) the NPN transistor closes and I effectively have 12V across Emitter-Base on the PNP transistor. I'm guessing that this is why it stops working as this is way more than the max 5V allowed.
Now, I just can't get my head around how I could switch 12V (or even up to 40V) with a PNP transistor that only allows 5V Emitter-Base voltage.
That circuit should work fine. It does not violate any of the absolute maximum ratings. The voltage across the emitter base junction of the PNP transistor (NPN, too) is about 0.7 V when conducting.
If there is a problem, you may have confused the EBC lead arrangement in wiring one or both of the transistors.
What do you mean by "It does work but not for very long". What happens? Please be more explicit.
The original button, -is it latching and all current passes through it or is it a momentary button that simply puts a short duration pulse to other controlling circuitry?
So, it did work fine when I tested it on breadboard with a single channel, then I soldered all 6 channels onto vero board, put it all in a box with the Arduino and the remote control board. I tested it and it was all still working fine. The next day I wanted to switch a socket on and it doesn't work. The Arduino is still responding and changing the output pin high/low as commanded. But the button of the remote is not being "pressed". Pressing the actual button on the remote still works.
I guess I have to get the meter out and measure where this has failed. I just wanted to find out if I made a general mistake with the circuit.
tinman13kup, the original buttons are momentary switches. The remote sends the on/off signal as long as the button is pressed. In the Arduino sketch when "pressing" a button I set the output pin for each channel high for 200ms, then low again
hmm..
You apply 5v to the 33k, hence the voltage on the gate Vg goes to 5V with respect to gnd, at this moment the source is at gnd. So Vgs is 5V.
The N-CH MOSFET begins to conduct, current flows and a voltage develops across the load, Vs (source) with respect to gnd.
NOW the voltage at the gate Vg is still 5 V but Vgs decreases as Vs increases and the MOSFET turns OFF.
The MOSFET will not turn fully ON, Vs cannot = Vd (drain) so load never has 12V across it.
Tom... :o
