Simple Latch Circuit using Transistors

How to build a tiny electrical switch that stays activated even after the signal is cut off is discussed in the article.

Because it makes use of transistors and other essential components it is affordable and easy to build.

One should give a quick signal to turn ON, it will stay ON until you use a separate circuit which is not discussed in this article to turn it off.

This type of switch serves in electronics for various kinds of activities including turning lights on and off.

What is a Transistor Latch Circuit:

An electrical circuit with two transistors that latches in response to a temporary positive signal at the input from the outside is known as a transistor latch circuit.

Either grounding the transistor base or switching the power OFF and then back ON will break this latch.

Circuit Working:

Parts List:

Transistors T1 and T2 are used in the shown latch circuit in such a way that T2 can either conduct or stop conduction depending on the input signal it receives.

Further T2 acts as a buffer to increase sensitivity to even little signals.

The base of T2 is pulled to ground by T1S conductance when a little positive signal is delivered to its input.

Then with a negative bias created by T1s conduction,T2 begins to conduct.

Note that T2 is a PNP device which reacts to the negative potential from T1s conduction whereas T1 is an NPN device that responds to positive inputs.

Feedback Mechanism:

An essential component of the circuit that allows for quick latching or freezing of the output with a constant positive supply is the presence of a feedback voltage through resistor R3.

This feedback takes place when T2 comes after T1 and R3 connects T2s collector pin to T1s base.

In most cases this feedback loop makes sure that T1 conducts constantly following the original trigger resulting in a latched state.

The circuit is protected by capacitor C1 against incorrect triggers brought on by accidental pick ups or switch ON transients.

Restoring the Circuit:

Restarting the power or grounding the base of T1 using a push button setup will return the circuit to its starting point.

This ability to adjust makes controlling and modifying the latch circuit simple.

Several security and alarm systems where a dependable and latching output is required can use the transistor latch circuit that is shown in this article.

Formulas:

The following formulas should be used for calculating the transistor biasing in the circuit:

VBE = 0.7V is the base emitter voltage.

• In forward active mode this formula represents the voltage drop across the base emitter junction of a silicon transistor.
• Thus when a voltage is applied to make the base positive with respect to the emitter (forward bias) there will be a normal voltage loss of around 0.7 volts across this junction.
• This voltage drop is required to turn on the transistor and allow current to flow through it.
• It is important to keep in mind that 0.7V is only an estimate and the actual Vbe can differ somewhat depending on the transistor type, manufacturing method and temperature.

Collector Current (IC) = (β + 1) times Base Current (IB)

• This formula (BJT) is commonly used to estimate the connection between the base current (Ib) and collector current (Ic) of a bipolar junction transistor.
• The parameter β (beta) represents a transistors current gain.
• It just shows how much the base current is amplified to generate the collector current.
• In other words if β is 100 then a base current of 1 mA will provide a collector current of around 100 mA (1 mA x 100).
• It uses the expression (β + 1) since a small amount of current also leaves the base terminal itself.
• In basic biasing calculations this is often ignored because it is often much smaller than the collector current.
• To provide more precise estimations this phrase is added.

IC = β times IB

• This formula gives a more accurate relationship between the collector current and the base current by ignoring the previously mentioned small base current.
• Changes in base current are directly connected to changes in collector current which are controlled by the transistors current gain (β).

Selecting the right Formula:

The voltage required for forward biasing in silicon transistors is calculated using the first formula which is a constant (Vbe = 0.7V).

By using the second or third collector current formula will depend on the level of accuracy needed.

A good approximation for most biasing calculations is given by the second formula (Ic = (β + 1) * IB).

The third formula (Ic = β * IB) is more accurate though it misses a small current component.

Remember that these calculations are the starting point for understanding transistor biasing.

Other factors like temperature variations and component tolerances would need to be taken into account for an effective design for actual circuits.

How to Build:

To build a Simple Latch Circuit using Transistors follow the below mentioned steps:

Identify and Assemble Components:

• Assemble all necessary parts including a power supply, resistors R1, R2, R3, capacitor C1 and NPN and PNP transistors T1 and T2.
• Verify that the resistor values and transistor types match the design requirements.

Connect Transistors T1 and T2:

• T1 and T2 emitters should be connected to the ground.
• The positive supply voltage need to be connected to T1s collector.
• T2s collector should be connected to the negative supply voltage.
• By connecting T1s base to the input signal source one can ensure proper biasing.

Configure Resistors R1, R2 and R3:

• Connect resistor R1 between the base of T1 and the positive supply voltage.
• Connect resistor R2 between the base of T2 and the emitter of T1.
• Connect resistor R3 between the collector of T2 and the base of T1.

Add Capacitor C1:

• Add capacitor C1 in parallel with resistor R1.
• When the circuit is activated this arrangement helps avoid short term effects and incorrect triggers.

Power Supply Connection:

• As you connect the power supply to the proper terminals make sure the voltage levels fall within the transistors specified working limit.

Testing and Adjustment:

• To start the latch send a little positive signal to T1s input.
• Watch how the circuit behaves.
• To get the right performance make adjustments based on testing results or alter resistor values as necessary.

Conclusion:

Using these procedures a transistor latch circuit that efficiently latches onto a high output state in response to a brief input signal may be built.

Thorough testing should be conducted for the best possible performance and modifications should be made in order to meet particular application needs.

References:

Why do all transistor latch circuits (that ive seen) use two transistors?