• ### 3 Transistor Latching circuit (push on – push off)

This is a very useful circuit, which can used to turn on and off things with single push of a button.
basically, a solid switch costs more than a few transistors(in high volume productions) and especially the high current switch costs a lot. so this circuit can be used to replace solid switch but not completely because sometimes solid switches are very useful for being very simple and you don’t need to troubleshoot for a broken transistor in a circuit.

The circuit basically has two parts, the upper part(transistors Q1 and Q2) acts like a thyristor* or more commonly known as “SCR”, which is made up of npn and pnp transistors here.
The lower part is the main switching part. The working principle is simple, when power is turned on, the SCR remains off and the capacitor C1 charges through R1 and R2.
When you push the button the current flows through the base of Q2(which is the gate of the SCR)from charged C1 and which then pulls current from Q1s base which gives more current to base of Q2, turning it on harder (it is like both npn and pnp love each other) and this transistor SCR turns on(SCR action) by passing current through the led or load, which is latched now. okay now you want to turn off the led, so what happens is when you first turned on SCR using C1, which is also connected to Q3, so the current flows through Q3s base which basically discharges C1 through R2 and the gate of the SCR has some voltage drop (<0.7v due to transistor Vbe) which keeps Q3 on, to discharge C1 completely.
so now when you turn off the load or led by pushing the button again, C1 acts like short circuit until capacitor fully charges (that is how capacitors work) and the current from gate of SCR is pulled to ground which basically turns off the SCR by not having SCR action(=> npn cant pull current from pnp because the base current is pulled which stops all this process and brings it back to first state) and another thing happens is, the Q3 transistor turns off, because its base current is also pulled to ground which stops discharging the C1 and it recharges again but here comes the interesting part, the gate of the SCR doesn’t need lot of current to turn on so the capacitor doesn’t have to be fully charged, it just has to reach 0.7v to turn on the SCR Again.

When it is turned off, the C1 has to discharge fast enough before you press the button again, so R2 as 10k is fast enough. the value of the capacitor can be between 1uF and 10uF because if its too low it cant give enough current to turn on the SCR. Or if its too high, the capacitor need more time to reach the voltage(0.7v) so it response time increases which we don’t want. The 10nF capacitor used just to be on the safe side, because we don’t want Q3 to turn on before SCR turns on otherwise SCR would never turn on and this capacitor wont effect the circuit that much even if its absent.
when you are using this circuit to turn on led, use current limiting resistor so that you wont kill the led and when using relay you will also need to limit the current and use antiparallel diode to protect the transistor from voltage spikes.
The input voltage can be used between 5 and 12 volts and it works better when using more than 5 volts.

*A real Thyristor or SCR doesn’t work like that and when you pull the current from gate, the thyristor wont turn off, but this SCR (in the circuit) is made up of transistors, so here it will turn off.
The real thyristor can only be turned off when you turn off the supply(or reach below latching current), so it is mostly used in controlling phase angle in AC and other AC applications.