LATCH CIRCUITMay 12th, 2021 by: Rick Sanchez
See the video tutorial
Part 1 - What we need?
Below you have all the parts needed for this circuit, and on the next step you have the schematic. You can test this circuit on a breadboard or maybe solder it directly to a small prototyping PCB. You can also add an extra LED with a 100 ohm resistor at the output, so you can see when the output is ON or OFF.
Part 2 - Schematic
But wait, the Q3 BJT is also connected with the base to the push button. So at the same time, this BJT will also turn ON. The collector of this transistor is also connected to taht R3 resistor and the C1 capacitor. So, since now this Q3 is ON and creates a path to ground, the C1 which was previously charged up by the R3 resistor, will now discharge to GND. So, the next time we push the button, the voltage at R3 wont be high anymore, it will be low. So the second time we push the button it will apply low signal at the bases of Q2 and Q3 and by that they turn OFF. Then the cycle repeats since now C1 can charge back to high value. That's how we can latch the circuit ON and OFF.