ESP8266 Community Forum

Pablo2048 wrote:To Markusonfire - check Your schematic please (emitter of NPN transistor connected to 3V3 instead of GND and so on...)

Thanks Pablo - corrected

Okay,

So I've done a revised the schematic.

Can you please give feedback on the below design, and possibly advise the spec of the Transistor and Diode I should use?

Also, if you've got any suggestions on other components (diodes, capacitors, etc) which could add to the devices reliability/resiliency, please share. I'd like this device to run 24/7 as long as possible without power-related lock-ups.

Mark

If the diode is supposed to work as flyback diode, it should be connected across the load in blocking direction, not across the transistor. You also need to join GND between the ESP and load.

Apart from that, your circuit should work, provided the transistor is capable of handling the required current and R1 is calculated appropriately. You probably need a darlington transistor to get a high enough gain.

Using a FET instead would look like this: http://www.eng.uwaterloo.ca/~dacananz/P ... witch.html (I would not recommend using Q1 without a base resistor, though)
If you need to isolate the load, you can replace Q1 with an optocoupler.

You don't say what spec the door strike has - probably needs lots of current so the advice to use a Mosfet is well worth taking.

This is the kind of circuit you could use NMOS are generally better than PMOS for simple interfaces...

If you want to keep your existing PCB you could possibly mount your SSR replacement on a square of stripboard. Check out the link to the stackexchange discussion for tips on selecting the Mosfet. Make sure it's a Logic-Level compatible device with a gate threshold below 3V.

And don't forget to take the advice to connect 12V and 3V3 grounds together!