Simple Latch Circuit using Transistors

The article describes how to construct a small electrical switch that remains active even after the signal is turned OFF.

It is cheap and easy to construct since it uses transistors and many other necessary parts.

If someone gives it a quick signal to turn it ON it will stay ON until some other circuit not covered in this article to turns it OFF.

In electronics this style of switch is used for many different kinds of tasks such as 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 act.

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 accurate 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 first formula a constant (Vbe = 0.7V) is used to find the voltage needed for forward biasing in silicon transistors.

The level of accuracy required depends on whether the second or third collector current formula is used.

For most of biasing calculations the second formula (Ic = (β + 1) * IB) provides a satisfactory estimate.

A little current component is missed by the third formula which is more correct (Ic = β * IB).

Keep in mind that understanding transistor biasing begins with these calculations.

An efficient design for actual circuits would require thinking about other elements such as component tolerances and temperature fluctuations.

How to Build:

To build a Simple Latch Circuit using Transistors following are the below mentioned steps for connections:

Choose and Put Together the Components:

• Put together all required components such as a power supply, NPN and PNP transistors T1 and T2, resistors R1, R2, and R3 and capacitor C1.
• One should ensure that the transistor types and resistor values match to the design standards.

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.

Set up R1, R2 and R3 resistors:

• Between the base of T1 and the positive supply voltage connect resistor R1.
• Connect resistor R2 between T1s emitter and T2s base.
• Join resistor R3 between T2s collector and T1s base.

Put Capacitor C1 in:

• Resistor R1 and capacitor C1 should be connected in parallel.
• This design helps prevent short term impacts and improper triggers when the circuit is enabled.

Connection to the Power Supply:

• Ensure that the voltage levels stay within the transistors specified operating limit as one can connect the power source to the proper terminals.

Testing and Modification:

• Give T1s input a little positive signal to start the latch.
• Observe the circuits behavior.
• As needed change the resistor values or make adjustments based on test results to get the desired result.

Conclusion:

A transistor latch circuit that safely latches onto a high output state in response to a temporary input signal could be a constructed using these steps.

For the best results complete testing should be carried out and adjustments should be made to meet certain application needs.

References:

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