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  P4
Thyristor
Latch

How the circuit works:

The latching circuit uses a Thyristor / Silicon Controlled Rectifier (SCR), to keep an output turned on after an input signal has triggered the device. When a positive going voltage is briefly applied to the input of the circuit, a current will start flowing into the Gate (G) of the Thyristor and this triggers the device to turn on permanently. So when the input is removed, the output will still remain switched on. To reset the circuit, the power supply to the device needs to be removed or an open-circuit applied across its output, to prevent current flowing into the Anode (see OP1).
An output device connected between the Anode of the Thyristor and the positive supply rail is sinked by the triggered Thyristor. The output device (load) allows the output current to flow into the Thyristor via its Anode (A) and down through the Cathode (C) to the negative supply rail.
The reset switch is used to interrupt the flow of current in the Anode/ Cathode circuit and by doing so, will switch the Thyristor "off".

 

Circuit Diagram

 




Avoiding Problems:

The Thyristor can become unlatched if a mechanical buzzer is used as an output. If buzzers are used, an LED or 1K resistor connected in parallel with it will help to prevent it reseting.(See OP2)
To avoid unwanted triggering of the Thyristor input by electrical "noise", a capacitor is used as a filter and a 67K resistor "pulls" the Gate down to 0 volts when there is no input voltage present.

Comparing The Thyristor with the Transistor:

The Thyristor is connected in a similar way to the transistor ( see P1 and P5). The input to the device is the Gate (equivalent to the Base), the output is connected to the Anode (equivalent to the Collector) and the Cathode is connected down to the negative power rail (like the Emitter).