A relay switch.
It’ll have four poles, two for the low and two for the high.
When you pass a current through the low side it’ll make contact on the high side and complete the circuit.
Nah m8, you want a MOSFET. A fraction of the cost of a relay, more reliable and much more efficient in this application. Relays only make sense these days for AC applications. Tuppence each from Farnell, or a quid for a big bag on eBay.
>>2881 Watch battery won't run a relay (for very long, or at all). MOSFET or just putting your switch in the right place are sensible options.
That's assuming you're not just copying stuff from t'other place for some demented reason.
If you're the lad from >>2882 then this should work.
But before you go modding the doorbell check the button isn't knackered and that the connections aren't loose or corroded.
Why would you take the switch connection from the low (switched) side of the solenoid? Are you trying to build an oscillator this way, to make the bell ring?
Rule 3 of electronics: Amplifiers oscillate, oscillators don't.
At least be prepared to put a capacitor between B & E , and even then, be prepared for the bastard thing to just fins a bias point and sit there burning power in the transistor until it fails.
>>2889 Because it's less of a pain in the arse to wire it that way when you take into account the physical construction of the doorbell and the existing wiring, and because it's a doorbell not a finely tuned SMPS.
Even if there's enough capacitative coupling in the cable to cause it to oscillate it should act as a charge pump via the B->E junction of Q1 until the base voltage gets pushed far enough below the switch on threshold to kill any oscillations.
sage because the proper answer is to replace the button and/or cable rather than mod the doorbell to work for a bit longer with failing wiring.
>>2890 The joy of electronics is that it doesn't give a damn if you're trying to build something trivial, the same rules still apply.
I think you should reduce R1 (possibly to zero), or you'll be pissing away volts across, and power into, the transistor. You want the transistor on hard. As it turns on, the voltage available through the switch will drop, as more volts appear across L1. No need for a current limiting resistor on the base, by the time the transistor is on enough to pull the bottom of the inductor to 0.6V or so, it'll all self-limit.
Bear in mind also that 9V batteries are shit. Alkalines start out at 10Ohms, end up at 1K. Make sure your doorbell solenoid won't just drain it in a couple of uses. A battery holder with 6 C cells has vastly better performance if you can find the space. (or 4 C cells, given that the pp3 will be wilting pretty much from the start).
