Simple Latch Circuit using Transistors

All-purpose Latch-up Switch Normally a latch-up switch needs either a relay, an SCR trigger circuit or some integrated circuit. The simple latch- up switch described here uses only a pair of general-purpose transistors. Initially. when there is no input. transistors Tl and T2 are cut-off and the output voltage is zero. Once DC voltage is applied at the input, say by momentarily pressing switch S l , transistor Tl gets base drive and conducts. driving T2 to saturation. The collector of T2 goes to almost DC supply voltage (about l V less due to the saturation VCE drop of T2). This voltage is fed back to the input. and therefore _the output persists even after removal of the external input. The circuit can be reset back to the original state only by disconnecting the DC supply, or by pressing the switch S2. Diode Dl is added to provide extra back bias to transistor Tl. Capacitor Cl prevents conduction of the transistors at the supply switch-on. T1 and T2 are general-purpose npn and pnp transistors preferably with high B. Transistor T2 should be adequately rated to be able to bear the load current. This electronic switch may be used for control of alarms etc working on DC voltages. It does not need any separate DC supply: it can be connected directly between the DC supply and the load to be controlled. It draws no idle current at all.