# Transistor as a one-time switch

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.?

• Use a (low current) thyristor, or make one from two transistors. Google is your friend. Commented Feb 12, 2014 at 17:45
• You can use one transistor and 2 or 3 resistors if you are allowed to blow a fuse to make action = one time.| Q1 = NPN or N Channel FET. Load in collector/drain. Pullup R1 Vcc to base/gate. Base via R2 to fuse to ground. R2 not needed if FET used. Switch from R2/fuse junction to Vcc. R1/R2 divider such as to hold transistor off when fuse is intact. Closing switch connects Vcc to ground via fuse. Add small R3 in series with fuse if desired to limit I_fuse_max. If a FET is used R2 not needed as pullup to Vcc during fuse blowing is OK (if Vgsmax < Vcc) and fuse clamps gate till blown. Commented Feb 18, 2014 at 23:01

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

• An inexpensive small TO-92 SCR is the 2N5064. 800mA onsemi.com/pub_link/Collateral/2N5060-D.PDF Commented Feb 12, 2014 at 18:35
• Perfect, thank you! Just two transistors, what do you know. Commented Feb 13, 2014 at 10:50

What you want is something that latches. You can do this with a couple of transistors, but for a one-off its probably simpler to use a flip-flop. A simple S-R (set/reset) flip-flop will do. The detector switch opening causes a set, and you have a reset pushbutton someplace so that you can shut it up once you know it's tripped.

However, while this can be done with transistors or a flip-flop, the simplest hardware-wise would be a microcontroller. You do the latching logic in firmware, and you can wiggle a pin to make the alarm sound to drive a speaker with directly. That way you an use a plain speaker, not a integrated buzzer unit.

A simple relay latch circuit would do: -

The red line is meant to indicate that when the relay coil is activated the contact at the top of the diagram shorts thus maintaining power to the relay contact until the push button is pressed. An alarm would simply connect to an unused contact or, if the relay contact were a changeover type, the normally closed contact would route power to the alarm.

Power comes in from the right.

An SCR may be used as per the above answers, however if you truly want to use FETs/Transistors then this circuit is a essentially a solid state latch.

R3 and R2 are selected to satisfy the VGS(th) conditions of the N-CH Power Fet. Choose a Power Fet suitable to drive your load.

• Might be a an idea to put a resistor between R3 and the reset switch. The current through the BSS84 should be momentary and only an ampere or three, but still. Commented Feb 12, 2014 at 19:31
• Good alternative. Essential difference though between your circuit and mine is that you use 'make' contacts where I use 'break' contacts. It depends on the application which version is most useful. Commented Feb 12, 2014 at 19:33
• @Martin Won't work that way. Horizontally between the gate of Q2 and R3 top, and the RESET switch goes right to the gate of Q2. Commented Feb 12, 2014 at 19:51

you need a latch. a simple transistor design would be

simulate this circuit – Schematic created using CircuitLab

Under normal circumstances switch SW1 would be closed, hence transistor Q4 wud be closed and Q5 and Q2 would be open in the absence on a bias. Q3 would be biased through R3 and hence would be closed. Hence entire voltage of 5V would be dropped across R4 and thus at the collector of Q3, voltage would be negligible (around 0.2-0.5V). Thus the speaker would not get sufficient voltage to turn on. but when SW1 is opened by the infiltrator, Q4 opens, hence Q5 closes which in turn opens Q3. This action proceeds to relieve the 5V through the speaker path and through the base of Q2 since both paths have almost equal impedance. Thus Q2 closes and this prevents Q3 from closing ever again even if SW1 is closed. This is because SW1 closing or opening only affects Q5, but now Q3's base is being drained by Q2's collector and this has no way of being stopped other than by disconnection of the power supply of the circuit. The permanent opening of Q3 on the opening of SW1 ensures that the voltage keeps dropping across the 2 equi-impedance paths namely the speaker and the base of Q2, hence the speaker keeps resounding till disconnection of circuit supply.