switchestransistors
I'm trying to develop some sort of a one-time switch.
Let's say when a criminal opens a switch – transistor opens and alarm sets on. I want it to continue working once switch is closed again.
I believe there are some special "clever" components for this purpose but can I achieve it by adding only simple stuff like transistors/capacitors/diodes/etc.?
No switch is perfect. Switch selection is based on the load that you are going to handle. No load has zero inductance or zero capacitance. Hence there will be surges and kick backs. It should be handled right - if not the circuit will fail under certain inputs.
Now-a-days you could easily find FETs with very low RDS ON (even 2.0 milli Ohms is not uncommon. Typically automotive industry pays pittance for those parts - but open market pricing could be expensive. I have never tried get those outside, but I guess even if it is triple it is still within your target). But once the RDS on is low, if you ever drive it in linear mode, it could easily smoke.
Your circuit concept is starting in the right direction but it requires some important changes for it to work.
You will require series base resistors for all of the PNP transistors. As drawn you will cook them with no base current limitation.
Secondly the NPN transistor is drawn backwards. Its emitter needs to connect to the GND.
Third you need to do some serious work reorganizing the way the bases of the various transistor are biased. You need to separate the 12V and 5V PNP bases so that the 12V one can shut off when the base drive goes away to the 5V one. After this fix it is necessary to provide also a separate pullup of the NPN transistor.
Don't cringe. Resistors are incredibly cheap.
Lastly this schematic, to be bluntly honest with you, is horribly drawn being that it does a bad job of following conventional standards for how a good schematic is drawn.
Best Answer
Simplest circuit is by using a SCR (Silicon Controlled Rectifier), these are pretty common components, but not as common as transistors.
simulate this circuit – Schematic created using CircuitLab
To sound the alarm, it is enough to disconnect the gate from ground using the TRIGGER switch. A small current will flow through R1, to the gate and latch the SCR. Once an SCR is latched, it can only be reset by interrupting the current through it (by pressing the RESET switch).
You can replace a SCR with two BJT's:
simulate this circuit
Circuit source