I have an alarm rated for 12V 90mA. I want to turn the alarm on/off with a 2N2222. The base of the 2N2222 is connected to an ESP32. Circuit diagram is attached. Will it work?
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Placing a resistor (10k to 47k) across the base-emitter of Q1 prevents false turn-on in case of a physical disconnection with the Wi-Fi module.
If the wire that connects the alarm to the Q1's collector is long (e.g. >1m) then unwanted spikes can be induced when you turn the alarm off and these spikes can damage Q1. So a diode (silicon such as 1N4148, 1N4007) can be placed: anode to Q1's collector, cathode to 12V (place as close to Q1 as possible).
And finally, make sure R9 is enough to force the Q1 saturate. Calculate R9 for a base current of one tenth to one twentieth of the load current, i.e. 4mA minimum. So R9 can be \$R_9=(3.3-0.6)/0.004=680\Omega\$ but 750R is okay.
answered Mar 16, 2023 at 8:09
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\$\begingroup\$ I dont have much info about current saturation. My alarm needs 90mA to operate. Can you suggest appropriate value of the base resistor for that? \$\endgroup\$– ROBERTCommented Mar 16, 2023 at 8:21